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Vaporization of Intact Neutral Biomolecules Using Laser-Based Thermal Desorption

  • Yerbolat Dauletyarov
    Yerbolat Dauletyarov
    Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
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  • , 
  • Siwen Wang
    Siwen Wang
    Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
    More by Siwen Wang
  • , and 
  • Daniel A. Horke*
    Daniel A. Horke
    Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
    *Email: [email protected]
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    Orcidhttps://orcid.org/0000-0002-9862-8108
Cite this: J. Am. Soc. Mass Spectrom. 2023, 34, 7, 1538–1542
Publication Date (Web):June 15, 2023
https://doi.org/10.1021/jasms.3c00194

Copyright © 2022 The Authors. Published by American Chemical Society. This publication is licensed under

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Abstract

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The production of a clean neutral molecular sample is a crucial step in many gas-phase spectroscopy and reaction dynamics experiments investigating neutral species. Unfortunately, conventional methods based on heating cannot be used with most nonvolatile biomolecules due to their thermal instability. In this paper, we demonstrate the application of laser-based thermal desorption (LBTD) to produce neutral molecular plumes of biomolecules such as dipeptides and lipids. Specifically, we report mass spectra of glycylglycine, glycyl-l-alanine, and cholesterol obtained using LBTD vaporization, followed by soft femtosecond multiphoton ionization (fs-MPI) at 400 nm. For all molecules, the signal from the intact precursor ion was observed, highlighting the softness and applicability of the LBTD and fs-MPI approach. In more detail, cholesterol underwent hardly any fragmentation. Both dipeptides fragmented significantly, although mostly through only a single channel, which we attribute to the fs-MPI process.

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Introduction

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Delivery of biomolecules, large and small alike, into the gas-phase is the main prerequisite for successful application of gas-phase spectroscopy and reaction dynamics techniques to molecular biology. Although the invention of electrospray ionization (ESI) and matrix-assisted laser desorption and ionization (MALDI) was a huge leap forward for ionic species, the progress for neutral species was fairly modest in comparison. The main problem with neutral species is non-applicability of ion-optics-based mass-filtering to isolate a molecule/cluster of interest from neutral contaminants resulting from fragmentation and/or clustering. This is especially relevant when an experiment involves so-called “universal probes” such as femtosecond (strong-field) ionization or X-ray/electron scattering, as opposed to selective methods such as resonance-enhanced multiphoton ionization.
Historically, neutral molecular beams of compounds of interest were produced by supersonic expansion of their vapors seeded, by heating, in a carrier gas. By its very nature, this method was limited to volatile and thermally stable compounds, precluding the study of large and many small biomolecules. A major advance here was the development of laser desorption jet-cooling (LDJC) (1) introduced in the late 1980s. In this method, a neutral molecular beam is produced through desorption of a target compound, placed on or mixed with a suitable surface material (typically graphite) by an IR laser pulse directly in front of an expanding supersonic jet of a carrier gas. Since then LDJC has been very successfully combined with various spectroscopic techniques to study a range of biomolecules including nucleobases, (2) nucleosides, (3) peptides, (4) and peptide aggregates. (5) However, it has been shown that it may cause considerable fragmentation of a target molecule, contaminating a molecular beam. (6) In addition, the surface material itself is a potential source of contamination. Furthermore, typical repetition rates for LDJC are on the order of 10 Hz, which considerably limits the experimental throughput.
More recently, Greenwood and co-workers (7) introduced an alternative approach, namely laser-based thermal desorption (LBTD), in their study of proton-impact fragmentation of neutral DNA nucleosides, known to be thermally labile. It is based on an older technique, laser-induced acoustic desorption (LIAD), (8) which has been used to desorb neutral nucleobases, dinucleoside phosphates, amino acids, lipids, and small peptides. (9−13) In both approaches, a sample is deposited on a substrate (typically a foil made of metal with a high melting point) and desorbed into the gas-phase by irradiating the back side of the substrate with a laser, a continuous one in LBTD and a pulsed one (typically with nanosecond pulsewidth) in LIAD. One of the advantages of LBTD/LIAD over LDJC is the absence of any additional material such as graphite, since the sample is typically deposited by drying its solution on a metal substrate. While technologically quite similar, the desorption mechanisms for LIAD and LBTD differ significantly. While LBTD is clearly a form of soft thermal vaporization, (14,15) the mechanism for LIAD, initially thought to be purely acoustic, is still actively debated. (16,17)
The LBTD approach has now been implemented by a number of groups around the world, and has successfully vaporized nucleobases, (14,18) amino acids, (19−21) and nucleosides. (7,22) Related approaches that use a glass surface instead of a metal substrate have also been demonstrated for desorption of nanoparticles (23) or phthalocyanine molecules. (24)
A major advantage of the LBTD approach is that the use of a continuous desorption source makes this an inherently continuous molecular source, in contrast with the pulsed LIAD approach. This allows its combination with high repetition-rate ionization lasers, as we demonstrate here, for high-throughput data collection. The availability of cheap continuous diode lasers furthermore makes LBTD much more cost-effective.
In this contribution, we demonstrate that the LBTD approach is also applicable for the vaporization of fragile and nonvolatile biomolecules. In particular, we report mass spectra of two dipeptides (glycylglycine and glycyl-l-alanine) and a lipid (cholesterol) obtained using LBTD combined with femtosecond multiphoton ionization (fs-MPI) at 400 nm. Notably, because of its thermal instability, the structure of glycylglycine was not known until very recently, when it was determined for the first time using rotational spectroscopy. (25) In all cases, the obtained mass spectra contained a significant contribution from intact precursor ions, significantly more than available reference spectra obtained using electron impact ionization. The remaining fragmentation of the dipeptide samples is primarily attributed to the fs-MPI process.

Experimental Methods

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A detailed description of our LBTD-coupled mass spectrometer was given previously, (15,17) and we detail here only the parameters pertinent to the current study. A schematic of the setup is shown in Figure 1. Sample molecules were deposited onto a 10 μm-thick titanium foil (Baoji Energy Titanium Co.) by spraying their solutions using a commercially available airbrush gun (Fengda, BD-208, 0.2 mm nozzle, ∼2 bar N2), and subsequently drying under air. Glycylglycine (Sigma-Aldrich G1002, 99%) was sprayed from its water/ethanol (50%/50% by volume) solution (80 mM) and glycyl-l-alanine (Sigma-Aldrich 50150, 99%) from its aqueous solution (15 mM). Cholesterol (Sigma-Aldrich C8667, 99%) was sprayed from its pure ethanol solution (3.0 mM). Deposited molecules were desorbed into the gas-phase by irradiating the uncoated side of the moving titanium foil (50 μm/s) with the continuous output of a diode laser (445 nm, Wavespectrum Laser Inc.) as shown in inset A in Figure 1. Two knife edges were used to reduce the effective irradiation area on the foil to an approximate size of 3 × 0.2 mm2 (see inset B in Figure 1). The diode laser power onto the foil (i.e., measured after transmission through the fiber and the knife edges) was set to 0.1 W for glycylglycine, 0.24 W for glycylalanine, and 0.05 W for cholesterol.

Figure 1

Figure 1. Schematic of the laser-based thermal desorption source. The sample is deposited onto a thin titanium foil, which is held by two rotating rollers constantly providing a fresh sample. A continuous desorption laser irradiated the back of the foil to vaporize samples. Molecules are ionized by femtosecond multiphoton ionization, and ions are detected in a custom time-of-flight mass spectrometer operated in ion counting mode. See the text for further details.

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The plume of desorbed molecules was ionized by the second harmonic (400 nm) of a Ti:sapphire femtosecond laser system (Spitfire Ace, Spectra Physics) with a fundamental output at 800 nm, operated at a 3 kHz repetition rate. Typical pulse durations were 150 fs (fwhm). The ionization laser was focused into the interaction region using a plano-convex spherical lens with a focal length of 500 mm, yielding a typical spot size of ∼50 μm in diameter. Laser polarization was kept linear and parallel to the extraction electrodes. Glycylglycine and cholesterol were ionized by 4.5 and 5 μJ pulses, respectively. Mass spectra of glycyl-l-alanine were obtained at three different pulse energies: 6, 10, and 22 μJ.
Produced ions were analyzed using a custom-built Wiley–McLaren time-of-flight mass spectrometer, operating in ion counting mode and with a typical mass resolution mΔm∼500. Single ion hits on the MCP detector were recorded and time-stamped by using a combination of constant fraction discriminator (Surface Concept GmbH) and time-to-digital converter (cronologic GmbH).

Results and Discussion

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In Figure 2, we show normalized mass spectra of (a) glycylglycine (gly-gly) and (b) glycyl-l-alanine (gly-ala) obtained by combining LBTD and fs-MPI at 400 nm, along with their molecular structures. For both molecules we clearly observe a strong precursor ion peak. The mass spectrum of gly-gly, ionized using 4.5 uJ pulses at 400 nm, is very clean, consisting of two major peaks: the precursor ion peak at m/z = 132 and the most intense fragment ion peak at m/z = 30. We assign the m/z = 30 peak to the immonium ion CH2NH2+ (a1), corresponding to the amine end of gly-gly. After losing CH2NH2+, gly-gly undergoes, with a low probability, further fragmentation producing CONH+ fragment ions corresponding to a small peak at m/z = 43. To the best of our knowledge, no fragmentation mass spectra of neutral gly-gly have been reported to date, except the one provided in the NIST Chemistry Webbook, (26) which shows a rich fragmentation pattern typical for electron-impact ionization mass spectra. Nonetheless, m/z = 30 was the most abundant ionic fragment peak present in the spectrum. The immonium ion m/z = 30 was also the main ionic fragment in collision-induced dissociation of protonated gly-gly in energy ranges from 3.5 to 7 eV, (27) and this is furthermore also consistent with computational studies of the fragmentation of protonated glycine. (28) In contrast, in collision-induced dissociation of protonated cyclic-glycylglycine it was significantly less abundant than other ionic fragment peaks. (29)

Figure 2

Figure 2. Molecular structures and normalized mass spectra of (a) glycylglycine and (b) glycyl-l-alanine obtained by using LBTD and fs-MPI at 400 nm. Glycylglycine was ionized by 4.5 μJ laser pulses. Mass spectra of glycyl-l-alanine were obtained at three different ionization laser pulse energies: 6, 10, and 22 μJ, corresponding to blue, black, and red traces, respectively. Spectra are normalized to the most abundant fragment and offset for clarity.

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The mass spectra for gly-ala are shown in Figure 2b for three different ionization laser energies (and hence intensities): 6, 10, and 22 μJ (blue, black, and red traces respectively). At the lowest intensity, the obtained spectrum is very similar to that of gly-gly, dominated by a single fragment channel of m/z = 30, which we again assign to the a1 fragment CH2NH2+, and still showing significant signal from intact precursor ions. Increasing the ionization laser intensity decreases the relative intensity of the intact precursor ion while leading to additional fragmentation. In particular, this leads to the opening of a second major fragmentation product at m/z = 44, corresponding to the immonium ion of y2a2 cleavage. (30,31) This increase in observed fragment yield as the ionization laser intensity is increased clearly indicates that, as in previous studies, the observed fragmentation is primarily due to the fs-MPI process, and not the LBTD source. (15) The observed fragmentation is again consistent with the available NIST reference spectrum following electron-impact ionization. (26)
In Figure 3, we show the molecular structure and normalized mass spectrum (blue) of cholesterol that we obtained using LBTD and fs-MPI at 400 nm. For comparison, we also show the normalized mass spectrum (red) of cholesterol from the NIST Chemistry Webbook, (26) obtained by means of electron-impact ionization. Whereas the NIST spectrum shows a very rich fragmentation pattern typical for electron-impact ionization of cholesterol, (32) fs-MPI combined with LBTD produced a very clean mass spectrum containing a very intense precursor ion peak with almost no fragmentation. Nevertheless, our mass spectrum does contain some of the bands present in the NIST spectrum, albeit in a very small amount, with the most prominent ones being at m/z = 43, 275, 368, and 371. Cholesterol has also been previously studied using LIAD desorption, coupled to a VUV atmospheric pressure photoionization source. (33) While these studies also observed a significant contribution from intact precursor ions, the mass spectra were dominated by the [M−H2O]+ peak. Nonetheless, they showed significantly reduced fragmentation compared to previous VUV-desorption ionization experiments. (34) This again highlights the softness of our LBTD/fs-MPI approach, and the clear difference between LIAD and LBTD; while the mechanism for LIAD is complicated and still not well understood, (16,17) LBTD is a thermal process relying on controlled and limited heating of the metal substrate. (14,15)

Figure 3

Figure 3. Normalized mass spectrum of cholesterol (blue) obtained using LBTD and fs-MPI at 400 nm. For comparison, the normalized mass spectrum of cholesterol from the NIST Chemistry Webbook, (26) obtained by means of electron-impact ionization, is shown in red.

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Conclusion

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We demonstrated that LBTD in combination with fs-MPI can be very effectively used to produce clean, intact, and continuous molecular samples of thermally labile and nonvolatile neutral biomolecules, such as (di)peptides and lipids. Our results show that the vaporization process is extremely soft, with most of the observed fragmentation attributed to the ionization rather than the desorption process.
The presented methodology for producing intact neutral biomolecules in the gas-phase will open up plenty of new opportunities to study biomolecular processes and interactions using the advanced tools of gas-phase spectroscopy and reaction dynamics. Since the produced samples are clean and free from contamination, they also allow the use of “universal probes”, such as ultrafast lasers for time-resolved dynamics studies or X-ray/electron beams for scattering experiments. (35) The continuous nature of this molecular source furthermore allows for very high-throughput experiments if high repetition-rate lasers are used. (18) Finally, decoupling the vaporization step from the ionization step allows for greater control and flexibility over the latter.

Author Information

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  • Corresponding Author
  • Authors
    • Yerbolat Dauletyarov - Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
    • Siwen Wang - Institute for Molecules and Materials, Radboud University, Heyendaalseweg 135, 6525 AJ Nijmegen, The Netherlands
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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This work was supported by The Netherlands Organization for Scientific Research (NWO) under grant numbers STU.019.009 and VIDI.193.037. We furthermore thank the Spectroscopy of Cold Molecules Department, and in particular Prof. Bas van de Meerakker, for continued support.

References

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    Wang, Qiaolin; Xu, Lihe; Qin, Zhengbo; Yang, Xinyan; Zheng, Xianfeng
    AIP Advances (2023), 13 (3), 035116CODEN: AAIDBI; ISSN:2158-3226. (American Institute of Physics)
    DNA (DNA) carries the genetic information necessary for the synthesis of RNA and proteins; it is a biol. macromol. essential for the development and proper functioning of living organisms and is composed of nucleobases, deoxyribose, and phosphate. The four nucleobases in DNA are adenine (AD), guanine (GU), thymine (TY), and cytosine (CY). Abnormal concns. of these four nucleobases in an organism have a significant impact on disease diagnosis. Therefore, the qual. and quant. detection of these DNA nucleobases in organisms is helpful to diagnose certain diseases. In this work, we report the simultaneous detn. of purine (AD, GU) and pyrimidine (TY, CY) nucleobases in DNA using laser-induced acoustic desorption (LIAD) with electron ionization (EI)/time-of-flight mass spectrometry (TOFMS). The purine (MW 120 Da) samples were used as model compds. to assess the sensitivity and quant. performance of the instrument. Its limits of detection assessed using the LIAD/EI/MS method were ∼0.5-1.2 pg under optimal conditions, and their calibration curves exhibited good linearity (R2 = 0.98). The LIAD/TOFMS was successfully applied in the simultaneous detection of AD, GU, TY, and CY in real DNA samples. The advantage of this technique is simple, fast, and without complex pre-treatment processes. In addn., a quartz-enhanced LIAD (QE-LIAD) source was used to improve the signal strength. The desorption for complex biomols. shows that the QE-LIAD is still a "gentle" desorption source. (c) 2023 American Institute of Physics.
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  10. 10
    Jarrell, T. M.; Owen, B. C.; Riedeman, J. S.; Prentice, B. M.; Pulliam, C. J.; Max, J.; Kenttämaa, H. I. Laser-Induced Acoustic Desorption/Electron Ionization of Amino Acids and Small Peptides. J. Am. Soc. Mass. Spectrom. 2017, 28, 1091– 1098,  DOI: 10.1007/s13361-017-1684-1
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    10
    Laser-Induced Acoustic Desorption/Electron Ionization of Amino Acids and Small Peptides
    Jarrell, Tiffany M.; Owen, Benjamin C.; Riedeman, James S.; Prentice, Boone M.; Pulliam, Chris J.; Max, Joann; Kenttamaa, Hilkka I.
    Journal of the American Society for Mass Spectrometry (2017), 28 (6), 1091-1098CODEN: JAMSEF; ISSN:1044-0305. (Springer)
    Laser-induced acoustic desorption (LIAD) allows for desorption of neutral nonvolatile compds. independent of their volatility or thermal stability. Many different ionization methods have been coupled with LIAD. Hence, this setup provides a better control over the types of ions formed than other mass spectrometry evapn./ionization methods commonly used to characterize biomols., such as ESI or MALDI. The utility of LIAD coupled with electron ionization (EI) was tested for the anal. of common amino acids with no derivatization. The results compared favorably with previously reported EI mass spectra obtained using thermal desorption/EI. Further, LIAD/EI mass spectra collected for hydrochloride salts of two amino acids are similar to those measured for the neutral amino acids with the exception of the appearance of an HCl+·ion. However, the hydrochloride salt of arginine showed a distinctly different LIAD/EI mass spectrum than the previously published literature EI mass spectrum, likely due to its highly basic side chain that makes a specific zwitterionic form particularly favorable. Finally, EI mass spectra were measured for seven small peptides, including di-, tri-, and tetrapeptides. These mass spectra show a variety of ion types. However, an type ions are prevalent. Also, electron-induced dissocn. (EID) of protonated peptides has been reported to form primarily an type ions. In addn., the loss of small neutral mols. and side-chain cleavages were obsd. that are reminiscent of other high-energy fragmentation methods, such as EID. Finally, the isomeric dipeptides LG and IG produce drastically different EI mass spectra, thus allowing differentiation of the leucine and isoleucine amino acids in these dipeptides.
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  11. 11
    Liu, J.-a.; Petzold, C. J.; Ramirez-Arizmendi, L. E.; Perez, J.; Kenttämaa, H. Phenyl Radicals React with Dinucleoside Phosphates by Addition to Purine Bases and H-Atom Abstraction from a Sugar Moiety. J. Am. Chem. Soc. 2005, 127, 12758– 12759,  DOI: 10.1021/ja052766a
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    11
    Phenyl Radicals React with Dinucleoside Phosphates by Addition to Purine Bases and H-Atom Abstraction from a Sugar Moiety
    Liu, Ji-Ang; Petzold, Christopher J.; Ramirez-Arizmendi, Luis E.; Perez, James; Kenttaemaa, Hilkka
    Journal of the American Chemical Society (2005), 127 (37), 12758-12759CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Laser-induced acoustic desorption combined with mass spectrometry has been used to demonstrate that Ph radicals can attack dinucleoside phosphates at both the sugar and base moieties, that purine bases are more susceptible to the attack than pyrimidine bases, and that the more electrophilic the radical, the more efficient the damage to dinucleoside phosphates.
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  12. 12
    Li, S.; Fu, M.; Habicht, S. C.; Pates, G. O.; Nash, J. J.; Kenttämaa, H. I. Phenyl Radical-Induced Damage to Dipeptides. J. Org. Chem. 2009, 74, 7724– 7732,  DOI: 10.1021/jo901470f
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    12
    Phenyl radical-induced damage to dipeptides
    Li, Sen; Fu, Mingkun; Habicht, Steven C.; Pates, George O.; Nash, John J.; Kenttamaa, Hilkka I.
    Journal of Organic Chemistry (2009), 74 (20), 7724-7732CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
    Laser-induced acoustic desorption (LIAD) incorporated with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR) has been utilized to investigate Ph radical-induced damage to dipeptides in the gas phase. On the basis of the product branching ratios measured for the reactions of two different pos. charged Ph radicals with 17 different dipeptides, the overall order of susceptibility to attack of the different sites in the dipeptides was detd. to be heteroarom. side chain ≈ S atom in SCH3 group > H atom in SH group > H atom in CH group > arom. side chain > S atom in SH group > NH2 in side chain > N-terminal NH2 > COOH in side chain ≈ C-terminal COOH. The amino acid sequence also influences the selectivity of these reactions. As expected, the ability of a Ph radical to damage dipeptides increases as the electrophilicity of the Ph radical increases.
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  13. 13
    Benham, K.; Fernández, F. M.; Orlando, T. M. Sweep Jet Collection Laser-Induced Acoustic Desorption Atmospheric Pressure Photoionization for Lipid Analysis Applications. J. Am. Soc. Mass. Spectrom. 2019, 30, 647– 658,  DOI: 10.1007/s13361-018-2118-4
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    13
    Sweep Jet Collection Laser-Induced Acoustic Desorption Atmospheric Pressure Photoionization for Lipid Analysis Applications
    Benham, Kevin; Fernandez, Facundo M.; Orlando, Thomas M.
    Journal of the American Society for Mass Spectrometry (2019), 30 (4), 647-658CODEN: JAMSEF; ISSN:1044-0305. (Springer)
    Laser-induced acoustic desorption coupled to microplasma-based atm. pressure photoionization (LIAD-APPI) using a nebulized sweep jet to aid in dopant introduction and ion transmission has been applied to the anal. of model, apolar lipid compds. Specifically, several sterols, sterol esters, and triacylglycerols were detected using dopants such as anisole and toluene. Addnl., several triacylglycerols, sterols, carboxylic acids, and hopanoids were detected from complex mixts. of olive oil and Australian shale rock ext. as a first demonstration of the applicability of LIAD-APPI on real-world samples. Detection limits using a sweep jet configuration for α-tocopherol and cholesterol are 609 ± 61 and 292 ± 29 fmol, resp. For sterol esters and triacylglycerols with a large no. of double bonds in the fatty acid chain, LIAD-APPI yields greater mol. ion or [M+NH4]+ abundances than those with satd. fatty acid chains. Dopants such as anisole and toluene, with ionization potentials (IPs) of 8.2 and 8.8 eV, resp., were tested. A greater degree of fragmentation with several of the more labile test compds. was obsd. using toluene. Overall, LIAD-APPI with a nebulized sweep jet requires minimal sample prepn. and is a generally useful and sensitive anal. technique for low-polarity mixts. of relevance to biochem. assays and geochem. profiling.
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  14. 14
    Ghafur, O.; Crane, S. W.; Ryszka, M.; Bockova, J.; Rebelo, A.; Saalbach, L.; De Camillis, S.; Greenwood, J. B.; Eden, S.; Townsend, D. Ultraviolet relaxation dynamics in uracil: Time-resolved photoion yield studies using a laser-based thermal desorption source. J. Chem. Phys. 2018, 149, 034301,  DOI: 10.1063/1.5034419
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    14
    Ultraviolet relaxation dynamics in uracil: Time-resolved photoion yield studies using a laser-based thermal desorption source
    Ghafur, Omair; Crane, Stuart W.; Ryszka, Michal; Bockova, Jana; Rebelo, Andre; Saalbach, Lisa; De Camillis, Simone; Greenwood, Jason B.; Eden, Samuel; Townsend, Dave
    Journal of Chemical Physics (2018), 149 (3), 034301/1-034301/13CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    Wavelength-dependent measurements of the RNA base uracil, undertaken with nanosecond UV laser pulses, have previously identified a fragment at m/z = 84 (corresponding to the C3H4N2O+ ion) at excitation wavelengths ≤232 nm. This has been interpreted as a possible signature of a theor. predicted ultrafast ring-opening occurring on a neutral excited state potential energy surface. To further investigate the dynamics of this mechanism, and also the non-adiabatic dynamics operating more generally in uracil, we have used a newly built ultra-high vacuum spectrometer incorporating a laser-based thermal desorption source to perform time-resolved ion-yield measurements at pump wavelengths of 267 nm, 220 nm, and 200 nm. We also report complementary data obtained for the related species 2-thiouracil following 267 nm excitation. Where direct comparisons can be made (267 nm), our findings are in good agreement with the previously reported measurements conducted on these systems using cold mol. beams, demonstrating that the role of initial internal energy on the excited state dynamics is negligible. Our 220 nm and 200 nm data also represent the first reported ultrafast study of uracil at pump wavelengths <250 nm, revealing extremely rapid (<200 fs) relaxation of the bright S3(1ππ*) state. These measurements do not, however, provide any evidence for the appearance of the m/z = 84 fragment within the first few hundred picoseconds following excitation. This key finding indicates that the detection of this specific species in previous nanosecond work is not directly related to an ultrafast ring-opening process. An alternative excited state process, operating on a more extended time scale, remains an open possibility. (c) 2018 American Institute of Physics.
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  15. 15
    Wang, S.; Abma, G. L.; Krüger, P.; van Roij, A.; Balster, M.; Janssen, N.; Horke, D. A. Comparing pulsed and continuous laser-induced acoustic desorption (LIAD) as sources for intact biomolecules. Eur. Phys. J. D 2022, 76, 128,  DOI: 10.1140/epjd/s10053-022-00459-7
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    15
    Comparing pulsed and continuous laser-induced acoustic desorption (LIAD) as sources for intact biomolecules
    Wang, Siwen; Abma, Grite L.; Krueger, Peter; van Roij, Andre; Balster, Michiel; Janssen, Niek; Horke, Daniel A.
    European Physical Journal D: Atomic, Molecular, Optical and Plasma Physics (2022), 76 (7), 128CODEN: EPJDF6; ISSN:1434-6060. (Springer International Publishing AG)
    Abstr.: A major obstacle to the gas-phase study of larger (bio)mol. systems is the vaporisation step, i.e., the introduction of intact sample mols. into the gas-phase. A promising approach is the use of laser-induced acoustic desorption (LIAD) sources, which have been demonstrated using both nanosecond pulsed and continuous desorption lasers. We directly compare here both approaches for the first time under otherwise identical conditions using adenine as a prototypical biol. mol., and study the produced mol. plumes using femtosecond multiphoton ionisation. We observe different desorption mechanisms at play for the two different desorption laser sources; however, we find no evidence in either case that the desorption process leads to fragmentation of the target mol. unless excessive desorption energy is applied. This makes LIAD a powerful approach for techniques that require high d. and high purity samples in the gas-phase, such as ultrafast dynamics studies or diffraction expts. Graphic abstr.: [graphic not available: see fulltext].
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  16. 16
    Zinovev, A. V.; Veryovkin, I. V.; Moore, J. F.; Pellin, M. J. Laser-driven acoustic desorption of organic molecules from back-irradiated solid foils. Anal. Chem. 2007, 79, 8232– 8241,  DOI: 10.1021/ac070584o
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    16
    Laser-Driven Acoustic Desorption of Organic Molecules from Back-Irradiated Solid Foils
    Zinovev, Alexander V.; Veryovkin, Igor V.; Moore, Jerry F.; Pellin, Michael J.
    Analytical Chemistry (Washington, DC, United States) (2007), 79 (21), 8232-8241CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    Laser-induced acoustic desorption (LIAD) from thin metal foils is a promising technique for gentle and efficient volatilization of intact org. mols. from surfaces of solid substrates. Using the single-photon ionization method combined with time-of-flight mass spectrometry, we have examd. the neutral component of the desorbed flux in LIAD and compared it to that from direct laser desorption. These basic studies of LIAD, conducted for mols. of various org. dyes (rhodamine B, fluorescein, anthracene, coumarin, BBQ), have demonstrated detection of intact parent mols. of the analyte even at its surface concns. corresponding to a submonolayer coating. In some cases (rhodamine B, fluorescein, BBQ), the parent mol. ion peak was accompanied by a few fragmentation peaks of comparable intensity, whereas for others, only peaks corresponding to intact parent mols. were detected. At all measured desorbing laser intensities (from 100 to 500 MW/cm2), the total amt. of desorbed parent mols. depended exponentially on the laser intensity. Translational velocities of the desorbed intact mols., detd. for the first time in this work, were of the order of hundreds of meters per s, less than what has been obsd. in our expts. for direct laser desorption, but substantially greater than the possible perpendicular velocity of the substrate foil surface due to laser-generated acoustic waves. Moreover, these velocities did not depend on the desorbing laser intensity, which implies the presence of a more sophisticated mechanism of energy transfer than direct mech. or thermal coupling between the laser pulse and the adsorbed mols. Also, the total flux of desorbed intact mols. as a function of the total no. of desorbing laser pulses, striking the same point on the target, decayed following a power law rather than an exponential function, as would have been predicted by the shake-off model. To summarize, the results of our expts. indicate that the LIAD phenomenon cannot be described in terms of simple mech. shake-off or direct laser desorption. Rather, they suggest that multistep energy-transfer processes are involved. Two possible (and not mutually exclusive) qual. mechanisms of LIAD that are based on formation of nonequil. energy states in the adsorbate-substrate system are proposed and discussed.
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    Huang, Z.; Ossenbrüggen, T.; Rubinsky, I.; Schust, M.; Horke, D. A.; Küpper, J. Development and characterization of a laser-induced acoustic desorption source. Anal. Chem. 2018, 90, 3920– 3927,  DOI: 10.1021/acs.analchem.7b04797
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    17
    Development and Characterization of a Laser-Induced Acoustic Desorption Source
    Huang, Zhipeng; Ossenbrueggen, Tim; Rubinsky, Igor; Schust, Matthias; Horke, Daniel A.; Kuepper, Jochen
    Analytical Chemistry (Washington, DC, United States) (2018), 90 (6), 3920-3927CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
    A laser-induced acoustic desorption source, developed for use at central facilities, such as free-electron lasers, is presented. It features prolonged measurement times and a fixed interaction point. A novel sample deposition method using aerosol spraying provides a uniform sample coverage and hence stable signal intensity. Using strong-field ionization as a universal detection scheme, the produced mol. plume was characterized in terms of no. d., spatial extend, fragmentation, temporal distribution, translational velocity, and translational temp. The effect of desorption laser intensity on these plume properties is evaluated. While translational velocity is invariant for different desorption laser intensities, pointing to a nonthermal desorption mechanism, the translational temp. increases significantly and higher fragmentation is obsd. with increased desorption laser fluence.
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    Wang, S.; Dauletyarov, Y.; Krüger, P.; Horke, D. A. High-throughput UV-photofragmentation studies of thymine and guanine. Phys. Chem. Chem. Phys. 2023, 25, 12322– 12330,  DOI: 10.1039/D3CP00328K
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    18
    High-throughput UV-photofragmentation studies of thymine and guanine
    Wang, Siwen; Dauletyarov, Yerbolat; Krueger, Peter; Horke, Daniel A.
    Physical Chemistry Chemical Physics (2023), 25 (17), 12322-12330CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    High-throughput photofragmentation studies of thymine and guanine were performed at 257 and 343 nm and for a wide range of ionisation laser intensities. Combining a continuous laser-based thermal desorption source with femtosecond multiphoton ionisation using a 50 kHz repetition rate laser allowed us to produce detailed 2D maps of fragmentation as a function of incident laser intensity. The fragmentation was distinctly soft, the parent ions being at least an order of magnitude more abundant than fragment ions. For thymine there was a single dominant fragmentation channel, which involves consecutive HNCO and CO losses. In contrast, for guanine there were several competing ones, the most probable channel corresponding to CH2N2 loss through opening of the pyrimidine ring. The dependence of parent ion abundance on the ionisation laser intensity showed that at 257 nm the ionisation of thymine is a 1 + 1 resonance enhanced process through its open-shell singlet state.
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    Calegari, F.; Ayuso, D.; Trabattoni, A.; Belshaw, L.; De Camillis, S.; Anumula, S.; Frassetto, F.; Poletto, L.; Palacios, A.; Decleva, P.; Greenwood, J. B.; Martín, F.; Nisoli, M. Ultrafast electron dynamics in phenylalanine initiated by attosecond pulses. Science 2014, 346, 336– 339,  DOI: 10.1126/science.1254061
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    19
    Ultrafast electron dynamics in phenylalanine initiated by attosecond pulses
    Calegari, F.; Ayuso, D.; Trabattoni, A.; Belshaw, L.; De Camillis, S.; Anumula, S.; Frassetto, F.; Poletto, L.; Palacios, A.; Decleva, P.; Greenwood, J. B.; Martin, F.; Nisoli, M.
    Science (Washington, DC, United States) (2014), 346 (6207), 336-339CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    In the past decade, attosecond technol. has opened up the investigation of ultrafast electronic processes in atoms, simple mols., and solids. Here, we report the application of isolated attosecond pulses to prompt ionization of the amino acid phenylalanine and the subsequent detection of ultrafast dynamics on a sub-4.5-fs temporal scale, which is shorter than the vibrational response of the mol. The ability to initiate and observe such electronic dynamics in polyat. mols. represents a crucial step forward in attosecond science, which is progressively moving toward the investigation of more and more complex systems.
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    Calegari, F.; Trabattoni, A.; Palacios, A.; Ayuso, D.; Castrovilli, M. C.; Greenwood, J. B.; Decleva, P.; Martín, F.; Nisoli, M. Charge migration induced by attosecond pulses in bio-relevant molecules. J. Phys. B 2016, 49, 142001,  DOI: 10.1088/0953-4075/49/14/142001
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    20
    Charge migration induced by attosecond pulses in bio-relevant molecules
    Calegari, Francesca; Trabattoni, Andrea; Palacios, Alicia; Ayuso, David; Castrovilli, Mattea C.; Greenwood, Jason B.; Decleva, Piero; Martin, Fernando; Nisoli, Mauro
    Journal of Physics B: Atomic, Molecular and Optical Physics (2016), 49 (14), 142001/1-142001/25CODEN: JPAPEH; ISSN:0953-4075. (IOP Publishing Ltd.)
    A review. After sudden ionization of a large mol., the pos. charge can migrate throughout the system on a sub-femtosecond time scale, purely guided by electronic coherences. The possibility to actively explore the role of the electron dynamics in the photo-chem. of bio-relevant mols. is of fundamental interest for understanding, and perhaps ultimately controlling, the processes leading to damage, mutation and, more generally, to the alteration of the biol. functions of the macromol. Attosecond laser sources can provide the extreme time resoln. required to follow this ultrafast charge flow. In this review we will present recent advances in attosecond mol. science: after a brief description of the results obtained for small mols., recent exptl. and theor. findings on charge migration in bio-relevant mols. will be discussed.
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    Sparling, C.; Crane, S. W.; Ireland, L.; Anderson, R.; Ghafur, O.; Greenwood, J. B.; Townsend, D. Velocity-map imaging of photoelectron circular dichroism in non-volatile molecules using a laser-based desorption source. Phys. Chem. Chem. Phys. 2023, 25, 6009– 6015,  DOI: 10.1039/D2CP05880D
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    21
    Velocity-map imaging of photoelectron circular dichroism in non-volatile molecules using a laser-based desorption source
    Sparling, Chris; Crane, Stuart W.; Ireland, Lewis; Anderson, Ross; Ghafur, Omair; Greenwood, Jason B.; Townsend, Dave
    Physical Chemistry Chemical Physics (2023), 25 (8), 6009-6015CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    We present an initial demonstration of a velocity-map imaging (VMI) expt. using a back-irradn. laser-based desorption source directly integrated into the electrode assembly. This has the potential to greatly expand the utility of the popular VMI approach by permitting its use with high d. plumes of non-volatile mol. samples. Photoelectron CD measurements on the phenylalanine mol. using 400 nm multiphoton ionization are used to illustrate this novel method, revealing forward-backward emission asymmetries on the order of 7%.
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    Camillis, S. D.; Miles, J.; Alexander, G.; Ghafur, O.; Williams, I. D.; Townsend, D.; Greenwood, J. B. Ultrafast Non-Radiative Decay of Gas-Phase Nucleosides. Phys. Chem. Chem. Phys. 2015, 17, 23643– 23650,  DOI: 10.1039/C5CP03806E
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    22
    Ultrafast non-radiative decay of gas-phase nucleosides
    Camillis, Simone De; Miles, Jordan; Alexander, Grace; Ghafur, Omair; Williams, Ian D.; Townsend, Dave; Greenwood, Jason B.
    Physical Chemistry Chemical Physics (2015), 17 (36), 23643-23650CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    The ultrafast photo-phys. properties of DNA are crucial in providing a stable basis for life. Although the DNA bases efficiently absorb UV radiation, this energy can be dissipated to the surrounding environment by the rapid conversion of electronic energy to vibrational energy within about a picosecond. The intrinsic nature of this internal conversion process has previously been demonstrated through gas phase expts. on the bases, supported by theor. calcns. De-excitation rates appear to be accelerated when individual bases are hydrogen bonded to solvent mols. or their complementary Watson-Crick pair. In this paper, the first gas-phase measurements of electronic relaxation in DNA nucleosides following UV excitation are reported. Using a pump-probe ionization scheme, the lifetimes for internal conversion to the ground state following excitation at 267 nm are found to be reduced by around a factor of two for adenosine, cytidine and thymidine compared with the isolated bases. These results are discussed in terms of a recent proposition that a charge transfer state provides an addnl. internal conversion pathway mediated by proton transfer through a sugar to base hydrogen bond.
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    Bulgakov, A. V.; Goodfriend, N.; Nerushev, O.; Bulgakova, N. M.; Starinskiy, S. V.; Shukhov, Y. G.; Campbell, E. E. B. Laser-induced transfer of nanoparticles for gas-phase analysis. J. Opt. Soc. Am. B 2014, 31, C15– C21,  DOI: 10.1364/JOSAB.31.000C15
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    23
    Laser-induced transfer of nanoparticles for gas-phase analysis
    Bulgakov, Alexander V.; Goodfriend, Nathan; Nerushev, Oleg; Bulgakova, Nadezhda M.; Starinskiy, Sergei V.; Shukhov, Yuri G.; Campbell, Eleanor E. B.
    Journal of the Optical Society of America B: Optical Physics (2014), 31 (11), C15-C21CODEN: JOBPDE; ISSN:0740-3224. (Optical Society of America)
    An exptl. study of laser-induced forward transfer of nanoparticles from a metal-coated glass substrate is presented. Nanoparticles are efficiently removed from the substrates due to transient blister formation. A combination of mass spectrometry, at. force microscopy studies of the irradiated substrates, and theor. considerations of temp. distributions and stress in the films during irradn. serves to provide insight into the mechanisms involved.
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    The energy thresholds for fragment ions from the collision-induced dissocn. of the protonated amino acid Gly, glycinamides H-Gly-NH2 and H-Gly-NHMe, and the peptides H-(Gly)n-OH (n = 2-4) and H-Gly-Gly-NH2 were detd. with a modified triple quadrupole mass spectrometer. The precursor ions were produced by electrospray. The threshold energies for the formation of the major fragment ions were detd. by fitting the exptl. detd. threshold curves to theor. threshold equations following the procedure developed by Armentrout and co-workers. To obtain corrections for the kinetic shifts, several of the transition states were modeled by using ab initio or semiempirical calcns. The immonium ion, CH2NH2+, which is an a1 ion, was obsd. as a major fragment ion from precursor ions (H2NCH2CO-X)H+, where X = OH, NH2, NHMe, and NH2CO2H. The activation energies obtained from the thresholds were between 40 and 50 kcal/mol. These energies were in good agreement with theor. evaluated activation energies for transition states based on a mechanism proposed by A. G. Harrison and co-workers. A second pathway leading to XH2+ (y1) ions was obsd. for X = NHMe and NHCH2CO2H. The threshold based activation energy was somewhat lower than that for the a1 ions. Agreement between the threshold based activation energy and the calcd. activation energy for a transition state involving an aziridinone intermediate provides support for the mechanism proposed by C. Wesdemiotis and co-workers. The lowest energy fragmentation pathway for ions with the general structure +H(Gly-Gly-X) was found to be the b2 (acylium) ion. In the case where X = NH2, the activation energy was only 20.4 kcal/mol. This very low energy was found consistent with the b2 ions being cyclic, protonated oxazalones, as proposed by A. G. Harrison and co-workers. The thresholds for the fragment ions from larger precursor ions were affected by very large kinetic shifts and suppression by competitive decompns. Accurate activation energies for these reactions could not be obtained. Nevertheless, the threshold curves provide a view of the evolution into the complex pathways occurring for the higher polypeptides.
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    Collision-induced dissocn. has been used to study the fragmentations of two protonated diketopiperazines, protonated cyclic-glycylglycine and cyclic-alanylalanine. Protonated cyclo-AA lost CO and (CO + NH3) at low collision energies, channels attributed to dissocn. of the O-protonated tautomer. Higher collision energies were required to dissoc. protonated cyclo-GG, and the two lowest-energy products were the result of losses of one CO and two CO mols. These occur from the higher-energy N-protonated tautomer, which is formed from the O-protonated tautomer by a 1,4-proton shift that has a high barrier (54.5 kcal mol-1) due to constraints imposed by the ring. Mechanistic schemes for four different dissocn. channels, three from the N-protonated tautomer and one from the O-protonated tautomer, have been computed using d. functional theory (DFT) at the B3LYP/6-311++G(d,p) level. Comparison of the potential energy surfaces for the two protonated diketopiperazines reveals the factors behind this dichotomy of fragmentation pathways. The IR multiple-photon dissocn. spectrum of the [M+H-NH3-CO]+ ion (m/z 98) from protonated cyclo-AA shows this product to be an oxazole, the lowest-energy isomer.
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    We demonstrate the first application of laser-induced acoustic desorption (LIAD) and atm. pressure photoionization (APPI) as a mass spectrometric method for detecting low-polarity orgs. This was accomplished using a Lyman-α (10.2 eV) photon generating microhollow cathode discharge (MHCD) microplasma photon source in conjunction with the addn. of a gas-phase mol. dopant. This combination provided a soft desorption and a relatively soft ionization technique. Selected compds. analyzed include α-tocopherol, perylene, cholesterol, phenanthrene, phylloquinone, and squalene. Detectable surface concns. as low as a few pmol per spot sampled were achievable using test mols. The combination of LIAD and APPI provided a soft desorption and ionization technique that can allow detection of labile, low-polarity, structurally complex mols. over a wide mass range with minimal fragmentation.
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    Mass spectrometry imaging of intact cholesterol in a mouse esophagus tissue section and mouse zygotes using VUV laser desorption/ionization method
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    International Journal of Mass Spectrometry (2018), 432 (), 9-13CODEN: IMSPF8; ISSN:1387-3806. (Elsevier B.V.)
    A new instrument based on VUV laser desorption/ionization (VUVDI) method was used to measure the mass spectrum of the cholesterol std. and the mass spectrometry images (MSI) of intact cholesterol in a mouse esophagus tissue section and mouse zygotes. The VUVDI mass spectrum of the cholesterol std. shows dominant peaks from the parent mols. In the case of the mouse esophagus tissue section, the ion images with a lateral resoln. of ∼ 4μm reveal that the cholesterol populates mainly in the mucosae. The exptl. results indicate that the VUVDI method may provide a new MSI tool to measure the intact cholesterol in tissues and cells.
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    Molecular imaging using X-ray free-electron lasers
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    A review. The opening of hard X-ray free-electron laser facilities, such as the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Lab. in the United States, has ushered in a new era in structural detn. With X-ray pulse durations down to 10 fs or shorter, and up to 1013 transversely coherent photons per pulse in a narrow spectral bandwidth, focused irradiances of 1018 to 1021 W cm-2 or higher can be produced at X-ray energies ranging from 500 eV to 10 keV. New techniques for detg. the structure of systems that cannot be crystd. and for studying the time-resolved behavior of irreversible reactions at femtosecond timescales are now available.
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  • Abstract

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    Figure 1

    Figure 1. Schematic of the laser-based thermal desorption source. The sample is deposited onto a thin titanium foil, which is held by two rotating rollers constantly providing a fresh sample. A continuous desorption laser irradiated the back of the foil to vaporize samples. Molecules are ionized by femtosecond multiphoton ionization, and ions are detected in a custom time-of-flight mass spectrometer operated in ion counting mode. See the text for further details.

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    Figure 2

    Figure 2. Molecular structures and normalized mass spectra of (a) glycylglycine and (b) glycyl-l-alanine obtained by using LBTD and fs-MPI at 400 nm. Glycylglycine was ionized by 4.5 μJ laser pulses. Mass spectra of glycyl-l-alanine were obtained at three different ionization laser pulse energies: 6, 10, and 22 μJ, corresponding to blue, black, and red traces, respectively. Spectra are normalized to the most abundant fragment and offset for clarity.

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    Figure 3

    Figure 3. Normalized mass spectrum of cholesterol (blue) obtained using LBTD and fs-MPI at 400 nm. For comparison, the normalized mass spectrum of cholesterol from the NIST Chemistry Webbook, (26) obtained by means of electron-impact ionization, is shown in red.

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      Double-resonant IR-UV spectroscopy of isolated gramicidin peptides in the gas phase shows evidence for a helical structure, similar to that in the condensed phase. This approach probes local details, free of interactions, in isolated mols.
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    5. 5
      Bakels, S.; Porskamp, S. B. A.; Rijs, A. M. Formation of Neutral Peptide Aggregates as Studied by Mass-Selective IR Action Spectroscopy. Angew. Chem., Int. Ed. 2019, 58, 10537– 10541,  DOI: 10.1002/anie.201902644
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      5
      Formation of Neutral Peptide Aggregates as Studied by Mass-Selective IR Action Spectroscopy
      Bakels, Sjors; Porskamp, Sebastiaan B. A.; Rijs, Anouk M.
      Angewandte Chemie, International Edition (2019), 58 (31), 10537-10541CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The spontaneous aggregation of proteins and peptides is widely studied owing to its relation to neurodegenerative diseases. To understand the underlying principles of peptide aggregation, elucidation of structure and structural changes upon their formation is key. This level of detail can be obtained by studying the peptide self-assembly in the gas phase. Structural characterization of aggregates is mainly done on charged species, as adding charges is an intrinsic part of the technique to bring mols. into the gas phase. Studying neutral peptide aggregates will complement the existing picture. These studies are restricted to dimers due to exptl. limitations. Herein, the authors present advances in laser desorption mol. beam spectroscopy to form neutral peptide aggregates consisting of up to 14 monomeric peptides in the gas phase. The combination of this technique with IR-UV spectroscopy allowed the authors to select each aggregate by size and subsequently characterize its structure.
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    6. 6
      Teschmit, N.; Długołęcki, K.; Gusa, D.; Rubinsky, I.; Horke, D. A.; Küpper, J. Characterizing and optimizing a laser-desorption molecular beam source. J. Chem. Phys. 2017, 147, 144204,  DOI: 10.1063/1.4991639
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      6
      Characterizing and optimizing a laser-desorption molecular beam source
      Teschmit, Nicole; Dlugolecki, Karol; Gusa, Daniel; Rubinsky, Igor; Horke, Daniel A.; Kuepper, Jochen
      Journal of Chemical Physics (2017), 147 (14), 144204/1-144204/9CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The design and characterization of a new laser-desorption mol. beam source, tailored for use in x-ray free-electron laser and ultrashort-pulse laser imaging expts., is presented. It consists of a single mech. unit contg. all source components, including the mol.-beam valve, the sample, and the fiber-coupled desorption laser, which is movable in 5 axes, as required for expts. at central facilities. Using strong-field ionization, the authors characterize the produced mol. beam and evaluate the influence of desorption laser pulse energy, relative timing of valve opening and desorption laser, sample bar height, and which part of the mol. packet is probed on the sample properties. Strong-field ionization acts as a universal probe and allows detecting all species present in the mol. beam, and hence enables one to analyze the purity of the produced mol. beam, including mol. fragments. Optimized exptl. parameters are presented for the prodn. of the purest mol. beam, contg. the highest yield of intact parent ions, which to be very sensitive to the placement of the desorbed-mol. plumes within the supersonic expansion were found. (c) 2017 American Institute of Physics.
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    7. 7
      Poully, J.-C.; Miles, J.; De Camillis, S.; Cassimi, A.; Greenwood, J. B. Proton irradiation of DNA nucleosides in the gas phase. Phys. Chem. Chem. Phys. 2015, 17, 7172– 7180,  DOI: 10.1039/C4CP05303F
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      7
      Proton irradiation of DNA nucleosides in the gas phase
      Poully, Jean-Christophe; Miles, Jordan; De Camillis, Simone; Cassimi, Amine; Greenwood, Jason B.
      Physical Chemistry Chemical Physics (2015), 17 (11), 7172-7180CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The four DNA nucleosides guanosine, adenosine, cytidine and thymidine have been produced in the gas phase by a laser thermal desorption source, and irradiated by a beam of protons with 5 keV kinetic energy. The mol. ions as well as energetic neutrals formed have been analyzed by mass spectrometry in order to shed light on the ionization and fragmentation processes triggered by proton collision. A range of 8-20 eV has been estd. for the binding energy of the electron captured by the proton. Glycosidic bond cleavage between the base and sugar has been obsd. with a high probability for all nucleosides, resulting in predominantly intact base ions for guanosine, adenosine, and cytidine but not for thymidine where intact sugar ions are dominant. This behavior is influenced by the ionization energies of the nucleobases (G < A < C < T), which seems to det. the localization of the charge following the initial ionization. This charge transfer process can also be inferred from the prodn. of protonated base ions, which have a similar dependence on the base ionization potential, although the base proton affinity might also play a role. Other dissocn. pathways have also been identified, including further fragmentation of the base and sugar moieties for thymidine and guanosine, resp., and partial breakup of the sugar ring without glycosidic bond cleavage mainly for adenosine and cytidine. These results show that charge localization following ionization by proton irradn. is important in detg. dissocn. channels of isolated nucleosides, which could in turn influence direct radiation damage in DNA.
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    8. 8
      Ma, X.; Zhang, Y.; Lei, H.-R.; Kenttämaa, H. I. Laser-induced acoustic desorption. MRS Bull. 2019, 44, 372– 381,  DOI: 10.1557/mrs.2019.105
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      8
      Laser-induced acoustic desorption
      Zhigilei, Leonid V.; Helvajian, Henry; Ma, Xin; Zhang, Yuyang; Lei, Hao-Ran; Kenttaemaa, Hilkka I.
      MRS Bulletin (2019), 44 (5), 372-381CODEN: MRSBEA; ISSN:0883-7694. (Cambridge University Press)
      Laser-induced acoustic desorption (LIAD) enables the desorption of nonvolatile and/or thermally labile neutral compds., such as asphaltenes, satd. hydrocarbons in base-oil fractions and biomols., from a metal surface into a mass spectrometer. This is a "gentle" evapn. technique and causes minimal fragmentation to the desorbed neutral mols., including oligonucleotides and polypeptides. LIAD can be coupled with a wide range of ionization methods to facilitate anal. of the desorbed analytes by using many different types of mass spectrometers, including Fourier transform ion cyclotron resonance, linear quadrupole ion trap and quadrupole time-of-flight instruments. The development and improvement of LIAD remains an active research area with diverse goals such as better desorption efficiencies, minimized analyte fragmentation and greater versatility. This article details the theory, exptl. methods, applications, and future directions of LIAD in combination with mass spectrometry.
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    9. 9
      Wang, Q.; Xu, L.; Qin, Z.; Yang, X.; Zheng, X. Potential use of LIAD time-of-flight mass spectrometry for the detection of biomolecules: An example of detecting nucleobases in DNA. AIP Adv. 2023, 13, 035116,  DOI: 10.1063/5.0137046
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      9
      Potential use of LIAD time-of-flight mass spectrometry for the detection of biomolecules: An example of detecting nucleobases in DNA
      Wang, Qiaolin; Xu, Lihe; Qin, Zhengbo; Yang, Xinyan; Zheng, Xianfeng
      AIP Advances (2023), 13 (3), 035116CODEN: AAIDBI; ISSN:2158-3226. (American Institute of Physics)
      DNA (DNA) carries the genetic information necessary for the synthesis of RNA and proteins; it is a biol. macromol. essential for the development and proper functioning of living organisms and is composed of nucleobases, deoxyribose, and phosphate. The four nucleobases in DNA are adenine (AD), guanine (GU), thymine (TY), and cytosine (CY). Abnormal concns. of these four nucleobases in an organism have a significant impact on disease diagnosis. Therefore, the qual. and quant. detection of these DNA nucleobases in organisms is helpful to diagnose certain diseases. In this work, we report the simultaneous detn. of purine (AD, GU) and pyrimidine (TY, CY) nucleobases in DNA using laser-induced acoustic desorption (LIAD) with electron ionization (EI)/time-of-flight mass spectrometry (TOFMS). The purine (MW 120 Da) samples were used as model compds. to assess the sensitivity and quant. performance of the instrument. Its limits of detection assessed using the LIAD/EI/MS method were ∼0.5-1.2 pg under optimal conditions, and their calibration curves exhibited good linearity (R2 = 0.98). The LIAD/TOFMS was successfully applied in the simultaneous detection of AD, GU, TY, and CY in real DNA samples. The advantage of this technique is simple, fast, and without complex pre-treatment processes. In addn., a quartz-enhanced LIAD (QE-LIAD) source was used to improve the signal strength. The desorption for complex biomols. shows that the QE-LIAD is still a "gentle" desorption source. (c) 2023 American Institute of Physics.
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    10. 10
      Jarrell, T. M.; Owen, B. C.; Riedeman, J. S.; Prentice, B. M.; Pulliam, C. J.; Max, J.; Kenttämaa, H. I. Laser-Induced Acoustic Desorption/Electron Ionization of Amino Acids and Small Peptides. J. Am. Soc. Mass. Spectrom. 2017, 28, 1091– 1098,  DOI: 10.1007/s13361-017-1684-1
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      10
      Laser-Induced Acoustic Desorption/Electron Ionization of Amino Acids and Small Peptides
      Jarrell, Tiffany M.; Owen, Benjamin C.; Riedeman, James S.; Prentice, Boone M.; Pulliam, Chris J.; Max, Joann; Kenttamaa, Hilkka I.
      Journal of the American Society for Mass Spectrometry (2017), 28 (6), 1091-1098CODEN: JAMSEF; ISSN:1044-0305. (Springer)
      Laser-induced acoustic desorption (LIAD) allows for desorption of neutral nonvolatile compds. independent of their volatility or thermal stability. Many different ionization methods have been coupled with LIAD. Hence, this setup provides a better control over the types of ions formed than other mass spectrometry evapn./ionization methods commonly used to characterize biomols., such as ESI or MALDI. The utility of LIAD coupled with electron ionization (EI) was tested for the anal. of common amino acids with no derivatization. The results compared favorably with previously reported EI mass spectra obtained using thermal desorption/EI. Further, LIAD/EI mass spectra collected for hydrochloride salts of two amino acids are similar to those measured for the neutral amino acids with the exception of the appearance of an HCl+·ion. However, the hydrochloride salt of arginine showed a distinctly different LIAD/EI mass spectrum than the previously published literature EI mass spectrum, likely due to its highly basic side chain that makes a specific zwitterionic form particularly favorable. Finally, EI mass spectra were measured for seven small peptides, including di-, tri-, and tetrapeptides. These mass spectra show a variety of ion types. However, an type ions are prevalent. Also, electron-induced dissocn. (EID) of protonated peptides has been reported to form primarily an type ions. In addn., the loss of small neutral mols. and side-chain cleavages were obsd. that are reminiscent of other high-energy fragmentation methods, such as EID. Finally, the isomeric dipeptides LG and IG produce drastically different EI mass spectra, thus allowing differentiation of the leucine and isoleucine amino acids in these dipeptides.
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    11. 11
      Liu, J.-a.; Petzold, C. J.; Ramirez-Arizmendi, L. E.; Perez, J.; Kenttämaa, H. Phenyl Radicals React with Dinucleoside Phosphates by Addition to Purine Bases and H-Atom Abstraction from a Sugar Moiety. J. Am. Chem. Soc. 2005, 127, 12758– 12759,  DOI: 10.1021/ja052766a
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      11
      Phenyl Radicals React with Dinucleoside Phosphates by Addition to Purine Bases and H-Atom Abstraction from a Sugar Moiety
      Liu, Ji-Ang; Petzold, Christopher J.; Ramirez-Arizmendi, Luis E.; Perez, James; Kenttaemaa, Hilkka
      Journal of the American Chemical Society (2005), 127 (37), 12758-12759CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Laser-induced acoustic desorption combined with mass spectrometry has been used to demonstrate that Ph radicals can attack dinucleoside phosphates at both the sugar and base moieties, that purine bases are more susceptible to the attack than pyrimidine bases, and that the more electrophilic the radical, the more efficient the damage to dinucleoside phosphates.
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    12. 12
      Li, S.; Fu, M.; Habicht, S. C.; Pates, G. O.; Nash, J. J.; Kenttämaa, H. I. Phenyl Radical-Induced Damage to Dipeptides. J. Org. Chem. 2009, 74, 7724– 7732,  DOI: 10.1021/jo901470f
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      12
      Phenyl radical-induced damage to dipeptides
      Li, Sen; Fu, Mingkun; Habicht, Steven C.; Pates, George O.; Nash, John J.; Kenttamaa, Hilkka I.
      Journal of Organic Chemistry (2009), 74 (20), 7724-7732CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
      Laser-induced acoustic desorption (LIAD) incorporated with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR) has been utilized to investigate Ph radical-induced damage to dipeptides in the gas phase. On the basis of the product branching ratios measured for the reactions of two different pos. charged Ph radicals with 17 different dipeptides, the overall order of susceptibility to attack of the different sites in the dipeptides was detd. to be heteroarom. side chain ≈ S atom in SCH3 group > H atom in SH group > H atom in CH group > arom. side chain > S atom in SH group > NH2 in side chain > N-terminal NH2 > COOH in side chain ≈ C-terminal COOH. The amino acid sequence also influences the selectivity of these reactions. As expected, the ability of a Ph radical to damage dipeptides increases as the electrophilicity of the Ph radical increases.
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    13. 13
      Benham, K.; Fernández, F. M.; Orlando, T. M. Sweep Jet Collection Laser-Induced Acoustic Desorption Atmospheric Pressure Photoionization for Lipid Analysis Applications. J. Am. Soc. Mass. Spectrom. 2019, 30, 647– 658,  DOI: 10.1007/s13361-018-2118-4
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      13
      Sweep Jet Collection Laser-Induced Acoustic Desorption Atmospheric Pressure Photoionization for Lipid Analysis Applications
      Benham, Kevin; Fernandez, Facundo M.; Orlando, Thomas M.
      Journal of the American Society for Mass Spectrometry (2019), 30 (4), 647-658CODEN: JAMSEF; ISSN:1044-0305. (Springer)
      Laser-induced acoustic desorption coupled to microplasma-based atm. pressure photoionization (LIAD-APPI) using a nebulized sweep jet to aid in dopant introduction and ion transmission has been applied to the anal. of model, apolar lipid compds. Specifically, several sterols, sterol esters, and triacylglycerols were detected using dopants such as anisole and toluene. Addnl., several triacylglycerols, sterols, carboxylic acids, and hopanoids were detected from complex mixts. of olive oil and Australian shale rock ext. as a first demonstration of the applicability of LIAD-APPI on real-world samples. Detection limits using a sweep jet configuration for α-tocopherol and cholesterol are 609 ± 61 and 292 ± 29 fmol, resp. For sterol esters and triacylglycerols with a large no. of double bonds in the fatty acid chain, LIAD-APPI yields greater mol. ion or [M+NH4]+ abundances than those with satd. fatty acid chains. Dopants such as anisole and toluene, with ionization potentials (IPs) of 8.2 and 8.8 eV, resp., were tested. A greater degree of fragmentation with several of the more labile test compds. was obsd. using toluene. Overall, LIAD-APPI with a nebulized sweep jet requires minimal sample prepn. and is a generally useful and sensitive anal. technique for low-polarity mixts. of relevance to biochem. assays and geochem. profiling.
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    14. 14
      Ghafur, O.; Crane, S. W.; Ryszka, M.; Bockova, J.; Rebelo, A.; Saalbach, L.; De Camillis, S.; Greenwood, J. B.; Eden, S.; Townsend, D. Ultraviolet relaxation dynamics in uracil: Time-resolved photoion yield studies using a laser-based thermal desorption source. J. Chem. Phys. 2018, 149, 034301,  DOI: 10.1063/1.5034419
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      14
      Ultraviolet relaxation dynamics in uracil: Time-resolved photoion yield studies using a laser-based thermal desorption source
      Ghafur, Omair; Crane, Stuart W.; Ryszka, Michal; Bockova, Jana; Rebelo, Andre; Saalbach, Lisa; De Camillis, Simone; Greenwood, Jason B.; Eden, Samuel; Townsend, Dave
      Journal of Chemical Physics (2018), 149 (3), 034301/1-034301/13CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      Wavelength-dependent measurements of the RNA base uracil, undertaken with nanosecond UV laser pulses, have previously identified a fragment at m/z = 84 (corresponding to the C3H4N2O+ ion) at excitation wavelengths ≤232 nm. This has been interpreted as a possible signature of a theor. predicted ultrafast ring-opening occurring on a neutral excited state potential energy surface. To further investigate the dynamics of this mechanism, and also the non-adiabatic dynamics operating more generally in uracil, we have used a newly built ultra-high vacuum spectrometer incorporating a laser-based thermal desorption source to perform time-resolved ion-yield measurements at pump wavelengths of 267 nm, 220 nm, and 200 nm. We also report complementary data obtained for the related species 2-thiouracil following 267 nm excitation. Where direct comparisons can be made (267 nm), our findings are in good agreement with the previously reported measurements conducted on these systems using cold mol. beams, demonstrating that the role of initial internal energy on the excited state dynamics is negligible. Our 220 nm and 200 nm data also represent the first reported ultrafast study of uracil at pump wavelengths <250 nm, revealing extremely rapid (<200 fs) relaxation of the bright S3(1ππ*) state. These measurements do not, however, provide any evidence for the appearance of the m/z = 84 fragment within the first few hundred picoseconds following excitation. This key finding indicates that the detection of this specific species in previous nanosecond work is not directly related to an ultrafast ring-opening process. An alternative excited state process, operating on a more extended time scale, remains an open possibility. (c) 2018 American Institute of Physics.
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    15. 15
      Wang, S.; Abma, G. L.; Krüger, P.; van Roij, A.; Balster, M.; Janssen, N.; Horke, D. A. Comparing pulsed and continuous laser-induced acoustic desorption (LIAD) as sources for intact biomolecules. Eur. Phys. J. D 2022, 76, 128,  DOI: 10.1140/epjd/s10053-022-00459-7
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      15
      Comparing pulsed and continuous laser-induced acoustic desorption (LIAD) as sources for intact biomolecules
      Wang, Siwen; Abma, Grite L.; Krueger, Peter; van Roij, Andre; Balster, Michiel; Janssen, Niek; Horke, Daniel A.
      European Physical Journal D: Atomic, Molecular, Optical and Plasma Physics (2022), 76 (7), 128CODEN: EPJDF6; ISSN:1434-6060. (Springer International Publishing AG)
      Abstr.: A major obstacle to the gas-phase study of larger (bio)mol. systems is the vaporisation step, i.e., the introduction of intact sample mols. into the gas-phase. A promising approach is the use of laser-induced acoustic desorption (LIAD) sources, which have been demonstrated using both nanosecond pulsed and continuous desorption lasers. We directly compare here both approaches for the first time under otherwise identical conditions using adenine as a prototypical biol. mol., and study the produced mol. plumes using femtosecond multiphoton ionisation. We observe different desorption mechanisms at play for the two different desorption laser sources; however, we find no evidence in either case that the desorption process leads to fragmentation of the target mol. unless excessive desorption energy is applied. This makes LIAD a powerful approach for techniques that require high d. and high purity samples in the gas-phase, such as ultrafast dynamics studies or diffraction expts. Graphic abstr.: [graphic not available: see fulltext].
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    16. 16
      Zinovev, A. V.; Veryovkin, I. V.; Moore, J. F.; Pellin, M. J. Laser-driven acoustic desorption of organic molecules from back-irradiated solid foils. Anal. Chem. 2007, 79, 8232– 8241,  DOI: 10.1021/ac070584o
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      16
      Laser-Driven Acoustic Desorption of Organic Molecules from Back-Irradiated Solid Foils
      Zinovev, Alexander V.; Veryovkin, Igor V.; Moore, Jerry F.; Pellin, Michael J.
      Analytical Chemistry (Washington, DC, United States) (2007), 79 (21), 8232-8241CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      Laser-induced acoustic desorption (LIAD) from thin metal foils is a promising technique for gentle and efficient volatilization of intact org. mols. from surfaces of solid substrates. Using the single-photon ionization method combined with time-of-flight mass spectrometry, we have examd. the neutral component of the desorbed flux in LIAD and compared it to that from direct laser desorption. These basic studies of LIAD, conducted for mols. of various org. dyes (rhodamine B, fluorescein, anthracene, coumarin, BBQ), have demonstrated detection of intact parent mols. of the analyte even at its surface concns. corresponding to a submonolayer coating. In some cases (rhodamine B, fluorescein, BBQ), the parent mol. ion peak was accompanied by a few fragmentation peaks of comparable intensity, whereas for others, only peaks corresponding to intact parent mols. were detected. At all measured desorbing laser intensities (from 100 to 500 MW/cm2), the total amt. of desorbed parent mols. depended exponentially on the laser intensity. Translational velocities of the desorbed intact mols., detd. for the first time in this work, were of the order of hundreds of meters per s, less than what has been obsd. in our expts. for direct laser desorption, but substantially greater than the possible perpendicular velocity of the substrate foil surface due to laser-generated acoustic waves. Moreover, these velocities did not depend on the desorbing laser intensity, which implies the presence of a more sophisticated mechanism of energy transfer than direct mech. or thermal coupling between the laser pulse and the adsorbed mols. Also, the total flux of desorbed intact mols. as a function of the total no. of desorbing laser pulses, striking the same point on the target, decayed following a power law rather than an exponential function, as would have been predicted by the shake-off model. To summarize, the results of our expts. indicate that the LIAD phenomenon cannot be described in terms of simple mech. shake-off or direct laser desorption. Rather, they suggest that multistep energy-transfer processes are involved. Two possible (and not mutually exclusive) qual. mechanisms of LIAD that are based on formation of nonequil. energy states in the adsorbate-substrate system are proposed and discussed.
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    17. 17
      Huang, Z.; Ossenbrüggen, T.; Rubinsky, I.; Schust, M.; Horke, D. A.; Küpper, J. Development and characterization of a laser-induced acoustic desorption source. Anal. Chem. 2018, 90, 3920– 3927,  DOI: 10.1021/acs.analchem.7b04797
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      17
      Development and Characterization of a Laser-Induced Acoustic Desorption Source
      Huang, Zhipeng; Ossenbrueggen, Tim; Rubinsky, Igor; Schust, Matthias; Horke, Daniel A.; Kuepper, Jochen
      Analytical Chemistry (Washington, DC, United States) (2018), 90 (6), 3920-3927CODEN: ANCHAM; ISSN:0003-2700. (American Chemical Society)
      A laser-induced acoustic desorption source, developed for use at central facilities, such as free-electron lasers, is presented. It features prolonged measurement times and a fixed interaction point. A novel sample deposition method using aerosol spraying provides a uniform sample coverage and hence stable signal intensity. Using strong-field ionization as a universal detection scheme, the produced mol. plume was characterized in terms of no. d., spatial extend, fragmentation, temporal distribution, translational velocity, and translational temp. The effect of desorption laser intensity on these plume properties is evaluated. While translational velocity is invariant for different desorption laser intensities, pointing to a nonthermal desorption mechanism, the translational temp. increases significantly and higher fragmentation is obsd. with increased desorption laser fluence.
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    18. 18
      Wang, S.; Dauletyarov, Y.; Krüger, P.; Horke, D. A. High-throughput UV-photofragmentation studies of thymine and guanine. Phys. Chem. Chem. Phys. 2023, 25, 12322– 12330,  DOI: 10.1039/D3CP00328K
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      18
      High-throughput UV-photofragmentation studies of thymine and guanine
      Wang, Siwen; Dauletyarov, Yerbolat; Krueger, Peter; Horke, Daniel A.
      Physical Chemistry Chemical Physics (2023), 25 (17), 12322-12330CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      High-throughput photofragmentation studies of thymine and guanine were performed at 257 and 343 nm and for a wide range of ionisation laser intensities. Combining a continuous laser-based thermal desorption source with femtosecond multiphoton ionisation using a 50 kHz repetition rate laser allowed us to produce detailed 2D maps of fragmentation as a function of incident laser intensity. The fragmentation was distinctly soft, the parent ions being at least an order of magnitude more abundant than fragment ions. For thymine there was a single dominant fragmentation channel, which involves consecutive HNCO and CO losses. In contrast, for guanine there were several competing ones, the most probable channel corresponding to CH2N2 loss through opening of the pyrimidine ring. The dependence of parent ion abundance on the ionisation laser intensity showed that at 257 nm the ionisation of thymine is a 1 + 1 resonance enhanced process through its open-shell singlet state.
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    19. 19
      Calegari, F.; Ayuso, D.; Trabattoni, A.; Belshaw, L.; De Camillis, S.; Anumula, S.; Frassetto, F.; Poletto, L.; Palacios, A.; Decleva, P.; Greenwood, J. B.; Martín, F.; Nisoli, M. Ultrafast electron dynamics in phenylalanine initiated by attosecond pulses. Science 2014, 346, 336– 339,  DOI: 10.1126/science.1254061
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      Ultrafast electron dynamics in phenylalanine initiated by attosecond pulses
      Calegari, F.; Ayuso, D.; Trabattoni, A.; Belshaw, L.; De Camillis, S.; Anumula, S.; Frassetto, F.; Poletto, L.; Palacios, A.; Decleva, P.; Greenwood, J. B.; Martin, F.; Nisoli, M.
      Science (Washington, DC, United States) (2014), 346 (6207), 336-339CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      In the past decade, attosecond technol. has opened up the investigation of ultrafast electronic processes in atoms, simple mols., and solids. Here, we report the application of isolated attosecond pulses to prompt ionization of the amino acid phenylalanine and the subsequent detection of ultrafast dynamics on a sub-4.5-fs temporal scale, which is shorter than the vibrational response of the mol. The ability to initiate and observe such electronic dynamics in polyat. mols. represents a crucial step forward in attosecond science, which is progressively moving toward the investigation of more and more complex systems.
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      Calegari, F.; Trabattoni, A.; Palacios, A.; Ayuso, D.; Castrovilli, M. C.; Greenwood, J. B.; Decleva, P.; Martín, F.; Nisoli, M. Charge migration induced by attosecond pulses in bio-relevant molecules. J. Phys. B 2016, 49, 142001,  DOI: 10.1088/0953-4075/49/14/142001
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      Charge migration induced by attosecond pulses in bio-relevant molecules
      Calegari, Francesca; Trabattoni, Andrea; Palacios, Alicia; Ayuso, David; Castrovilli, Mattea C.; Greenwood, Jason B.; Decleva, Piero; Martin, Fernando; Nisoli, Mauro
      Journal of Physics B: Atomic, Molecular and Optical Physics (2016), 49 (14), 142001/1-142001/25CODEN: JPAPEH; ISSN:0953-4075. (IOP Publishing Ltd.)
      A review. After sudden ionization of a large mol., the pos. charge can migrate throughout the system on a sub-femtosecond time scale, purely guided by electronic coherences. The possibility to actively explore the role of the electron dynamics in the photo-chem. of bio-relevant mols. is of fundamental interest for understanding, and perhaps ultimately controlling, the processes leading to damage, mutation and, more generally, to the alteration of the biol. functions of the macromol. Attosecond laser sources can provide the extreme time resoln. required to follow this ultrafast charge flow. In this review we will present recent advances in attosecond mol. science: after a brief description of the results obtained for small mols., recent exptl. and theor. findings on charge migration in bio-relevant mols. will be discussed.
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      Sparling, C.; Crane, S. W.; Ireland, L.; Anderson, R.; Ghafur, O.; Greenwood, J. B.; Townsend, D. Velocity-map imaging of photoelectron circular dichroism in non-volatile molecules using a laser-based desorption source. Phys. Chem. Chem. Phys. 2023, 25, 6009– 6015,  DOI: 10.1039/D2CP05880D
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      Velocity-map imaging of photoelectron circular dichroism in non-volatile molecules using a laser-based desorption source
      Sparling, Chris; Crane, Stuart W.; Ireland, Lewis; Anderson, Ross; Ghafur, Omair; Greenwood, Jason B.; Townsend, Dave
      Physical Chemistry Chemical Physics (2023), 25 (8), 6009-6015CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      We present an initial demonstration of a velocity-map imaging (VMI) expt. using a back-irradn. laser-based desorption source directly integrated into the electrode assembly. This has the potential to greatly expand the utility of the popular VMI approach by permitting its use with high d. plumes of non-volatile mol. samples. Photoelectron CD measurements on the phenylalanine mol. using 400 nm multiphoton ionization are used to illustrate this novel method, revealing forward-backward emission asymmetries on the order of 7%.
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      Camillis, S. D.; Miles, J.; Alexander, G.; Ghafur, O.; Williams, I. D.; Townsend, D.; Greenwood, J. B. Ultrafast Non-Radiative Decay of Gas-Phase Nucleosides. Phys. Chem. Chem. Phys. 2015, 17, 23643– 23650,  DOI: 10.1039/C5CP03806E
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      Ultrafast non-radiative decay of gas-phase nucleosides
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      Physical Chemistry Chemical Physics (2015), 17 (36), 23643-23650CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      The ultrafast photo-phys. properties of DNA are crucial in providing a stable basis for life. Although the DNA bases efficiently absorb UV radiation, this energy can be dissipated to the surrounding environment by the rapid conversion of electronic energy to vibrational energy within about a picosecond. The intrinsic nature of this internal conversion process has previously been demonstrated through gas phase expts. on the bases, supported by theor. calcns. De-excitation rates appear to be accelerated when individual bases are hydrogen bonded to solvent mols. or their complementary Watson-Crick pair. In this paper, the first gas-phase measurements of electronic relaxation in DNA nucleosides following UV excitation are reported. Using a pump-probe ionization scheme, the lifetimes for internal conversion to the ground state following excitation at 267 nm are found to be reduced by around a factor of two for adenosine, cytidine and thymidine compared with the isolated bases. These results are discussed in terms of a recent proposition that a charge transfer state provides an addnl. internal conversion pathway mediated by proton transfer through a sugar to base hydrogen bond.
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      Bulgakov, A. V.; Goodfriend, N.; Nerushev, O.; Bulgakova, N. M.; Starinskiy, S. V.; Shukhov, Y. G.; Campbell, E. E. B. Laser-induced transfer of nanoparticles for gas-phase analysis. J. Opt. Soc. Am. B 2014, 31, C15– C21,  DOI: 10.1364/JOSAB.31.000C15
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      Laser-induced transfer of nanoparticles for gas-phase analysis
      Bulgakov, Alexander V.; Goodfriend, Nathan; Nerushev, Oleg; Bulgakova, Nadezhda M.; Starinskiy, Sergei V.; Shukhov, Yuri G.; Campbell, Eleanor E. B.
      Journal of the Optical Society of America B: Optical Physics (2014), 31 (11), C15-C21CODEN: JOBPDE; ISSN:0740-3224. (Optical Society of America)
      An exptl. study of laser-induced forward transfer of nanoparticles from a metal-coated glass substrate is presented. Nanoparticles are efficiently removed from the substrates due to transient blister formation. A combination of mass spectrometry, at. force microscopy studies of the irradiated substrates, and theor. considerations of temp. distributions and stress in the films during irradn. serves to provide insight into the mechanisms involved.
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      Cotter, J. P.; Brand, C.; Knobloch, C.; Lilach, Y.; Cheshnovsky, O.; Arndt, M. In search of multipath interference using large molecules. Science Advances 2017, 3, e1602478  DOI: 10.1126/sciadv.1602478
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      In search of multipath interference using large molecules
      Cotter, Joseph P.; Brand, Christian; Knobloch, Christian; Lilach, Yigal; Cheshnovsky, Ori; Arndt, Markus
      Science Advances (2017), 3 (8), e1602478/1-e1602478/6CODEN: SACDAF; ISSN:2375-2548. (American Association for the Advancement of Science)
      The superposition principle is fundamental to the quantum description of both light and matter. Recently, a no. of expts. have sought to directly test this principle using coherent light, single photons, and nuclear spin states. We extend these expts. to massive particles for the first time. We compare the interference patterns arising from a beam of large dye mols. diffracting at single, double, and triple slit material masks to place limits on any high-order, or multipath, contributions. We observe an upper bound of less than one particle in a hundred deviating from the expectations of quantum mechanics over a broad range of transverse momenta and de Broglie wavelength.
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      Cabezas, C.; Varela, M.; Alonso, J. L. The Structure of the Elusive Simplest Dipeptide Gly-Gly. Angew. Chem., Int. Ed. 2017, 56, 6420– 6425,  DOI: 10.1002/anie.201702425
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      The Structure of the Elusive Simplest Dipeptide Gly-Gly
      Cabezas, Carlos; Varela, Marcelino; Alonso, Jose L.
      Angewandte Chemie, International Edition (2017), 56 (23), 6420-6425CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Among the hundreds of peptide compds. for which conformations have been detd. by using different spectroscopic techniques, the structure of the simplest dipeptide glycylglycine (Gly-Gly) is conspicuously absent. Herein, for the first time, solid samples of Gly-Gly have been vaporized by laser ablation and three different structures have been revealed in a supersonic expansion by Fourier transform microwave spectroscopy. The intramol. hydrogen bonding interactions that stabilize the obsd. forms have been established based on the 14N nuclear quadrupole hyperfine structure. We have illustrated how conformer interconversion distorts the equil. conformational distribution, giving rise to missing conformers in the conformational landscape.
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      Klassen, J. S.; Kebarle, P. Collision-Induced Dissociation Threshold Energies of Protonated Glycine, Glycinamide, and Some Related Small Peptides and Peptide Amino Amides. J. Am. Chem. Soc. 1997, 119, 6552– 6563,  DOI: 10.1021/ja962813m
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      Collision-Induced Dissociation Threshold Energies of Protonated Glycine, Glycinamide, and Some Related Small Peptides and Peptide Amino Amides
      Klassen, John S.; Kebarle, Paul
      Journal of the American Chemical Society (1997), 119 (28), 6552-6563CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The energy thresholds for fragment ions from the collision-induced dissocn. of the protonated amino acid Gly, glycinamides H-Gly-NH2 and H-Gly-NHMe, and the peptides H-(Gly)n-OH (n = 2-4) and H-Gly-Gly-NH2 were detd. with a modified triple quadrupole mass spectrometer. The precursor ions were produced by electrospray. The threshold energies for the formation of the major fragment ions were detd. by fitting the exptl. detd. threshold curves to theor. threshold equations following the procedure developed by Armentrout and co-workers. To obtain corrections for the kinetic shifts, several of the transition states were modeled by using ab initio or semiempirical calcns. The immonium ion, CH2NH2+, which is an a1 ion, was obsd. as a major fragment ion from precursor ions (H2NCH2CO-X)H+, where X = OH, NH2, NHMe, and NH2CO2H. The activation energies obtained from the thresholds were between 40 and 50 kcal/mol. These energies were in good agreement with theor. evaluated activation energies for transition states based on a mechanism proposed by A. G. Harrison and co-workers. A second pathway leading to XH2+ (y1) ions was obsd. for X = NHMe and NHCH2CO2H. The threshold based activation energy was somewhat lower than that for the a1 ions. Agreement between the threshold based activation energy and the calcd. activation energy for a transition state involving an aziridinone intermediate provides support for the mechanism proposed by C. Wesdemiotis and co-workers. The lowest energy fragmentation pathway for ions with the general structure +H(Gly-Gly-X) was found to be the b2 (acylium) ion. In the case where X = NH2, the activation energy was only 20.4 kcal/mol. This very low energy was found consistent with the b2 ions being cyclic, protonated oxazalones, as proposed by A. G. Harrison and co-workers. The thresholds for the fragment ions from larger precursor ions were affected by very large kinetic shifts and suppression by competitive decompns. Accurate activation energies for these reactions could not be obtained. Nevertheless, the threshold curves provide a view of the evolution into the complex pathways occurring for the higher polypeptides.
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      O’Hair, R. A. J.; Broughton, P. S.; Styles, M. L.; Frink, B. T.; Hadad, C. M. The fragmentation pathways of protonated glycine: A computational study. J. Am. Soc. Mass. Spectrom. 2000, 11, 687– 696,  DOI: 10.1016/S1044-0305(00)00143-4
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      The fragmentation pathways of protonated glycine: a computational study
      O'Hair, R. A. J.; Broughton, P. S.; Styles, M. L.; Frink, B. T.; Hadad, C. M.
      Journal of the American Society for Mass Spectrometry (2000), 11 (8), 687-696CODEN: JAMSEF; ISSN:1044-0305. (Elsevier Science Inc.)
      Numerous studies have demonstrated that protonated aliph. amino acids, [H2NCHRCO2H + H]+, fragment in the gas phase to form iminium ions, H2N:CHR+. Unfortunately none of these studies have probed the structure of the neutral(s) lost as well as the mechanism of fragmentation. Three main mechanisms have been previously proposed: (1) loss of the combined elements of H2O and CO; (2) loss of dihydroxycarbene (HO)2C: and (3) loss of formic acid, HC(:O)OH. Herein, ab initio and d. functional theory calcns. have been used to calc. the key reactants, transition states, and products of these and several other competing reaction channels in the fragmentation of protonated glycine. The loss of the combined elements of H2O and CO is thermodynamically and kinetically favored over the alternative formic acid or (HO)2C fragmentation processes.
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      Shek, P. I.; Lau, J. K.-C.; Zhao, J.; Grzetic, J.; Verkerk, U. H.; Oomens, J.; Hopkinson, A. C.; Siu, K. M. Fragmentations of protonated cyclic-glycylglycine and cyclic-alanylalanine. Int. J. Mass Spectrom. 2012, 316-318, 199– 205,  DOI: 10.1016/j.ijms.2012.02.011
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      Fragmentations of protonated cyclic-glycylglycine and cyclic-alanylalanine
      Shek, P. Y. Iris; Lau, Justin Kai-Chi; Zhao, Junfang; Grzetic, Josipa; Verkerk, Udo H.; Oomens, Jos; Hopkinson, Alan C.; Siu, K. W. Michael
      International Journal of Mass Spectrometry (2012), 316-318 (), 199-205CODEN: IMSPF8; ISSN:1387-3806. (Elsevier B.V.)
      Collision-induced dissocn. has been used to study the fragmentations of two protonated diketopiperazines, protonated cyclic-glycylglycine and cyclic-alanylalanine. Protonated cyclo-AA lost CO and (CO + NH3) at low collision energies, channels attributed to dissocn. of the O-protonated tautomer. Higher collision energies were required to dissoc. protonated cyclo-GG, and the two lowest-energy products were the result of losses of one CO and two CO mols. These occur from the higher-energy N-protonated tautomer, which is formed from the O-protonated tautomer by a 1,4-proton shift that has a high barrier (54.5 kcal mol-1) due to constraints imposed by the ring. Mechanistic schemes for four different dissocn. channels, three from the N-protonated tautomer and one from the O-protonated tautomer, have been computed using d. functional theory (DFT) at the B3LYP/6-311++G(d,p) level. Comparison of the potential energy surfaces for the two protonated diketopiperazines reveals the factors behind this dichotomy of fragmentation pathways. The IR multiple-photon dissocn. spectrum of the [M+H-NH3-CO]+ ion (m/z 98) from protonated cyclo-AA shows this product to be an oxazole, the lowest-energy isomer.
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      Eckart, K.; Holthausen, M. C.; Koch, W.; Spiess, J. Mass spectrometric and quantum mechanical analysis of gas-phase formation, structure, and decomposition of various b2 ions and their specifically deuterated analogs. J. Am. Soc. Mass. Spectrom. 1998, 9, 1002– 1011,  DOI: 10.1016/S1044-0305(98)00076-2
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      30
      Mass spectrometric and quantum mechanical analysis of gas-phase formation, structure, and decomposition of various b2 ions and their specifically deuterated analogs
      Eckart, Klaus; Holthausen, Max C.; Koch, Wolfram; Spiess, Joachim
      Journal of the American Society for Mass Spectrometry (1998), 9 (10), 1002-1011CODEN: JAMSEF; ISSN:1044-0305. (Elsevier Science Inc.)
      B ions represent an important type of fragment ions derived from protonated peptides by cleavage of an amide bond with N-terminal charge retention. Such species have also been discussed as key intermediates during cyclic peptide fragmentation. Detailed structural information on such ion types can facilitate the interpretation of multiple step fragmentations such as the formation of inner chain fragments from linear peptides or the fragmentation of cyclic peptides. The structure of different b2 ion isomers was investigated with collision-induced dissocns. (CID) in combination with hydrogen/deuterium (H/D) exchange of the acidic protons. Special care was taken to investigate fragment ions derived from pure gas-phase processes. Structures deduced from the results of the CID anal. were compared with structures predicted on the basis of quantum chem. d. functional theory (DFT) calcns. to be most stable. The results pointed to different types of structures for b2 ion isomers of complementary amino acid sequences. Either the protonated oxazolone structure or the N-terminally protonated immonium ion structure were proposed on the basis of the CID results and the DFT calcns. In addn., the anal. of different selectively N-alkylated peptide analogs revealed mechanistic details of the processes generating b ions.
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      Harrison, A. G.; Csizmadia, I. G.; Tang, T.-H. Structure and fragmentation of b2 ions in peptide mass spectra. J. Am. Soc. Mass. Spectrom. 2000, 11, 427– 436,  DOI: 10.1016/S1044-0305(00)00104-5
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      Structure and fragmentation of b2 ions in peptide mass spectra
      Harrison, A. G.; Csizmadia, I. G.; Tang, T.-H.
      Journal of the American Society for Mass Spectrometry (2000), 11 (5), 427-436CODEN: JAMSEF; ISSN:1044-0305. (Elsevier Science Inc.)
      A Polemic. In a no. of cases the b2 ion obsd. in peptide mass spectra fragments directly to the a1 ion. The present study examines the scope of this reaction and provides evidence as to the structures of the b2 ions undergoing fragmentation to the a1 ion. The b2 ion H-Ala-Gly+ fragments, in part, to the a1 ion, whereas the isomeric b2 ion H-Gly-Ala+ does not fragment to the a1 ion. Ab initio calcns. of ion energies show that this different behavior can be rationalized in terms of protonated oxazolone structures for the b2 ions provided one assumes a reverse activation energy of ∼1 eV for the reaction b2 → a2; such a reverse activation energy is consistent with exptl. kinetic energy release measurements. Exptl., the H-Aib-Ala+ b2 ion, which must have a protonated oxazolone structure, fragments extensively to the a1 ion. We conclude that the proposal by Eckart et al. (J. Am. Soc. Mass Spectrum. 1998, 9, 1002) that the b2 ions which undergo fragmentation to a1 ions have an immonium ion structure is not necessary to rationalize the results, but that the fragmentation does occur from a protonated oxazolone structure. It is shown that the b2 → a1 reaction occurs extensively when the C-terminus residue in the b2 ion is Gly and with less facility when the C-terminus residue is Ala. When the C-terminus residue is Val or larger, the b2 → a1 reaction cannot compete with the b2 → a2 fragmentation reaction. Some preliminary results on the fragmentation of a2 ions are reported.
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      Wyllie, S. G.; Amos, B. A.; Tokes, L. Electron impact induced fragmentation of cholesterol and related C-5 unsaturated steroids. J. Org. Chem. 1977, 42, 725– 732,  DOI: 10.1021/jo00424a033
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      Electron impact induced fragmentation of cholesterol and related C-5 unsaturated steroids
      Wyllie, S. G.; Amos, Bernard A.; Tokes, Laszlo
      Journal of Organic Chemistry (1977), 42 (4), 725-32CODEN: JOCEAH; ISSN:0022-3263.
      The mass spectra of cholest-5-ene and various C-5 unsatd. 3β-hydroxy sterols indicated that the fragmentations leading to the characteristic (M - 85)+ and (M - 111)+ ions in these sterols are triggered solely by the double bond. With the aid of 11 deuterium labeled analogs all diagnostic cleavages of this class of compds. were identified. Both (M - 85)+ and (M - 111)+ ions of cholesterol were due to very complex fragmentations involving the loss of ring A and part of ring B by cleavages of the C-1-C-10, C-5-C-10, and C-5-C-6 or C-7-C-8 bonds, resp., with a H transfer mainly from C-6 in the (M - 85)+ ion.
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      Benham, K.; Hodyss, R.; Fernández, F. M.; Orlando, T. M. Laser-Induced Acoustic Desorption Atmospheric Pressure Photoionization via VUV-Generating Microplasmas. J. Am. Soc. Mass. Spectrom. 2016, 27, 1805– 1812,  DOI: 10.1007/s13361-016-1467-0
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      Laser-Induced Acoustic Desorption Atmospheric Pressure Photoionization via VUV-Generating Microplasmas
      Benham, Kevin; Hodyss, Robert; Fernandez, Facundo M.; Orlando, Thomas M.
      Journal of the American Society for Mass Spectrometry (2016), 27 (11), 1805-1812CODEN: JAMSEF; ISSN:1044-0305. (Springer)
      We demonstrate the first application of laser-induced acoustic desorption (LIAD) and atm. pressure photoionization (APPI) as a mass spectrometric method for detecting low-polarity orgs. This was accomplished using a Lyman-α (10.2 eV) photon generating microhollow cathode discharge (MHCD) microplasma photon source in conjunction with the addn. of a gas-phase mol. dopant. This combination provided a soft desorption and a relatively soft ionization technique. Selected compds. analyzed include α-tocopherol, perylene, cholesterol, phenanthrene, phylloquinone, and squalene. Detectable surface concns. as low as a few pmol per spot sampled were achievable using test mols. The combination of LIAD and APPI provided a soft desorption and ionization technique that can allow detection of labile, low-polarity, structurally complex mols. over a wide mass range with minimal fragmentation.
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      Wang, J.; Wang, Z.; Liu, F.; Mo, Y.; Cai, L.; Sun, C.; Zhang, S.; Zhang, R.; Abliz, Z.; Zhang, X. Mass spectrometry imaging of intact cholesterol in a mouse esophagus tissue section and mouse zygotes using VUV laser desorption/ionization method. Int. J. Mass Spectrom. 2018, 432, 9– 13,  DOI: 10.1016/j.ijms.2018.06.008
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      Mass spectrometry imaging of intact cholesterol in a mouse esophagus tissue section and mouse zygotes using VUV laser desorption/ionization method
      Wang, Jia; Wang, Zhaoying; Liu, Feng; Mo, Yuxiang; Cai, Lesi; Sun, Chenglong; Zhang, Sichun; Zhang, Ruiping; Abliz, Zeper; Zhang, Xinrong
      International Journal of Mass Spectrometry (2018), 432 (), 9-13CODEN: IMSPF8; ISSN:1387-3806. (Elsevier B.V.)
      A new instrument based on VUV laser desorption/ionization (VUVDI) method was used to measure the mass spectrum of the cholesterol std. and the mass spectrometry images (MSI) of intact cholesterol in a mouse esophagus tissue section and mouse zygotes. The VUVDI mass spectrum of the cholesterol std. shows dominant peaks from the parent mols. In the case of the mouse esophagus tissue section, the ion images with a lateral resoln. of ∼ 4μm reveal that the cholesterol populates mainly in the mucosae. The exptl. results indicate that the VUVDI method may provide a new MSI tool to measure the intact cholesterol in tissues and cells.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC1cXhtlCnurzJ&md5=5f38cb0f5bf6e22e61358998cc005b01
    35. 35
      Barty, A.; Küpper, J.; Chapman, H. N. Molecular Imaging Using X-Ray Free-Electron Lasers. Annu. Rev. Phys. Chem. 2013, 64, 415– 435,  DOI: 10.1146/annurev-physchem-032511-143708
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      35
      Molecular imaging using X-ray free-electron lasers
      Barty, Anton; Kuepper, Jochen; Chapman, Henry N.
      Annual Review of Physical Chemistry (2013), 64 (), 415-435CODEN: ARPLAP; ISSN:0066-426X. (Annual Reviews Inc.)
      A review. The opening of hard X-ray free-electron laser facilities, such as the Linac Coherent Light Source (LCLS) at SLAC National Accelerator Lab. in the United States, has ushered in a new era in structural detn. With X-ray pulse durations down to 10 fs or shorter, and up to 1013 transversely coherent photons per pulse in a narrow spectral bandwidth, focused irradiances of 1018 to 1021 W cm-2 or higher can be produced at X-ray energies ranging from 500 eV to 10 keV. New techniques for detg. the structure of systems that cannot be crystd. and for studying the time-resolved behavior of irreversible reactions at femtosecond timescales are now available.
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      https://chemport.cas.org/services/resolver?origin=ACS&resolution=options&coi=1%3ACAS%3A528%3ADC%252BC3sXntVCrs74%253D&md5=72098939f6d92e65cd4cb41374079f3f
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