COMMUNICATIONS
Directed DNA Metallization
Glenn A. Burley - ,
Johannes Gierlich - ,
Mohammad R. Mofid - ,
Hadar Nir - ,
Shay Tal - ,
Yoav Eichen - , and
Thomas Carell
Genes of interest can be selectively metallized via the incorporation of modified triphosphates. These triphosphates bear functions that can be further derivatized with aldehyde groups via the use of click chemistry. Treatment of the aldehyde-labeled gene mixture with the Tollens reagent, followed by a development process, results in the selective metallization of the gene of interest in the presence of natural DNA strands.
Analysis of Lanthanide-Induced NMR Shifts of the Ce@C82 Anion
Michio Yamada - ,
Takatsugu Wakahara - ,
Yongfu Lian - ,
Takahiro Tsuchiya - ,
Takeshi Akasaka - ,
Markus Waelchli - ,
Naomi Mizorogi - ,
Shigeru Nagase - , and
Karl M. Kadish
The mapping of bond connectivity in the carbon cage of [Ce@C82]- and full assignment of the NMR lines were successfully achieved by means of 2D INADEQUATE NMR measurement. Paramagnetic NMR analysis shows that the Ce atom in [Ce@C82]- is located at an off-centered position adjacent to a hexagonal ring along the C2 axis of the C2v−C82 cage.
Synthesis and Structural Characterization of Endohedral Pyrrolidinodimetallofullerene: La2@C80(CH2)2NTrt
Michio Yamada - ,
Takatsugu Wakahara - ,
Tsukasa Nakahodo - ,
Takahiro Tsuchiya - ,
Yutaka Maeda - ,
Takeshi Akasaka - ,
Kenji Yoza - ,
Ernst Horn - ,
Naomi Mizorogi - , and
Shigeru Nagase
The endohedral pyrrolidinodimetallofullerene, La2@C80(CH2)2NTrt (Trt = triphenylmethyl), was successfully synthesized and characterized. X-ray crystallographic and NMR spectroscopic analyses reveal that two La atoms in the 6,6-adduct are localized at the stable site on the mirror plane. Theoretical calculation also suggests the localization of two La atoms in the 6,6-adduct.
PNA-Based Reagents for the Direct and Site-Specific Synthesis of Thymine Dimer Lesions in Genomic DNA
J. Carsten Pieck - ,
David Kuch - ,
Friederike Grolle - ,
Uwe Linne - ,
Clemens Haas - , and
Thomas Carell
Palladium(0)-Catalyzed Alkylative Cyclization of Alkynals and Alkynones: Remarkable trans-Addition of Organoboronic Reagents
Hirokazu Tsukamoto - ,
Tatsuhiko Ueno - , and
Yoshinori Kondo
Palladium/monophosphine complexes catalyze trans-selective arylative, alkenylative, and alkylative cyclization reactions of alkynals and alkynones with organoboronic reagents. These reactions afford six-membered allylic alcohols with endo-tri- or tetrasubstituted olefin groups and/or five-membered counterparts with exo olefin groups. The ratios of these products are dramatically affected by alkyne substituents as well as the phosphine ligand. The remarkable trans-selectivity of the process results from the novel reaction mechanism involving oxidative addition without oxametallacycle formation.
Free Energies of Molecular Crystal Surfaces by Computer Simulation: Application to Tetrathiophene
Valentina Marcon - and
Guido Raos
We describe a general simulation protocol for the evaluation of the surface free energies of molecular crystals, which are of broad interest for phenomena such as polymorphism and crystal growth. The method has been applied to selected surfaces of two polymorphs of tetrathiophene. The simulations highlight an important temperature-dependent entropic contribution to the surface free energies, which is not included in widely used static simulations of surface structure and energetics.
Non-SELEX Selection of Aptamers
Maxim Berezovski - ,
Michael Musheev - ,
Andrei Drabovich - , and
Sergey N. Krylov
Aptamers are typically selected from libraries of random DNA (or RNA) sequences by SELEX, which involves multiple rounds of alternating steps of partitioning and PCR amplification. Here we report, for the first time, non-SELEX selection of aptamersa process that involves repetitive steps of partitioning with no amplification between them. A highly efficient affinity method, non-equilibrium capillary electrophoresis of equilibrium mixtures (NECEEM), was used for partitioning. We found that three steps of NECEEM-based partitioning in the non-SELEX approach were sufficient to improve the affinity of a DNA library to a target protein by more than 4 orders of magnitude. The resulting affinity was higher than that of the enriched library obtained in three rounds of NECEEM-based SELEX. Remarkably, NECEEM-based non-SELEX selection took only 1 h in contrast to several days or several weeks required for a typical SELEX procedure by conventional partitioning methods. In addition, NECEEM-based non-SELEX allowed us to accurately measure the abundance of aptamers in the library. Not only does this work introduce an extremely fast and economical method for aptamer selection, but it also suggests that aptamers may be much more abundant than they are thought to be. Finally, this work opens the opportunity for selection of drug candidates from libraries of small molecules, which cannot be PCR-amplified and thus are not approachable by SELEX.
Multifunctional Chemical Vapor Sensors of Aligned Carbon Nanotube and Polymer Composites
Chen Wei - ,
Liming Dai - ,
Ajit Roy - , and
Tia Benson Tolle
Partially coating perpendicularly aligned carbon nanotube arrays with an appropriate polymer thin film along their tube length provides a novel concept for developing new sensors of high sensitivity, good selectivity, and excellent environmental stability for the detection of a broad class of chemical vapors with low power consumption. The absorption and desorption of chemical vapors by the polymer matrix cause changes in the inter-tube distance and, hence, the surface resistance across the nanotube film. Simple measurements of the resistance change, therefore, constitute the nanotube−polymer chemical vapor sensors. These rationally designed, aligned carbon nanotube−polymer composite films are flexible and can be effectively integrated into many systems for a wide range of potential applications, including their use as multifunctional sensors for sensing chemical vapors, mechanical deformations, thermal and optical exposures.
Self-Adhesion among Phospholipid Vesicles
F. M. Menger - and
Hailing Zhang
A compound was synthesized that binds to a phospholipid bilayer via a hydrophobic steroid thereby projecting a strong multi-hydrogen bonding unit into the surrounding water. As shown by light scattering, light microscopy, and cryo-HRSEM, this latter unit self-adheres and induces membrane−membrane attachments, as found in many biological systems.
Further Study of the Reaction of Fe2+ with CN-: Synthesis and Characterization of cis and trans [FeII,III(CN)4L2]n- Complexes
Gina M. Chiarella - ,
Doris Y. Melgarejo - , and
Stephen A. Koch
The reaction of Fe2+ with CN-, which was first performed in 1704, has been used to synthesize a new series of basic [FeII,III(CN)4L2]n- complexes, where L is a monodentate ligand. trans-Na2[FeII(CN)4(DMSO)2] and cis-[NEt4]2[FeII(CN)4(pyridine)2] are synthesized by the direct reaction of FeCl2 with 4 equiv of CN- in DMSO or pyridine. Air oxidation of the latter compound gives cis-[NEt4][FeIII(CN)4(pyridine)2]. The non-cyanide ligands in these complexes undergo facile ligand exchange reactions with solvent. Reaction of cis-[NEt4]2[FeII(CN)4(pyridine)2] with CO at room temperature gives trans-[NEt4]2[FeII(CN)4(pyridine)(CO)].
One-Dimensional Metallic Conducting Pathway of Cyclohexyl-Substituted Spiro-Biphenalenyl Neutral Radical Molecular Crystal
Jingsong Huang - and
Miklos Kertesz
The unprecedented metallic character of the cyclohexyl-substituted spiro-biphenalenyl neutral radical molecular crystal (5) suggested by its Pauli paramagnetism [Science 2005, 309, 281] is contradicted by the thermally activated conduction measured along the needle axis of crystal 5 and by an optical gap of Eg = 0.34 eV. Herein we provide the first high quality ab initio electronic structure calculations using density functional theory to reconcile these properties. The calculations point toward 5 being a quasi one-dimensional (1-D) material, with a 1-D conducting pathway along the [101] π-chain direction. Along any directions other than the π-chain, conduction is impeded by the small interchain overlap. 5 has a quarter-filled band structure with a density of states of N(Ef) = 7.5 states eV-1 at the Fermi level, leading to a metallic character along the π-chain.
The Dehydratase Activity of Lacticin 481 Synthetase is Highly Processive
Leah M. Miller - ,
Champak Chatterjee - ,
Wilfred A. van der Donk - , and
Neil L. Kelleher
Lacticin 481 synthetase (LctM) is a bifunctional enzyme that undertakes dehydration and cyclization in the structural region of the pre-lacticin peptide (LctA) to introduce three thioether rings and one dehydrobutyrine residue. The order and timing of these events has been investigated employing high-resolution ESI−FTMS-based tandem MS/MS techniques and chemical derivatization. LctM demonstrates highly processive behavior as seen by MS analysis of the reaction course of dehydration. Furthermore, cyclization is not tightly coupled to dehydration and follows at a later stage of the enzymatic reaction.
Thionitroxides, RSNHO•: The Structure of the SNO Moiety in “S-Nitrosohemoglobin”, A Possible NO Reservoir and Transporter
Yi-Lei Zhao - and
K. N. Houk
Nitric oxide (NO) plays important roles in many biological processes. S-Nitrosothiols have long been believed to have significant roles in NO biochemistry. The modified cysteine residue of hemoglobin was previously identified as a distorted S-nitrosothiol (RSNO) or an S-hydroxyamino radical (RSN•OH). Here we show that a thionitroxide (RSNHO•, S-aminyloxyl radical) is likely the observed species. Computational studies show that the thionitroxide is the only structure consistent with the electron density in the hemoglobin Cysβ93-SNO structure previously reported. Although a metastable adduct, the thionitroxide in a hydrogen-bonding environment can form readily and release NO upon exposure to an aqueous environment. The thionitroxides could be responsible for the biological effects attributed to S-nitrosothiols or could serve as precursors to S-nitrosothiols in oxidative conditions.
Highly Enantioselective Synthesis of Tetrahydro-β-Carbolines and Tetrahydro-γ-Carbolines Via Pd-Catalyzed Intramolecular Allylic Alkylation
Marco Bandini - ,
Alfonso Melloni - ,
Fabio Piccinelli - ,
Riccardo Sinisi - ,
Simona Tommasi - , and
Achille Umani-Ronchi
A New Approach to Promoting Sluggish Diels−Alder Reactions: Dihapto-Coordination of the Diene
Weijun Liu - ,
Fei You - ,
Christopher J. Mocella - , and
W. Dean Harman
The cycloaddition between 1,3-cyclohexadiene and various enones and enals (methyl vinyl ketone, ethyl vinyl ketone, methacrolien) is accomplished at room temperature in yields ranging from 51 to 68% without the use of Lewis acids, high pressures, or microwave reactors. This normally sluggish cyclization is accomplished by precoordination of the diene to a π-basic molybdenum complex. The η2-bound metal is thought to promote a Michael reaction between the uncoordinated portion of the diene and the enone, and the resulting enolate then closes to form the cycloalkene product. The organic cycloadduct is removed by oxidation with air or with silver triflate in nearly quantitative yield. For more sterically hindered enones (e.g., mesityl oxide) and for methyl acrylate, the desired outcome requires the use of BF3·OEt2, and yields are significantly lower (15−35%)
Priming Type II Polyketide Synthases via a Type II Nonribosomal Peptide Synthetase Mechanism
Miho Izumikawa - ,
Qian Cheng - , and
Bradley S. Moore
Benzoic acid priming of the enterocin and actinorhodin type II polyketide synthase complexes was accomplished in vitro via an unprecedented type II nonribosomal peptide synthetase-like mechanism involving the benzoate:acyl carrier protein (ACP) ligase EncN and the ACP EncC. The transfer of the aryl acid to the ACP is ATP-dependent, yet coenzyme A-independent, as characterized with radiolabeled substrates and protein mass spectrometry. Subsequent transport of the ACP-bound aryl group to the native enterocin and the aberrant actinorhodin ketosynthase chain length factor heterodimers was further demonstrated, thereby demonstrating the potential of this biocatalyst for engineering diverse aryl-primed aromatic polyketide agents.
Ion Pair Driven Self-Assembly of a Flexible Bis-Zwitterion in Polar Solution: Formation of Discrete Nanometer-Sized Cyclic Dimers
Carsten Schmuck - ,
Thomas Rehm - ,
Franziska Gröhn - ,
Katja Klein - , and
Frank Reinhold
The self-complementary flexible bis-zwitterion 1 forms discrete nanometer-sized cyclic dimers via ion pair driven self-assembly even in polar solvents. The existence of such dimers was confirmed by DOSY NMR, FAB-MS, and scattering experiments (DLS, SANS) which all indicate the concentration-dependent formation of cyclic dimers with a hydrodynamic radius of rH ≈ 2.5 nm in solution.
A Two-Stage One-Pot Enzymatic Synthesis of TDP-l-mycarose from Thymidine and Glucose-1-phosphate
Haruko Takahashi - ,
Yung-nan Liu - , and
Hung-wen Liu
This report describes a procedure combining six enzymes native to Escherichia coli or Salmonella typhi, such as thymidine kinase (TK), thymidylate kinase (TMK), nucleoside diphosphate kinase (NDK), pyruvate kinase (PK; for ATP regeneration), TDP-glucose synthetase (RfbA), and TDP-glucose 4,6-dehydratase (RfbB), with five enzymes from Streptomyces fradiae, such as TylX3, TylC1, TylC3, TylK, and TylC2, that resulted in the biosynthesis of TDP-l-mycarose from glucose-1-phosphate and thymidine. This two-stage one-pot approach can be readily applied to the synthesis of other unusual sugars.
Low-Valent Niobium-Mediated Double Activation of C−F/C−H Bonds: Fluorene Synthesis from o-Arylated α,α,α-Trifluorotoluene Derivatives
Kohei Fuchibe - and
Takahiko Akiyama
Highly Porous Fibers by Electrospinning into a Cryogenic Liquid
Jesse T. McCann - ,
Manuel Marquez - , and
Younan Xia
Highly porous fibers of various polymers were created by electrospinning with a modified collector. A bath of liquid nitrogen was used to freeze the fibers, inducing a phase separation between the polymer and the solvent. When the solvent was removed in vacuo, highly porous fibers were obtained. Poly(styrene), poly(acrylonitrile), poly(vinylidene fluoride), and poly(ε-caprolactone) were all electrospun into porous fibers using this simple method. These porous fibers have a range of potential applications in encapsulation, controlled release, superhydrophobic coating, and lightweight reinforcement.
The Role of Methyl Groups in the Formation of Hydrogen Bond in DMSO−Methanol Mixtures
Qingzhong Li - ,
Guoshi Wu - , and
Zhiwu Yu
When examining the formation energetics of a hydrogen-bonded complex R−X−H···Y−R‘, focus has been almost always on the atoms directly involved, namely the atoms X, Y, and H. Little attention has been paid to the effects of the secondary alkyl groups R and R'. Taking dimethyl sulfoxide (DMSO)−methanol binary system as an example, we have studied the roles of the alkyl groups in stabilizing the hydrogen bonds by employing FTIR and NMR techniques and quantum chemical calculations. We found that methyl groups play different roles in response to the hydrogen-bonding interactions. The methyl groups of DMSO are electron-donating, whereas that of methanol is electron-withdrawing, both making positive contributions. The findings reveal non-negligible effects of secondary alkyl groups in hydrogen bonding interaction and may shed light on the understanding of other more complicated hydrogen-bonded systems in chemical and biological systems.
Oximate-Bridged Trinuclear Dy−Cu−Dy Complex Behaving as a Single-Molecule Magnet and Its Mechanistic Investigation
Fumihito Mori - ,
Tetsuya Nyui - ,
Takayuki Ishida - ,
Takashi Nogami - ,
Kwang-Yong Choi - , and
Hiroyuki Nojiri
Lanthanide ions are supposed to be promising candidates for the elements of single-molecule magnets (SMMs) because of the large magnetic momentum and anisotropy. We have established the [Dy2Cu] complex as a new SMM. A plausible mechanism for quantum tunneling of magnetization is proposed for the first time among the 4f−3d heterometallic SMMs. The magnetic coupling parameter between Dy and Cu ions was well-defined as −0.155 K.
Efficient Synthesis of Cyclopentenones from Enynyl Acetates via Tandem Au(I)-Catalyzed 3,3-Rearrangement and the Nazarov Reaction
Liming Zhang - and
Shaozhong Wang
A highly efficient method for the synthesis of versatile cyclopentenones from readily available enynyl acetates via tandem Au(I)-catalyzed 3,3-rearrangement and the Nazarov reaction is developed. Significant substrate flexibility and excellent control of the double bond position in the cyclopentenone ring render this an attractive method for cyclopentenone synthesis.
Dynamics of Nitric Oxide Rebinding and Escape in Horseradish Peroxidase
Xiong Ye - ,
Anchi Yu - , and
Paul M. Champion
Ultrafast kinetic measurements of NO rebinding to horseradish peroxidase (HRP) are reported for the first time. The geminate kinetics are found to be exponential for all HRP samples studied. The ferric forms of HRP have NO geminate recombination time constants in the range of 15−30 ps, while the ferrous form has a time constant of ∼7 ps. The simple exponential NO geminate kinetics found for HRP demonstrate that heme relaxation is not the underlying source of the nonexponential NO rebinding in myoglobin (Mb). The NO ligand escape rates from HRP are also determined, and they are found to depend dramatically on the presence or absence of the competitive inhibitor benzohydroxamic acid (BHA). The kinetic results indicate that, in contrast to Mb, there is direct solvent access to the distal heme pocket of HRP.
Anti-Markovnikov N−H and O−H Additions to Electron-Deficient Olefins Catalyzed by Well-Defined Cu(I) Anilido, Ethoxide, and Phenoxide Systems
Colleen Munro-Leighton - ,
Elizabeth D. Blue - , and
T. Brent Gunnoe
The monomeric Cu(I) complexes (IPr)Cu(Z) (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene, Z = NHPh, OEt, or OPh) react with YH (Y = PhNH, PhCH2NH, EtO, or PhO) to catalytically add Y−H bonds across the CC bond of electron-deficient olefins to yield anti-Markovnikov organic products. Catalytic activity has been observed for olefins CH2C(H)(X) with X = CN, C(O)Me, or CO2Me as well as crotononitrile. Preliminary studies implicate an intermediate in which the C−Y bond forms through a nucleophilic addition pathway.
Total Synthesis of (±)-Welwitindolinone A Isonitrile
Sarah E. Reisman - ,
Joseph M. Ready - ,
Atsushi Hasuoka - ,
Catherine J. Smith - , and
John L. Wood
A highly stereoselective total synthesis of the alkaloid natural product welwitindolinone A isonitrile has been completed. The synthesis utilizes a chloronium ion mediated semi-pinacol rearrangement to simultaneously install the C10 quaternary center and neopentyl chlorine and a novel anionic cyclization to construct the spiro-oxindole with complete stereocontrol.
Ti Coordination in Titanium Silicalite-1
Wallace O. Parker - and
Roberto Millini
Progressive isomorphous incorporation of TiIV (or BIII) heteroatoms into the MFI structure of as-synthesized silicalite-1 caused a decrease in the amount of siloxy groups (anions), requisite for counter-balancing the structural directing agent (cation), as determined using 1H MAS NMR to quantify the silanol protons H-bonded to the siloxy oxygen. This revealed the negative charge on the incorporated heteroatoms, identifying them as TiO5 (or BO4) sites.
Iterative, Aqueous Synthesis of β3-Oligopeptides without Coupling Reagents
Nancy Carrillo - ,
Eric A. Davalos - ,
Justin A. Russak - , and
Jeffrey W. Bode
The chemoselective synthesis of amides by decarboxylative couplings of α-ketoacids and isoxazolidines makes possible an iterative approach to poly-β3-peptides. Peptide assembly occurs under aqueous conditions and requires no coupling reagents. The requisite isoxazolidine monomers are prepared in enantiopure form by a convenient two-step protocol starting from the appropriate aldehydes.
Axially Chiral Guanidine as Enantioselective Base Catalyst for 1,4-Addition Reaction of 1,3-Dicarbonyl Compounds with Conjugated Nitroalkenes
Masahiro Terada - ,
Hitoshi Ube - , and
Yusuke Yaguchi
A new strategy for designing chiral guanidine molecules is presented, which features the introduction of an axially chiral binaphthyl backbone. The axially chiral guanidine catalysts thus developed facilitated the highly enantioselective 1,4-addition reaction of 1,3-dicarbonyl compounds with a broad range of conjugated nitroalkenes and showed extremely high catalytic activity.
Checkerboard-Type Charge-Ordered State of a Pressure-Induced Superconductor, β-(meso-DMBEDT-TTF)2PF6
Shinya Kimura - ,
Hideaki Suzuki - ,
Tomoko Maejima - ,
Hatsumi Mori - ,
Jun-Ichi Yamaura - ,
Toru Kakiuchi - ,
Hiroshi Sawa - , and
Hiroshi Moriyama
To investigate the insulating state of the pressure-induced superconductor, β-(meso-DMBEDT-TTF)2PF6, we have carried out X-ray analysis at 11.5 K. In an asymmetric unit, there exist two crystallographically independent donor molecules, caused by charge separation. In the column structure, the arrangement of the charge-rich (r) and -poor (p) donor molecules is as rrpprrpp, which affords “checkerboard”-type charge ordering.
Multiple C−H Bond Activation in Group 3 Chemistry: Synthesis and Structural Characterization of an Yttrium−Aluminum−Methine Cluster
H. Martin Dietrich - ,
Hilde Grove - ,
Karl W. Törnroos - , and
Reiner Anwander
Complete donor-induced alkylaluminate cleavage of halfmetallocene complex Cp*Y(AlMe4)2, that is, treatment of Cp*Y(AlMe4)2 with 2 equiv of diethyl ether, produces [Cp*Y(μ2-Me)2]3 in high yield (95%). In contrast, the equimolar reaction of Cp*Y(AlMe4)2 with diethyl ether reproducibly formed complex [Cp*4Y4(μ2-CH3)2{(CH3)Al(μ2-CH3)2}4(μ4-CH)2] in low yield (10−30%) via a multiple C−H bond activation. The synthesis of the heterooctametallic yttrium−aluminum−methine cluster was also accomplished in moderate yield (47%) by the equimolar reaction of discrete Cp*Y(AlMe4)2 and [Cp*Y(μ2-Me)2]3 in the absence of any donor solvent and “free” AlMe3. This gives strong evidence that preformed heterometal-bridged Y−CH3−Al moieties are prone to multiple hydrogen abstraction in the presence of a highly basic reagent such as [Cp*Y(μ2-Me)2]3. The monocylopentadienyl complexes [Cp*Y(μ2-Me)2]3 and [Cp*4Y4(μ2-CH3)2{(CH3)Al(μ2-CH3)2}4(μ4-CH)2] were structurally characterized.
Palladium(II)-Catalyzed Aerobic Dialkoxylation of Styrenes: A Profound Influence of an o-Phenol
Mitchell J. Schultz - and
Matthew S. Sigman
Macroporous Manganese Oxides with Regenerative Mesopores
Eric S. Toberer - ,
Thomas D. Schladt - , and
Ram Seshadri
We demonstrate a simple route to making hierarchically porous MnO. Macropores in Mn3O4 are induced through a process of powder sintering. The loss of volume associated with the reduction of Mn3O4 to MnO results in the formation of mesopores in the walls of a macroporous monolith of MnO. The mesopores are regenerative, in that oxidation closes them up and reduction opens them again.
Catalytic Four-Component Assembly Based on Allenylboronate Platform: New Access to Privileged Allylic Amine Structures
Keisuke Tonogaki - ,
Kenichiro Itami - , and
Jun-ichi Yoshida
We developed a novel palladium-catalyzed four-component assembly based on allenylboronate platform, by which privileged allylic amine structures can be constructed in a regioselective, stereoselective, and diversity-oriented manner. The boryl group acts not only as a useful group that can be transformed to various functional groups afterward but also as a stereochemical controller in the generation of key (π-allyl)palladium intermediates. A short synthesis of rolipram (selective phosphodiesterase-4 inhibitor) is also demonstrated.
Springs, Rings, and Spirals of Rutile-Structured Tin Oxide Nanobelts
Rusen Yang - and
Zhong Lin Wang
Single-crystalline springs, rings, and spirals have been discovered for the first time in rutile-structured SnO2. The formation process is proposed for minimizing the electrostatic energy due to the polar charges on the (011) surfaces. These structures are not only ideal systems for fundamental understanding of the polarization effect on the morphology at the nanoscale level but also have potential applications as nanoscale sensors, resonators, and transducers.
Photoinduced Phase Separation
Ana Vesperinas - ,
Julian Eastoe - ,
Paul Wyatt - ,
Isabelle Grillo - ,
Richard K. Heenan - ,
Jonathan M. Richards - , and
Gordon A. Bell
A novel approach of photoinduced phase separation has been demonstrated with a photolabile anionic surfactant, mixed with an inert nonionic surfactant in the presence of salting-out electrolyte. Breakdown of the photolyzable surfactant results in hydrophobic photoproducts, which are emulsified by the remaining inert surfactant; added electrolyte resolves the emulsion into macroscopic oily and aqueous phases. The initial micellar systems can disperse an insoluble additive marker dye (shown), which may be spatially segregated from the aqueous environment by the action of UV light.
Iron-Assisted Vapor-Phase Hydrothermal Method: A Low-Temperature Approach To Synthesize Blue Light Emissive SiOx Nanowires with Single-Crystal Structure of P21212
Ping Chen - ,
Songhai Xie - ,
Nan Ren - ,
Yahong Zhang - ,
Angang Dong - ,
Ying Chen - , and
Yi Tang
A novel and facile approach, named iron-assisted vapor-phase hydrothermal method, is proposed to fabricate the long and stable bright blue light emitting SiOx nanowires (SiONWs) at a low temperature. The obtained SiONWs possess a brand-new single-crystalline structure of space group P21212. This new type of bright blue light emitting SiONW with a new single-crystalline structure may initiate a new 1D optical material and offer new opportunities for the research in science and technology.
Umpolung of Michael Acceptors Catalyzed by N-Heterocyclic Carbenes
Christian Fischer - ,
Sean W. Smith - ,
David A. Powell - , and
Gregory C. Fu
N-Heterocyclic carbenes can catalyze β-alkylations of a range of α,β-unsaturated esters, amides, and nitriles that bear pendant leaving groups to form a variety of ring sizes. In this process, the nucleophilic catalyst transiently transforms the normally electrophilic β carbon into a nucleophilic site through an unanticipated addition−tautomerization sequence.
ARTICLES
The “Somersault” Mechanism for the P-450 Hydroxylation of Hydrocarbons. The Intervention of Transient Inverted Metastable Hydroperoxides
Robert D. Bach - and
Olga Dmitrenko
A series of model theoretical calculations are described that suggest a new mechanism for the oxidation step in enzymatic cytochrome P450 hydroxylation of saturated hydrocarbons. A new class of metastable metal hydroperoxides is described that involves the rearrangement of the ground-state metal hydroperoxide to its inverted isomeric form with a hydroxyl radical hydrogen bonded to the metal oxide (MO−OH → MO····HO). The activation energy for this somersault motion of the FeO−OH group is 20.3 kcal/mol for the P450 model porphyrin iron(III) hydroperoxide [Por(SH)Fe(III)−OOH-] to produce the isomeric ferryl oxygen hydrogen bonded to an •OH radical [Por(SH)Fe(III)-O····HO-]. This isomeric metastable hydroperoxide, the proposed primary oxidant in the P450 hydroxylation reaction, is calculated to be 17.8 kcal/mol higher in energy than the ground-state iron(III) hydroperoxide Cpd 0. The first step of the proposed mechanism for isobutane oxidation is abstraction of a hydrogen atom from the C−H bond of isobutane by the hydrogen-bonded hydroxyl radical to produce a water molecule strongly hydrogen bonded to anionic Cpd II. The hydroxylation step involves a concerted but nonsynchronous transfer of a hydrogen atom from this newly formed, bound, water molecule to the ferryl oxygen with a concomitant rebound of the incipient •OH radical to the carbon radical of isobutane to produce the C−O bond of the final product, tert-butyl alcohol. The TS for the oxygen rebound step is 2 kcal/mol lower in energy than the hydrogen abstraction TS (ΔE⧧ = 19.5 kcal/mol). The overall proposed new mechanism is consistent with a lot of the ancillary experimental data for this enzymatic hydroxylation reaction.
Operating Molecular Elevators
Jovica D. Badjic - ,
Célia M. Ronconi - ,
J. Fraser Stoddart - ,
Vincenzo Balzani - ,
Serena Silvi - , and
Alberto Credi
Inspired by the concept of multivalency in living systems, two mechanically interlocked molecules have been conceived that incorporate not once or twice but thrice the features of a pH-switchable [2]rotaxane with two orthogonal recognition sites for dibenzo[24]crown-8 (DB24C8), and 2,3-dinaphtho[24]crown-8 (DN24C8)one a dialkylammonium ion (CH2NH2+CH2) and the other a bipyridinium dication (BIPY2+). Whereas at low pH, the CH2NH2+CH2 sites bind the DB24C8/DN24C8 macrocycles preferentially, at high pH, deprotonation occurs with loss of hydrogen bonding and the macrocycles will move to the BIPY2+ sites, where they can acquire some stabilizing [π−π] stacking interactions. Such mechanically interlocked molecules have been assembled from a trifurcated rig-like component wherein the dumbbell-like components of three [2]rotaxanes have one of their ends fused onto alternate positions (1,3,5) around a benzenoid core. The rig is mechanically interlocked by a platform based on a tritopic receptor, wherein either three benzo[24]crown-8 or three 2,3-naphtho[24]crown-8 macrocycles are fused onto a hexaoxatriphenylene core. The synthesis of these molecular elevators involves 1:1 complexation, followed by stoppering, i.e., feet are added to the rig. 1H NMR spectroscopy and cyclic voltammetry, aided and abetted by absorption spectroscopy, have been employed to unravel the details of the mechanism by which the rig and platform components move on the alternate addition of base and acid. For each molecular elevator, the platform operates by taking three distinct steps associated with each of the three deprotonation/reprotonation processes. Thus, molecular elevators are more reminiscent of a legged animal than they are of passengers on freight elevators.
Synthesis and Photoelectron Spectroscopic Studies of N(CH2CH2NMe)3PE (E = O, S, NH, CH2)
Tamás Kárpáti - ,
Tamás Veszprémi - ,
Natesan Thirupathi - ,
Xiaodong Liu - ,
Zhigang Wang - ,
Arkady Ellern - ,
László Nyulászi - , and
John G. Verkade
The synthesis and the crystal and molecular structure of N(CH2CH2NMe)3PCH2 is reported. The P−Nax distance is rather long in N(CH2CH2NMe)3PCH2. The ylide N(CH2CH2NMe)3PCH2 proved to be a stronger proton acceptor than proazaphosphatrane N(CH2CH2NMe)3P, since it was shown to deprotonate N(CH2CH2NMe)3PH+. The extremely strong basicity of the ylide is in accordance with its low ionization energy (6.3 eV), which is the lowest in the presently investigated series N(CH2CH2NMe)3PE (E: CH2, NH, lone pair, O and S), and to the best of our knowledge it is the smallest value observed for a non-conjugated phosphorus ylide. Computations reveal the existence of two bond strech isomers, and the stabilization of the phosphorus centered cation by electron donation from the equatorial and the axial nitrogens. Similar stabilizing effects operate in the case of protonation of E. A fine balance of these different interactions determines the P−Nax distance, which is thus very sensitive to the level of the theory applied. According to the quantum mechanical calculations, methyl substitution at the equatorial nitrogens flattens the pyramidality of this atom, increasing its electron donor capability. As a consequence, the PNax distance in the short-transannular bonded protonated systems and the radical cations is longer by about 0.5 Å in the Neq(Me) than in the Neq(H) systems. Accordingly, isodesmic reaction energies show that a stabilization of about 25 and 10 kcal/mol is attributable to the formation of the transannular bond in case of Neq(H) and the experimentally realizable Neq(Me) species, respectively.
Ruthenium Molecular Wires with Conjugated Bridging Ligands: Onset of Band Formation in Linear Inorganic Conjugated Oligomers
Samuel Flores-Torres - ,
Geoffrey R. Hutchison - ,
Leonard J. Soltzberg - , and
Héctor D. Abruña
We have prepared and characterized a series of multimetallic oligomers of Ru using the π-conjugated bridging ligand tetra-2-pyridyl-1,4-pyrazine (tppz), as well as mixed-ligand complexes with terpyridine end caps, and analyzed their electrochemical and spectroscopic properties, comparing them with modern computational electronic structure methods. The results suggest that the high degree of metal−metal interunit communication in these linear oligomers yields low HOMO−LUMO gaps, high delocalization, and the onset of “quasi-band” features, all indicative that these compounds should be excellent molecular wire materials. Recent spectroscopic and excited-state analyses of these and related compounds focus on optically accessible states, which ignore optically silent frontier electronic states more relevant to nanoelectronic applications.
Exploring the Energy Landscape of a Small RNA Hairpin
Hairong Ma - ,
David J. Proctor - ,
Elzbieta Kierzek - ,
Ryszard Kierzek - ,
Philip C. Bevilacqua - , and
Martin Gruebele
The energy landscape of a small RNA tetraloop hairpin is explored by temperature jump kinetics and base-substitution. The folding kinetics are single-exponential near the folding transition midpoint Tm. An additional fast phase appears below the midpoint, and an additional slow phase appears above the midpoint. Stem mutation affects the high-temperature phase, while loop mutation affects the low-temperature phase. An adjusted 2-D lattice model reproduces the temperature-dependent phases, although it oversimplifies the structural interpretation. A four-state free energy landscape model is generated based on the lattice model. This model explains the thermodynamics and multiphase kinetics over the full temperature range of the experiments. An analysis of three variants shows that one of the intermediate RNA structures is a stacking-related trap affected by stem but not loop modification, while the other is an early intermediate that forms some stem and loop structure. Even a very fast-folding 8-mer RNA with an ideal tetraloop sequence has a rugged energy landscape, ideal for testing analytical and computational models.
Selective Formation of a Self-Assembling Homo or Hetero Cavitand Cage via Metal Coordination Based on Thermodynamic or Kinetic Control
Masamichi Yamanaka - ,
Yoshifumi Yamada - ,
Yoshihisa Sei - ,
Kentaro Yamaguchi - , and
Kenji Kobayashi
The selective formation of a homo or hetero cavitand cage composed of two molecules of tetra(4-pyridyl)-cavitand (1), tetrakis(4-cyanophenyl)-cavitand (2), or tetrakis(4-pyridylethynyl)-cavitand (3), and four molecules of Pd(dppp)(OTf)2 (4) or Pt(dppp)(OTf)2 (5) has been studied. A 1:1:4 mixture of 1 with more steric restriction, 2 with less coordination ability, and 4 or 5 specifically self-assembled into a hetero cavitand cage 6 or 7, respectively. In contrast, a 1:1:4 mixture of 2, 3, and 4 in CDCl3 at room temperature assembled into the most labile homo cyanophenyl cavitand cage 8 and the most stable homo pyridylethynyl cavitand cage 9 in a 1:1 ratio. Upon heating at 50 °C, the thermodynamic equilibrium was shifted to a 1:1:1 mixture of 8, 9, and a hetero cavitand cage 10. When 1 equiv of 3 was added to 8 at room temperature, 8, 9, and 10 were formed initially in a 1:1:3 ratio and finally shifted to a 1:1:1 ratio. In the Pt-system, upon addition of 1 equiv of 3 to homo cyanophenyl cavitand cage 11 in CDCl3 at room temperature, the ratio of hetero to homo cavitand cage (13/12) initially attained was 8.7 and remained above 5.6 at room temperature. Upon heating at 50 °C, 13 was finally converted to 11 and 12. Thus, the selectivity for the self-assembly of the homo or hetero cavitand cage is controlled by the balance between kinetic and thermodynamic stabilities of cages based on a combination of factors such as coordination ability and steric demand of the cavitands.
Probing the Decay Coordinate of the Green Fluorescent Protein: Arrest of Cis−Trans Isomerization by the Protein Significantly Narrows the Fluorescence Spectra
Solomon S. Stavrov - ,
Kyril M. Solntsev - ,
Laren M. Tolbert - , and
Dan Huppert
The fluorescence spectra of the wild-type green fluorescence protein (wt-GFP) and the anionic form of p-hydroxybenzylidenedimethylimidazolone (p-HBDI), which models the protein chromophore, were obtained in the 80−300 K temperature range in glycerol/water solvent. The protein spectra have pronounced and well-resolved vibronic structure, at least at lower temperatures. In contrast, the chromophore spectra are very broad and structureless even at the lowest temperatures. Analysis of the spectra shows that the experimentally observed red-shift of the protein spectrum upon heating is apparently caused by quadratic vibronic coupling of the torsional deformation (TD) of the phenyl single bond of the chromophore to the electronic transition. The broad spectra of the chromophore manifest the contribution of different conformations in the glycerol/water solvent. In particular, the lowest-temperature spectrum reflects the distribution over the same TD coordinate in the excited electronic state, which essentially contributes to the asymmetry of the spectrum. Upon heating, motion along this coordinate leads to a configuration from which the radiationless transition takes place. This narrows the distribution along the TD coordinate, causing a more symmetric fluorescence spectrum. We were able to reconstruct the broad, structureless fluorescence spectra of p-HBDI in glycerol/water solutions at various temperatures by convoluting the original wt-GFP spectra with the function describing the distribution of the transition energies of the p-HBDI chromophore. Thus, both the fluorescence broadening and increase in radiationless transition upon removal of the protein chromophore to bulk solvent are consistent with decay by a barrierless TD of the phenyl single bond.
Characterizing Challenging Microcrystalline Solids with Solid-State NMR Shift Tensor and Synchrotron X-ray Powder Diffraction Data: Structural Analysis of Ambuic Acid
James K. Harper - ,
David M. Grant - ,
Yuegang Zhang - ,
Peter L. Lee - , and
Robert Von Dreele
Synchrotron X-ray powder diffraction and solid-state 13C NMR shift tensor data are combined to provide a unique path to structure in microcrystalline organic solids. Analysis is demonstrated on ambuic acid powder, a widely occurring natural product, to provide the complete crystal structure. The NMR data verify phase purity, specify one molecule per asymmetric unit, and provide an initial structural model including relative stereochemistry and molecular conformation. A refinement of X-ray data from the initial model establishes that ambuic acid crystallizes in the P21 space group with unit cell parameters a = 15.5047(7), b = 4.3904(2), and c = 14.1933(4) Å and β = 110.3134(3)°. This combined analysis yields structural improvements at two dihedral angles over prior NMR predictions with differences of 103° and 37° found. Only minor differences of ±5.5°, on average, are observed at all remaining dihedral angles. Predicted hydroxyl hydrogen-bonding orientations also fit NMR predictions within ±6.9°. This refinement corrects chemical shift assignments at two carbons and reduces the NMR error by ∼16%. This work demonstrates that the combination of long-range order information from synchrotron powder diffraction data together with the accurate shorter range structure given by solid-state NMR measurements is a powerful tool for studying challenging organic solids.
A DFT/CDM Study of Metal−Carboxylate Interactions in Metalloproteins: Factors Governing the Maximum Number of Metal-Bound Carboxylates
Todor Dudev - and
Carmay Lim
The number of negatively charged metal-bound Asp/Glu residues determines the net charge of the carboxylate-rich metal-binding site, which has been found to play a role in enhancing the affinity and/or selectivity of a protein cavity for a given metal cofactor. Therefore, it is of interest to know the maximum number of carboxylates that could bind to a given metal (Mq+) of charge q and the key factors determining this upper limit in protein cavities, which are usually relatively buried. Using density functional theory combined with the continuum dielectric method to compute the H2O → CH3COO- exchange free energies, the maximum number of carboxylates bound to Mq+ in a relatively buried metal-binding site is found to depend on (i) the metal charge, q, (ii) the carboxylate-binding mode, and (iii) the first-shell carboxylate−second-shell ligand interactions. The maximum number of carboxylates bound to Mq+ in a fully/partially solvent inaccessible protein cavity would not likely exceed q + 2 if (a) the metal-bound Asp/Glu side chains are hydrogen bonded to a Lys/Arg side chain or several peptide backbone amides/Asn/Gln side chains in the metal's second coordination shell or (b) at least one acidic residue binds bidentately, as opposed to monodentately, to the metal cofactor. This number is reduced to q + 1 in the absence of stabilizing interactions from outer-shell ligand(s) and if all the carboxylates are bound monodentately to the metal cofactor in a buried cavity. The computational results are consistent with findings from a PDB survey of uni-, di-, and trivalent metal-binding sites containing Asp/Glu residues.
pH Rate Profiles of FnY356−R2s (n = 2, 3, 4) in Escherichia coli Ribonucleotide Reductase: Evidence that Y356 Is a Redox-Active Amino Acid along the Radical Propagation Pathway
Mohammad R. Seyedsayamdost - ,
Cyril S. Yee - ,
Steven Y. Reece - ,
Daniel G. Nocera - , and
JoAnne Stubbe
The Escherichia coli ribonucleotide reductase (RNR), composed of two subunits (R1 and R2), catalyzes the conversion of nucleotides to deoxynucleotides. Substrate reduction requires that a tyrosyl radical (Y122•) in R2 generate a transient cysteinyl radical (C439•) in R1 through a pathway thought to involve amino acid radical intermediates [Y122• → W48 → Y356 within R2 to Y731 → Y730 → C439 within R1]. To study this radical propagation process, we have synthesized R2 semisynthetically using intein technology and replaced Y356 with a variety of fluorinated tyrosine analogues (2,3-F2Y, 3,5-F2Y, 2,3,5-F3Y, 2,3,6-F3Y, and F4Y) that have been described and characterized in the accompanying paper. These fluorinated tyrosine derivatives have potentials that vary from −50 to +270 mV relative to tyrosine over the accessible pH range for RNR and pKas that range from 5.6 to 7.8. The pH rate profiles of deoxynucleotide production by these FnY356−R2s are reported. The results suggest that the rate-determining step can be changed from a physical step to the radical propagation step by altering the reduction potential of Y356• using these analogues. As the difference in potential of the FnY• relative to Y• becomes >80 mV, the activity of RNR becomes inhibited, and by 200 mV, RNR activity is no longer detectable. These studies support the model that Y356 is a redox-active amino acid on the radical-propagation pathway. On the basis of our previous studies with 3-NO2Y356−R2, we assume that 2,3,5-F3Y356, 2,3,6-F3Y356, and F4Y356−R2s are all deprotonated at pH > 7.5. We show that they all efficiently initiate nucleotide reduction. If this assumption is correct, then a hydrogen-bonding pathway between W48 and Y356 of R2 and Y731 of R1 does not play a central role in triggering radical initiation nor is hydrogen-atom transfer between these residues obligatory for radical propagation.
Mono-, Di-, Tri-, and Tetra-Substituted Fluorotyrosines: New Probes for Enzymes That Use Tyrosyl Radicals in Catalysis†
Mohammad R. Seyedsayamdost - ,
Steven Y. Reece - ,
Daniel G. Nocera - , and
JoAnne Stubbe
A set of N-acylated, carboxyamide fluorotyrosine (FnY) analogues [Ac−3-FY−NH2, Ac−3,5-F2Y−NH2, Ac−2,3-F2Y−NH2, Ac−2,3,5-F3Y−NH2, Ac−2,3,6-F3Y−NH2 and Ac−2,3,5,6-F4Y−NH2] have been synthesized from their corresponding amino acids to interrogate the detailed reaction mechanism(s) accessible to FnY•s in small molecules and in proteins. These Ac−FnY−NH2 derivatives span a pKa range from 5.6 to 8.4 and a reduction potential range of 320 mV in the pH region accessible to most proteins (6−9). DFT electronic-structure calculations capture the observed trends for both the reduction potentials and pKas. Dipeptides of the methyl ester of 4-benzoyl-l-phenylalanyl-FnYs at pH 4 were examined with a nanosecond laser pulse and transient absorption spectroscopy to provide absorption spectra of FnY•s. The EPR spectrum of each FnY• has also been determined by UV photolysis of solutions at pH 11 and 77 K. The ability to vary systematically both pKa and radical reduction potential, together with the facility to monitor radical formation with distinct absorption and EPR features, establishes that FnYs will be useful in the study of biological charge-transport mechanisms involving tyrosine. To demonstrate the efficacy of the fluorotyrosine method in unraveling charge transport in complex biological systems, we report the global substitution of tyrosine by 3-fluorotyrosine (3-FY) in the R2 subunit of ribonucleotide reductase (RNR) and present the EPR spectrum along with its simulation of 3-FY122•. In the companion paper, we demonstrate the utility of FnYs in providing insight into the mechanism of tyrosine oxidation in biological systems by incorporating them site-specifically at position 356 in the R2 subunit of Escherichia coli RNR.
The A-FX to FA/B Step in Synechocystis 6803 Photosystem I Is Entropy Driven
Harvey J. M. Hou - and
David Mauzerall
We have previously reported the enthalpy and volume changes of charge separation in photosystem I from Synechocystis 6803 using pulsed photoacoustics on the microsecond time scale, assigned to the electron-transfer reaction from excited-state P700* to FA/B iron sulfur clusters. In the present work, we focus on the thermodynamics of two steps in photosystem I: (1) P700* → A1-FX (<10 ns) and (2) A1-FX → FA/B- (20−200 ns). The fit by convolution of photoacoustic waves on the nanosecond and microsecond time scales resolved two kinetic components: (1) a prompt component (<10 ns) with large negative enthalpy (−0.8 ± 0.1 eV) and large volume change (−23 ± 2 A3), which are assigned to the P700* → A1-FX step, and (2) a component with ∼200 ns lifetime, which has a positive enthalpy (+0.4 ± 0.2 eV) and a small volume change (−3 ± 2 A3) that are attributed to the A1-FX → FA/B- step. For the fast reaction using the redox potentials of A1FX (−0.67 V) and P700 (+0.45 V) and the energy of P700* (1.77 eV), the free energy change for the P700* → A1-FX step is −0.63 eV, and thus the entropy change (TΔS, T = 25 °C) is −0.2 ± 0.3 eV. For the slow reaction, A1-FX → FA/B-, taking the free energy of −0.14 eV [Santabara, S.; Heathcote, P; Evans, C. W. Biochim. Biophys. Acta 2005, 1708, 283−310], the entropy change (TΔS) is positive, +0.54 ± 0.3 eV. The positive entropy contribution is larger than the positive enthalpy, which indicates that the A-FX to FA/B- step in photosystem I is entropy driven. Other possible contributions to the measured values are discussed.
Aerosol Assisted Chemical Vapor Deposition Using Nanoparticle Precursors: A Route to Nanocomposite Thin Films
Robert G. Palgrave - and
Ivan P. Parkin
Gold nanoparticle and gold/semiconductor nanocomposite thin films have been deposited using aerosol assisted chemical vapor deposition (CVD). A preformed gold colloid in toluene was used as a precursor to deposit gold films onto silica glass. These nanoparticle films showed the characteristic plasmon absorption of Au nanoparticles at 537 nm, and scanning electron microscopic (SEM) imaging confirmed the presence of individual gold particles. Nanocomposite films were deposited from the colloid concurrently with conventional CVD precursors. A film of gold particles in a host tungsten oxide matrix resulted from co-deposition with [W(OPh)6], while gold particles in a host titania matrix resulted from co-deposition with [Ti(OiPr)4]. The density of Au nanoparticles within the film could be varied by changing the Au colloid concentration in the original precursor solution. Titania/gold composite films were intensely colored and showed dichromism: blue in transmitted light and red in reflected light. They showed metal-like reflection spectra and plasmon absorption. X-ray photoelectron spectroscopy and energy-dispersive X-ray analysis confirmed the presence of metallic gold, and SEM imaging showed individual Au nanoparticles embedded in the films. X-ray diffraction detected crystalline gold in the composite films. This CVD technique can be readily extended to produce other nanocomposite films by varying the colloids and precursors used, and it offers a rapid, convenient route to nanoparticle and nanocomposite thin films.
Determination of Equilibrium Constants for Atom Transfer Radical Polymerization
Wei Tang - ,
Nicolay V. Tsarevsky - , and
Krzysztof Matyjaszewski
Atom transfer radical polymerization (ATRP) equilibrium constants (KATRP) were determined using modified Fischer's equations for the persistent radical effect. The original Fischer's equations could be used only for low conversion of CuI to X−CuII and consequently for relatively low values of KATRP. At higher conversion to X−CuII (>10%) and for larger values of KATRP (>10-7), modified equations that take into account the changes in catalyst and initiator concentrations should be used. The validity of new equations was confirmed by detailed kinetic simulations. UV−vis spectrometric and GC measurements were used to follow the evolution of X−CuII species and the initiator concentration, respectively, and to successfully determine values of KATRP for several catalysts and alkyl halides. The effect of structure on reactivities of ATRP components is presented.
A Comprehensive Approach to the Synthesis of Sulfate Esters
Levi S. Simpson - and
Theodore S. Widlanski
A comprehensive approach to the synthesis of sulfate esters was developed. This approach permits the direct and high-yielding synthesis of protected sulfate monoesters. Subsequent deblocking to reveal sulfate monoesters is accomplished in near-quantitative yield. The exceptionally stable neopentyl protecting group and the labile isobutyl protecting group were utilized in the synthesis of aromatic and aliphatic sulfate monoesters. Strategies for tuning protecting group reactivity were also explored and developed.
Bismuth-Catalyzed Intermolecular Hydroamination of 1,3-Dienes with Carbamates, Sulfonamides, and Carboxamides
Hongbo Qin - ,
Noriyuki Yamagiwa - ,
Shigeki Matsunaga - , and
Masakatsu Shibasaki
A Bi(OTf)3/Cu(CH3CN)4PF6 system efficiently promoted intermolecular 1:1 hydroamination of 1,3-dienes with various carbamates, sulfonamides, and carboxamides to afford allylic amines in good yield (up to 96%). Reaction proceeded with 0.5−10 mol % catalyst loading at 25−100 °C (generally at 50 °C) in 1,4-dioxane within 24 h. The Bi(OTf)3/Cu(CH3CN)4PF6 system constitutes a new entry into series of intermolecular hydroamination catalysis. Mechanistic studies and the postulated reaction mechanism are also discussed.
Substrate Specificity of an Active Dinuclear Zn(II) Catalyst for Cleavage of RNA Analogues and a Dinucleoside
AnnMarie O'Donoghue - ,
Sang Yong Pyun - ,
Meng-Yin Yang - ,
Janet R. Morrow - , and
John P. Richard
The cleavage of the diribonucleoside UpU (uridylyl-3‘-5‘-uridine) to form uridine and uridine (2‘,3‘)-cyclic phosphate catalyzed by the dinuclear Zn(II) complex of 1,3-bis(1,4,7-triazacyclonon-1-yl)-2-hydroxypropane (Zn2(1)(H2O)) has been studied at pH 7−10 and 25 °C. The kinetic data are consistent with the accumulation of a complex between catalyst and substrate and were analyzed to give values of kc (s-1), Kd (M), and kc/Kd (M-1 s-1) for the Zn2(1)(H2O)-catalyzed reaction. The pH rate profile of values for log kc/Kd for Zn2(1)(H2O)-catalyzed cleavage of UpU shows the same downward break centered at pH 7.8 as was observed in studies of catalysis of cleavage of 2-hydroxypropyl-4-nitrophenyl phosphate (HpPNP) and uridine-3‘-4-nitrophenyl phosphate (UpPNP). At low pH, where the rate acceleration for the catalyzed reaction is largest, the stabilizing interaction between Zn2(1)(H2O) and the bound transition states is 9.3, 7.2, and 9.6 kcal/mol for the catalyzed reactions of UpU, UpPNP, and HpPNP, respectively. The larger transition-state stabilization for Zn2(1)(H2O)-catalyzed cleavage of UpU (9.3 kcal/mol) compared with UpPNP (7.2 kcal/mol) provides evidence that the transition state for the former reaction is stabilized by interactions between the catalyst and the C-5‘-oxyanion of the basic alkoxy leaving group.
Marinomycins A−D, Antitumor-Antibiotics of a New Structure Class from a Marine Actinomycete of the Recently Discovered Genus “Marinispora”
Hak Cheol Kwon - ,
Christopher A. Kauffman - ,
Paul R. Jensen - , and
William Fenical
Four antitumor-antibiotics of a new structure class, the marinomycins A−D (1−4), were isolated from the saline culture of a new group of marine actinomycetes, for which we have proposed the name “Marinispora”. The structures of the marinomycins, which are unusual macrodiolides composed of dimeric 2-hydroxy-6-alkenyl-benzoic acid lactones with conjugated tetraene-pentahydroxy polyketide chains, were assigned by combined spectral and chemical methods. In room light, marinomycin A slowly isomerizes to its geometrical isomers marinomycins B and C. Marinomycins A−D show significant antimicrobial activities against drug resistant bacterial pathogens and demonstrate impressive and selective cancer cell cytotoxicities against six of the eight melanoma cell lines in the National Cancer Institute's 60 cell line panel. The discovery of these new compounds from a new, chemically rich genus further documents that marine actinomycetes are a significant resource for drug discovery.
NMR Structure and Dynamic Studies of an Anion-Binding, Channel-Forming Heptapeptide
Gabriel A. Cook - ,
Robert Pajewski - ,
Mahalaxmi Aburi - ,
Paul E. Smith - ,
Om Prakash - ,
John M. Tomich - , and
George W. Gokel
The synthetic peptide (C18H37)2NCOCH2OCH2CON−(Gly)3−Pro−(Gly)3−OCH2Ph forms chloride-selective channels in liposomes and exhibits voltage-gating properties in planar phospholipid bilayers. The peptide fragment of the channel is based on a conserved motif in naturally occurring chloride transporters. Membrane-anchoring residues at the N- and C-terminal ends augment the peptide. NMR spectra (1D and 2D) of the channel in CDCl3 showed significant variation in the absence and presence of stoichiometric tetrabutylammonium chloride (Bu4NCl). One-dimensional solution-state NMR titration studies combined with computational molecular simulation studies indicate that the peptide interacts with the salt as an ion pair and H-bonds chloride. To our knowledge, this is the first structural analysis of any synthetic anion-channel salt complex.
Rotational and Translational Diffusion of Peptide-Coated CdSe/CdS/ZnS Nanorods Studied by Fluorescence Correlation Spectroscopy
James M. Tsay - ,
Sören Doose - , and
Shimon Weiss
CdSe/CdS/ZnS nanorods (NRs) of three aspect ratios were coated with phytochelatin-related peptides and studied using fluorescence correlation spectroscopy (FCS). Theoretical predictions of the NRs' rotational diffusion contribution to the correlation curves were experimentally confirmed. We monitored rotational and translational diffusion of NRs and extracted hydrodynamic radii from the extracted diffusion constants. Translational and rotational diffusion constants (Dtrans and Drot) for NRs were in good agreement with Tirado and Garcia de la Torre's as well as with Broersma's theories when accounting for the ligand dimensions. NRs fall in the size range where rotational diffusion can be monitored with higher sensitivity than translational diffusion due to a steeper length dependence, Drot ∼ L-3 versus Dtrans ∼ L-1. By titrating peptide-coated NRs with bovine serum albumin, we monitored (nonspecific) binding through rotational diffusion and showed that Drot is an advantageous observable for monitoring binding. Monitoring rotational diffusion of bioconjugated NRs using FCS might prove to be useful for observing binding and conformational dynamics in biological systems.
Double-Ink Dip-Pen Nanolithography Studies Elucidate Molecular Transport
Jennifer R. Hampton - ,
Arrelaine A. Dameron - , and
Paul S. Weiss
We have investigated the transport mechanism of the inks most typically used in dip-pen nanolithography by patterning both 16-mercaptohexadecanoic acid (MHDA) and 1-octadecanethiol (ODT) on the same Au{111} substrate. Several pattern geometries were used to probe ink transport from the tip to the sample during patterning of both dots (stationary tip) and lines (moving tip). When ODT was written on top of a pre-existing MHDA structure, the ODT was observed at the outsides of the MHDA structure, and the transport rate increased. In the reverse case, the MHDA was also observed on the outsides of the previously patterned ODT features; however, the transport rate was reduced. Furthermore, the shapes of pre-existing patterns of one ink were not changed by deposition of the other ink. These results highlight the important role hydrophobicity plays, both of the substrate as well as of the inks, in determining transport properties and thereby patterns produced in dip-pen nanolithography.
The Chemistry of Dinuclear Analogues of the Anticancer Drug Cisplatin. A DFT/CDM Study†
Dirk V. Deubel
The mechanism of the formation of dinuclear platinum(II) μ-hydroxo complexes from cisplatin hydrolysis products, their interconversion, decomposition, and reactions with biomolecules has been explored using a combined DFT/CDM approach. All activation barriers for the formation of [cis-{Pt(NH3)2(X)}−(μ-OH)−cis-{Pt(NH3)2(Y)}]n+ (X, Y = Cl, OH2, OH) via nucleophilic attack of a hydroxo complex on an aqua complex are lower than the activation barriers for cisplatin hydrolysis. Considering therapeutic Pt(II) concentrations in tumors, however, only the reaction between two molecules of cis-[Pt(NH3)2(OH2)(OH)]+ (E) yielding [cis-{Pt(NH3)2(OH2)}−(μ-OH)−cis-{Pt(NH3)2(OH)}]2+ (5) remains kinetically superior to cisplatin hydrolysis. 5 is strongly stabilized by intramolecular hydrogen bonding between the terminal aqua and hydroxo ligands, resulting in an unusually high pKa of 5 and a low pKa of its conjugate acid. Unimolecular cyclization of 5 yields the dimers [cis-{Pt(NH3)2}(μ-OH)]22+ (7a with antiperiplanar OH groups and 7b with synperiplanar OH groups). The electronic structure of several diplatinum(II) complexes has been analyzed to clarify whether there are metal−metal interactions. The overall reactivity to guanine (Gua) and dimethyl sulfide (Met, representing the thioether functional group of methionine) increases in the order 5 < 7a ≈ 7b < mononuclear complexes, whereas the kinetic selectivity to Gua relative to Met increases in the order 7a ≈ 5 < 7b ≈ monocationic mononuclear complexes < dicationic mononuclear complex. The results of this work (i) help assess whether dinuclear metabolites play a role in cisplatin chemotherapy, (ii) elucidate the toxicity and pharmacological inactivity of [cis-{Pt(NH3)2}(μ-OH)]22+, and (iii) suggest future investigations of dinuclear anticancer complexes that contain one μ-hydroxo ligand.
Design of Highly Active Binary Catalyst Systems for CO2/Epoxide Copolymerization: Polymer Selectivity, Enantioselectivity, and Stereochemistry Control
Xiao-Bing Lu - ,
Lei Shi - ,
Yi-Ming Wang - ,
Rong Zhang - ,
Ying-Ju Zhang - ,
Xiao-Jun Peng - ,
Zhi-Chao Zhang - , and
Bo Li
Asymmetric, regio- and stereoselective alternating copolymerization of CO2 and racemic aliphatic epoxides proceeds effectively under mild temperature and pressure by using a binary catalyst system of a chiral tetradentate Schiff base cobalt complex [SalenCoIIIX] as the electrophile in conjunction with an ionic organic ammonium salt or a sterically hindered strong organic base as the nucleophile. The substituent groups on the aromatic rings, chiral diamine backbone, and axial X group of the electrophile, as well as the nucleophilicity, leaving ability, and coordination ability of the nucleophile, all significantly affect the catalyst activity, polymer selectivity, enantioselectivity, and stereochemistry. A bulky chiral cyclohexenediimine backbone complex [SalcyCoIIIX] with an axial X group of poor leaving ability as the electrophile, combined with a bulky nuclephile with poor leaving ability and low coordination ability, is an ideal binary catalyst system for the copolymerization of CO2 and a racemic aliphatic epoxide to selectively produce polycarbonates with relatively high enantioselectivity, >95% head-to-tail connectivity, and >99% carbonate linkages. A fast copolymerization of CO2 and epoxides was observed when the concentration of the electrophile or/and the nucleophile was increased, and the number of polycarbonate chains was proportional to the concentration of the nucleophile. Electrospray ionization mass spectrometry, in combination with a kinetic study, showed that the copolymerization involved the coordination activation of the monomer by the electrophile and polymer chain growth predominately occurring in the nucleophile. Both the enantiomorphic site effect resulting from the chiral electrophile and the polymer chain end effect mainly from the bulky nucleophile cooperatively control the stereochemistry of the CO2/epoxide copolymerization.
The Concept of Delayed Nucleation in Nanocrystal Growth Demonstrated for the Case of Iron Oxide Nanodisks
Maria F. Casula - ,
Young-wook Jun - ,
David J. Zaziski - ,
Emory M. Chan - ,
Anna Corrias - , and
A. Paul Alivisatos
A comprehensive study of iron oxide nanocrystal growth through non-hydrolitic, surfactant-mediated thermal reaction of iron pentacarbonyl and an oxidizer has been conducted, which includes size control, anisotropic shape evolution, and crystallographic phase transition of monodisperse iron oxide colloidal nanocrystals. The reaction was monitored via in situ UV−vis spectroscopy, taking advantage of the color change accompanying the iron oxide colloid formation, allowing measurement of the induction time for nucleation. Features of the synthesis such as the size control and reproducibility are related to the occurrence of the observed delayed nucleation process. As a separate source of iron and oxygen is adopted, phase control could also be achieved by sequential injections of oxidizer.
Effect of Single-Point Sequence Alterations on the Aggregation Propensity of a Model Protein
Dusan Bratko - ,
Troy Cellmer - ,
John M. Prausnitz - , and
Harvey W. Blanch
Sequences of contemporary proteins are believed to have evolved through a process that optimized their overall fitness, including their resistance to deleterious aggregation. Biotechnological processing may expose therapeutic proteins to conditions that are much more conducive to aggregation than those encountered in a cellular environment. An important task of protein engineering is to identify alternative sequences that would protect proteins when processed at high concentrations without altering their native structure associated with specific biological function. Our computational studies exploit parallel tempering simulations of coarse-grained model proteins to demonstrate that isolated amino acid residue substitutions can result in significant changes in the aggregation resistance of the protein in a crowded environment while retaining protein structure in isolation. A thermodynamic analysis of protein clusters subject to competing processes of folding and association shows that moderate mutations can produce effects similar to those caused by changes in system conditions, including temperature, concentration, and solvent composition, that affect the aggregation propensity. The range of conditions where a protein can resist aggregation can therefore be tuned by sequence alterations, although the protein generally may retain its generic ability for aggregation.
Polymer Crystallization-Driven, Periodic Patterning on Carbon Nanotubes
Lingyu Li - ,
Christopher Y. Li - , and
Chaoying Ni
We report herein a unique means to periodically pattern polymeric materials on individual carbon nanotubes (CNTs) using a controlled polymer crystallization method. One-dimensional (1D) CNTs were periodically decorated with polymer lamellar crystals, resulting in nano-hybrid shish-kebab (NHSK) structures. The periodicity of the polymer lamellae varies from 20 to 150 nm. The kebabs are approximately 5−10 nm thick (along CNT direction) with a lateral size of ∼20 nm to micrometers, which can be readily controlled by varying crystallization conditions. Both polyethylene and Nylon 66 were successfully decorated on single-walled carbon nanotubes (SWNTs), multiwalled carbon nanotubes (MWNTs), as well as vapor grown carbon nanofibers (CNFs). The formation mechanism was attributed to “size-dependent soft epitaxy”. Because NHSK formation conditions depend on CNT structures, it further provides a unique opportunity for CNT separation. The reported method opens a gateway to periodically patterning polymers and different functional groups on individual CNTs in an ordered and controlled manner, an attractive research field that is yet to be explored.
Relationship between Two-Photon Absorption and the π-Conjugation Pathway in Porphyrin Arrays through Dihedral Angle Control
Tae Kyu Ahn - ,
Kil Suk Kim - ,
Deok Yun Kim - ,
Su Bum Noh - ,
Naoki Aratani - ,
Chusaku Ikeda - ,
Atsuhiro Osuka - , and
Dongho Kim
Recently, covalently linked or self-assembled porphyrin array systems have attracted much attention for their enhanced two-photon absorption (TPA) behaviors. In this study, we have investigated the TPA properties of various dihedral angle controlled, directly linked porphyrin dimers and arrays to elucidate the relationship between the π-conjugation pathway and TPA properties. We have demonstrated a strong correlation between π-conjugation (aromaticity) and TPA properties in porphyrin assemblies.
Toward Single Molecule DNA Sequencing: Direct Identification of Ribonucleoside and Deoxyribonucleoside 5‘-Monophosphates by Using an Engineered Protein Nanopore Equipped with a Molecular Adapter
Yann Astier - ,
Orit Braha - , and
Hagan Bayley
Individual nucleic acid molecules might be sequenced by the identification of nucleoside 5‘-monophosphates as they are released by processive exonucleases. Here, we show that single molecule detection with a modified protein nanopore can be used to identify ribonucleoside and 2‘-deoxyribonucleoside 5‘-monophosphates, thereby taking a step along this path. Distinct levels of current block are observed for each of the four members of a set of nucleoside 5‘-monophosphates when the molecules bind within a mutant α-hemolysin pore, (M113R)7, equipped with the molecular adapter heptakis-(6-deoxy-6-amino)-β-cyclodextrin. While our results compare favorably with alternative approaches, further work will be required to improve the accuracy of identification of the nucleic acid bases, to feed each released nucleotide into the pore, and to ensure that every nucleotide is captured by the adapter.
Redox Enzymes in Tethered Membranes
Lars J. C. Jeuken - ,
Simon D. Connell - ,
Peter J. F. Henderson - ,
Robert B. Gennis - ,
Stephen D. Evans - , and
Richard J. Bushby
An electrode surface is presented that enables the characterization of redox-active membrane enzymes in a native-like environment. An ubiquinol oxidase from Escherichia coli, cytochrome bo3 (cbo3), has been co-immobilized into tethered bilayer lipid membranes (tBLMs). The tBLM is formed on gold surfaces functionalized with cholesterol tethers which insert into the lower leaflet of the membrane. The planar membrane architecture is formed by self-assembly of proteoliposomes, and its structure is characterized by surface plasmon resonance (SPR), electrochemical impedance spectroscopy (EIS), and tapping-mode atomic force microscopy (TM-AFM). The functionality of cbo3 is investigated by cyclic voltammetry (CV) and is confirmed by the catalytic reduction of oxygen. Interfacial electron transfer to cbo3 is mediated by the membrane-localized ubiquinol-8, the physiological electron donor of cbo3. Enzyme coverages observed with TM-AFM and CV coincide (2−8.5 fmol·cm-2), indicating that mostif not allcbo3 on the surface is catalytically active and thus retains its integrity during immobilization.
Utilization of the Pleiotropy of a Peptidic Aptamer To Fabricate Heterogeneous Nanodot-Containing Multilayer Nanostructures
Ken-Ichi Sano - ,
Hiroyuki Sasaki - , and
Kiyotaka Shiba
Peptide aptamers (=binders) against inorganic materials often show a capacity for mineralization of their target atoms; thus they are able to function both as binding molecules and as mediators for mineralization. Although the mechanisms underlying these two properties of peptide aptamers are not yet fully understood, they have been used separately to fabricate various nanostructures. Here, we present a novel method of nanofabrication, in which binding and mineralization by a peptide aptamer are alternately utilized to assemble multilayered nanostructures comprised of metal loaded cage proteins ornamented with Ti-binding peptides.
A Comprehensive Study of Self-Assembled Monolayers of Anthracenethiol on Gold: Solvent Effects, Structure, and Stability
Daniel Käfer - ,
Gregor Witte - ,
Piotr Cyganik - ,
Andreas Terfort - , and
Christof Wöll
The formation and molecular structure of self-assembled monolayers (SAMs) of anthracene-2-thiol (AnT) on Au(111) have been characterized by reflection adsorption infrared spectroscopy, thermal desorption spectroscopy, X-ray photoelectron spectroscopy, near-edge X-ray absorption spectroscopy, scanning tunneling microscopy, and low energy electron diffraction. It is demonstrated that highly ordered monolayer films are formed upon immersion, but their quality depends critically on the choice of solvents and rinsing conditions. The saturated monolayer is characterized by a closed packed arrangement of upright standing molecules forming a (2 × 4)rect unit cell. At about 450 K a partial desorption takes place and the remaining molecules form a dilute (4 × 2)-phase with an almost planar adsorption geometry, while further heating above 520 K causes a thermally induced fragmentation. According to their different densities both phases reveal very diverse chemical reactivities. Whereas the saturated monolayer is stable and inert under ambient conditions, the dilute phase does not warrant any protection of the sulfur headgroups which oxidize rapidly in air.
A Threaded Loop Conformation Adopted by a Family of Peptoid Nonamers
Kai Huang - ,
Cindy W. Wu - ,
Tracy J. Sanborn - ,
James A. Patch - ,
Kent Kirshenbaum - ,
Ronald N. Zuckermann - ,
Annelise E. Barron - , and
Ishwar Radhakrishnan
Non-natural polymers with well-defined three-dimensional folds offer considerable potential for engineering novel functions that are outside the scope of biological polymers. Here we describe a family of N-substituted glycine or “peptoid” nonamers that folds into an unusual “threaded loop” structure of exceptional thermal stability and conformational homogeneity in acetonitrile. The structure is chain-length-specific and relies on bulky, chiral side chains and chain-terminating functional groups for stability. Notable elements of the structure include the engagement of the positively charged amino terminus by carbonyl groups of the backbone through hydrogen bonding interactions and shielding of polar groups from and near-complete exposure of hydrophobic groups to solvent, in a manner resembling a folded polypeptide globular domain turned inside-out. The structure is stable in a variety of organic solvents but is readily denatured in any solvent/cosolvent milieu with hydrogen bonding potential. The structure could serve as a scaffold for the elaboration of novel functions and could be used to test methodologies for predicting solvent-dependent polymer folding.
Tuning the Reactivity of Osmium(II) and Ruthenium(II) Arene Complexes under Physiological Conditions
Anna F. A. Peacock - ,
Abraha Habtemariam - ,
Rafael Fernández - ,
Victoria Walland - ,
Francesca P. A. Fabbiani - ,
Simon Parsons - ,
Rhona E. Aird - ,
Duncan I. Jodrell - , and
Peter J. Sadler
The OsII arene ethylenediamine (en) complexes [(η6-biphenyl)Os(en)Cl][Z], Z = BPh4 (4) and BF4 (5), are inactive toward A2780 ovarian cancer cells despite 4 being isostructural with an active RuII analogue, 4R. Hydrolysis of 5 occurred 40 times more slowly than 4R. The aqua adduct 5A has a low pKa (6.3) compared to that of [(η6-biphenyl)Ru(en)(OH2)]2+ (7.7) and is therefore largely in the hydroxo form at physiological pH. The rate and extent of reaction of 5 with 9-ethylguanine were also less than those of 4R. We replaced the neutral en ligand by anionic acetylacetonate (acac). The complexes [(η6-arene)Os(acac)Cl], arene = biphenyl (6), benzene (7), and p-cymene (8), adopt piano-stool structures similar to those of the RuII analogues and form weak dimers through intermolecular (arene)CH···O(acac) H-bonds. Remarkably, these OsII acac complexes undergo rapid hydrolysis to produce not only the aqua adduct, [(η6-arene)Os(acac)(OH2)]+, but also the hydroxo-bridged dimer, [(η6-arene)Os(μ2-OH)3Os(η6-arene)]+. The pKa values for the aqua adducts 6A, 7A, and 8A (7.1, 7.3, and 7.6, respectively) are lower than that for [(η6-p-cymene)Ru(acac)(OH2)]+ (9.4). Complex 8A rapidly forms adducts with 9-ethylguanine and adenosine, but not with cytidine or thymidine. Despite their reactivity toward nucleobases, complexes 6−8 were inactive toward A549 lung cancer cells. This is attributable to rapid hydrolysis and formation of unreactive hydroxo-bridged dimers which, surprisingly, were the only species present in aqueous solution at biologically relevant concentrations. Hence, the choice of chelating ligand in OsII (and RuII) arene complexes can have a dramatic effect on hydrolysis behavior and nucleobase binding and provides a means of tuning the reactivity and the potential for discovery of anticancer complexes.
Electrochemical Growth of Highly Oriented Organic−Inorganic Superlattices Using Solid-Supported Multilamellar Membranes as Templates
Li-Li Xing - ,
Da-Peng Li - ,
Shu-Xin Hu - ,
Huai-Yu Jing - ,
Honglan Fu - ,
Zhen-Hong Mai - , and
Ming Li
Controllable depositing of relatively thick inorganic sublayers into organic templates to fabricate organic−inorganic superlattices is of great importance. We report a novel approach to fabricating phospholipid/Ni(OH)2 superlattices by electrochemical deposition of the inorganic component into solid-supported multilamellar templates. The well-ordered and highly oriented multilamellar templates are produced by spreading small drops of lipid solution on silicon surfaces and letting the solvent evaporate slowly. The templates which are used as working electrodes preserve the lamellar structure in the electrolyte solution. The resulting superlattices are highly oriented. The thickness of the nickel hydroxide is controlled by the concentration of nickel ions in the electrolyte bath. The electron density profiles derived from the X-ray diffraction data reveal that the thickness of the nickel hydroxide sublayers increases from 15 to 27 Å as the concentration of nickel nitrate increases from 0.005 mol/L to 0.08 mol/L. We expect that the new method can be extended to depositing a variety of inorganic components including metals, oxides, and semiconductors.
Controlled Clustering of Superparamagnetic Nanoparticles Using Block Copolymers: Design of New Contrast Agents for Magnetic Resonance Imaging
Jean-François Berret - ,
Nicolas Schonbeck - ,
Florence Gazeau - ,
Delphine El Kharrat - ,
Olivier Sandre - ,
Annie Vacher - , and
Marc Airiau
When polyelectrolyte-neutral block copolymers are mixed in aqueous solutions with oppositely charged species, stable complexes are found to form spontaneously. The mechanism is based on electrostatics and on the compensation between the opposite charges. Electrostatic complexes exhibit a core−shell microstructure. In the core, the polyelectrolyte blocks and the oppositely charged species are tightly bound and form a dense coacervate microphase. The shell is made of the neutral chains and surrounds the core. In this paper, we report on the structural and magnetic properties of such complexes made from 6.3 nm diameter superparamagnetic nanoparticles (maghemite γ-Fe2O3) and cationic-neutral copolymers. The copolymers investigated are poly(trimethylammonium ethylacrylate methyl sulfate)-b-poly(acrylamide), with molecular weights 5000-b-30000 g mol-1 and 110000-b-30000 g mol-1. The mixed copolymer−nanoparticle aggregates were characterized by a combination of light scattering and cryo-transmission electron microscopy. Their hydrodynamic diameters were found in the range 70−150 nm, and their aggregation numbers (number of nanoparticles per aggregate) from tens to hundreds. In addition, Magnetic Resonance Spin−Echo measurements show that the complexes have a better contrast in Magnetic Resonance Imaging than single nanoparticles and that these complexes could be used for biomedical applications.
Aliphatic/Aromatic Polyimide Ionomers as a Proton Conductive Membrane for Fuel Cell Applications
Naoki Asano - ,
Makoto Aoki - ,
Shinsuke Suzuki - ,
Kenji Miyatake - ,
Hiroyuki Uchida - , and
Masahiro Watanabe
To produce a proton conductive and durable polymer electrolyte membrane for fuel cell applications, a series of sulfonated polyimide ionomers containing aliphatic groups both in the main and in the side chains have been synthesized. The title polyimide ionomers 1 with the ion exchange capacity of 1.78−2.33 mequiv/g were obtained by a typical polycondensation reaction as transparent, ductile, and flexible membranes. The proton conductivity of 1 was slightly lower than that of the perfluorinated ionomer (Nafion) below 100 °C, but comparable at higher temperature and 100% RH. The highest conductivity of 0.18 S cm-1 was obtained for 1 at 140 °C. Ionomer 1 with high IEC and branched chemical structure exhibited improved proton conducting behavior without sacrificing membrane stability. Microscopic analyses revealed that smaller (<5 nm) and well-dispersed hydrophilic domains contribute to better proton conducting properties. Hydrogen and oxygen permeability of 1 was 1−2 orders of magnitude lower than that of Nafion under both dry and wet conditions. Fuel cell was fabricated with 1 membrane and operated at 80 °C and 0.2 A/cm2 supplying H2 and air both at 60% or 90% RH. Ionomer 1 membrane showed comparable performance to Nafion and was durable for 5000 h without distinct degradation.
ADDITIONS AND CORRECTIONS
Structural and Spectroscopic Demonstration of Agostic C−C Interactions in Electron-Deficient Metallacyclobutanes and Related Cage Complexes: Possible Implications for Olefin Polymerizations and Metatheses [J. Am. Chem. Soc. 2005, 127, 12426−12435].
Benjamin G. Harvey - ,
Charles L. Mayne - ,
Atta M. Arif - ,
Robert Tomaszewski - , and
Richard D. Ernst
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Tris(chloranilato)ferrate(III) Anionic Building Block Containing the (Dihydroxo)oxodiiron(III) Dimer Cation: Synthesis and Characterization of [(TPA)(OH)FeIIIOFeIII(OH)(TPA)][Fe(CA)3]0.5(BF4)0.5·1.5MeOH·H2O [TPA = tris(2-pyridylmethyl)amine; CA = chloranilate] [J. Am. Chem. Soc. 2006, 128, 40−41].
Kil Sik Min - ,
Arnold L. Rheingold - , and
Joel S. Miller
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BOOK REVIEWS
Enantiomer Separation: Fundamentals and Practical Methods Edited by Fumio Toda (Okayama University of Science, Japan). Kluwer Academic Publishers: Dordrecht, Boston, London. 2004. viii + 334 pp. $149.00. ISBN 1-4020-2336-7.
Thomas K. Green
Microwaves in Organic and Medicinal Chemistry, Volume 25 By C. Oliver Kappe and Alexander Stadler (Karl-Franzens-University Graz). From the Series, Methods and Principles in Medicinal Chemistry. Edited by R. Mannhold, H. Kubinyi, and G. Folkers. Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim. 2005. xii + 410 pp. $170.00. ISBN 3-527-31210-2.
James J. Kiddle
Dictionary of Microscopy By Julian P. Heath (Cambridge, UK). John Wiley & Sons, Ltd: Chichester, UK. 2005. 358 pp. $49.95. ISBN 0-470-01199-8.
Advances in Heterocyclic Chemistry, Volume 88 Edited by Alan R. Katritzky (University of Florida, Gainesville). Elsevier Academic Press: San Diego, CA. 2005. x + 324 pp. $180.00. ISBN 0-12-020788-5.
Eric Block
Poly(arylene ethynylene)s: From Synthesis to Application. Advances in Polymer Science, Volume 177 Edited by Christoph Weder (Case Western Reserve University). Springer: Berlin, Heidelberg, New York. xii + 272 pp. $226.00. ISBN 3-540-23366-0.
Kirk S. Schanze