
About the Cover:
In a high-viscosity solvent a DNA G-quadruplex requires over 6 months to reach its thermodynamically favored structure, compared to <2 min in aqueous solvent. This tremendous alteration of folding kinetics, which is understood using Kramers rate theory, inspired a tribute to the 1960s American television series “The Twilight Zone”. See Hud and co-workers, p 15324. View the article.
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Communications

Copper(I)-Catalyzed Formal Carboboration of Alkynes: Synthesis of Tri- and Tetrasubstituted Vinylboronates
Ricardo Alfaro - ,
Alejandro Parra - ,
José Alemán - ,
José Luis García Ruano - , and
Mariola Tortosa *
The first copper-catalyzed formal carboboration of alkynes, in which a C–B bond and a C–C bond are created in a single catalytic cycle, is presented. The reaction proceeds with high regioselectivity and syn-stereoselectivity to form tri- and tetrasubstituted vinylboronic esters from commercially available bis(pinacolato)diboron. A subsequent cross-coupling reaction gives access to highly substituted alkenes.

Pushing the Ir-Catalyzed C–H Polyborylation of Aromatic Compounds to Maximum Capacity by Exploiting Reversibility
Maria N. Eliseeva - and
Lawrence T. Scott *
Small amounts of base (e.g., 10% potassium t-butoxide or sodium methoxide) have been found to promote equilibration of the kinetically favored products from Ir-catalyzed C–H polyborylations of aromatic compounds. In the presence of excess borylating agent, bis(pinacolato)diborane (B2pin2), repetitive deborylation/reborylations reposition the Bpin substituents until a pattern that accommodates the maximum number of Bpin substituents is achieved. A high-yield, one-step synthesis of 1,3,5,7,9-pentakis(Bpin)corannulene is reported that illustrates this useful extension of the Ir-catalyzed borylation reaction.

Solvent-Free Synthesis of Zeolites from Solid Raw Materials
Limin Ren - ,
Qinming Wu - ,
Chengguang Yang - ,
Longfeng Zhu - ,
Caijin Li - ,
Pengling Zhang - ,
Haiyan Zhang - ,
Xiangju Meng *- , and
Feng-Shou Xiao *
As important industrial materials, microporous zeolites are necessarily synthesized in the presence of solvents such as in hydrothermal, solvothermal, and ionothermal routes. We demonstrate here a simple and generalized solvent-free route for synthesizing various types of zeolites by mixing, grinding, and heating solid raw materials. Compared with conventional hydrothermal route, the avoidance of solvents in the synthesis not only significantly reduces the waste production, but also greatly increases the yield of zeolite products. In addition, the use of starting solid raw materials remarkably enhances the synthesis efficiency and reduces the use of raw materials, energy, and costs.

Rational Design of Allosteric Inhibitors and Activators Using the Population-Shift Model: In Vitro Validation and Application to an Artificial Biosensor
Francesco Ricci - ,
Alexis Vallée-Bélisle - ,
Alessandro Porchetta - , and
Kevin W. Plaxco *
The population-shift mechanism can be used for rational re-engineering of structure-switching biosensors to enable their allosteric inhibition and activation. As a proof-of-principle example of this, we have introduced distal allosteric sites into molecular beacons, which are optical sensors for the detection of specific nucleic acid sequences. The binding of inhibitors and activators to these sites enabled the rational modulation of the sensor’s target affinity—and thus its useful dynamic range—over 3 orders of magnitude. The convenience with which this was done suggests that the population-shift mechanism may prove to be a useful method by which allosteric regulation can be introduced into biosensors, “smart” biomaterials, and other artificial biotechnologies.

Disappearance of Superconductivity in the Solid Solution between (Ca4Al2O6)(Fe2As2) and (Ca4Al2O6)(Fe2P2) Superconductors
Parasharam M. Shirage *- ,
Kunihiro Kihou - ,
Chul-Ho Lee - ,
Nao Takeshita - ,
Hiroshi Eisaki - , and
Akira Iyo *
The effect of alloying the two perovskite-type iron-based superconductors (Ca4Al2O6)(Fe2As2) and (Ca4Al2O6)(Fe2P2) was examined. While the two stoichiometric compounds possess relatively high Tc’s of 28 and 17 K, respectively, their solid solutions of the form (Ca4Al2O6)(Fe2(As1–xPx)2) do not show superconductivity over a wide range from x = 0.50 to 0.95. The resultant phase diagram is thus completely different from those of other typical iron-based superconductors such as BaFe2(As,P)2 and LaFe(As,P)O, in which superconductivity shows up when P is substituted for As in the non-superconducting “parent” compounds. Notably, the solid solutions in the non-superconducting range exhibit resistivity anomalies at temperatures of 50–100 K. The behavior is reminiscent of the resistivity kink commonly observed in various non-superconducting parent compounds that signals the onset of antiferromagnetic/orthorhombic long-range order. The similarity suggests that the suppression of the superconductivity in the present case also has a magnetic and/or structural origin.

A Dynamic FRET Reporter of Gene Expression Improved by Functional Screening
Martina Schifferer - and
Oliver Griesbeck *
Here, we describe a reporter system that consists of a FRET biosensor and its corresponding aptamer. The FRET biosensor employs the synthetic aptamer binding peptide Rsg1.2 sandwiched between mutants of the Green Fluorescent Protein and undergoes FRET when binding its corresponding Rev Responsive Element (RRE) RNA aptamer. We developed a novel approach to engineer FRET biosensors by linker extension and screening to improve signal strength of the biosensor which we called VAmPIRe (Viral Aptamer binding Peptide based Indicator for RNA detection). We demonstrate that the system is quantitative, reversible and works with high specificity in vitro and in vivo in living bacteria and mammalian cells. Thus, VAmPIRe may become valuable for RNA localizations and as a dynamic RNA-based reporter for live cell imaging. Moreover, functional screening of large libraries as demonstrated here may become applicable to optimize some of the many FRET biosensors of cellular signaling.

Highly Efficient Blue Electrophosphorescent Polymers with Fluorinated Poly(arylene ether phosphine oxide) as Backbone
Shiyang Shao - ,
Junqiao Ding *- ,
Lixiang Wang *- ,
Xiabin Jing - , and
Fosong Wang
In view of the tolerance of F atoms in FIrpic to the nucleophilic aromatic substitution polymerization, an activated fluorinated poly(arylene ether phosphine oxide) backbone is used to construct novel blue electrophosphorescent polymers containing FIrpic as the blue emitter, because they can be synthesized under a milder temperature of 120 °C. Compared with the counterparts prepared at high temperature (165 °C), unexpected bathochromic shift is successfully avoided, and a state-of-art luminous efficiency as high as 19.4 cd A–1 is achieved. The efficiency is comparable to the corresponding physical blend system, which indicates that the fluorinated poly(arylene ether phosphine oxide) has the potential to be used as the platform for the development of high-performance all-phosphorescent white polymer based on single polymer system.

Nanoscale Arrangement of Proteins by Single-Molecule Cut-and-Paste
Mathias Strackharn - ,
Diana A. Pippig *- ,
Philipp Meyer - ,
Stefan W. Stahl - , and
Hermann E. Gaub
Protein-based nanostructures are key to the organization of life and it is their precise arrangement, which determines their specific functions. A single-molecule approach for the directed assembly of protein arrangements allows for a controlled composition of systems based on protein components. Applying antibodies and antigenic peptide tags we utilized the Single-Molecule Cut-and-Paste (SMC&P) technique for the handling of single proteins. Protein–DNA complexes could be arranged to complex patterns with the functionality of the protein part remaining unimpaired.

Bioelectrochemical Switches for the Quantitative Detection of Antibodies Directly in Whole Blood
Alexis Vallée-Bélisle - ,
Francesco Ricci - ,
Takanori Uzawa - ,
Fan Xia - , and
Kevin W. Plaxco *
The development of rapid, low-cost point-of-care approaches for the quantitative detection of antibodies would drastically impact global health by shortening the delay between sample collection and diagnosis and by improving the penetration of modern diagnostics into the developing world. Unfortunately, however, current methods for the quantitative detection of antibodies, including ELISAs, Western blots, and fluorescence polarization assays, are complex, multiple-step processes that rely on well-trained technicians working in well-equipped laboratories. In response, we describe here a versatile, DNA-based electrochemical “switch” for the rapid, single-step measurement of specific antibodies directly in undiluted whole blood at clinically relevant low-nanomolar concentrations.

Tuning the Gate Opening Pressure of Metal–Organic Frameworks (MOFs) for the Selective Separation of Hydrocarbons
Nour Nijem - ,
Haohan Wu - ,
Pieremanuele Canepa - ,
Anne Marti - ,
Kenneth J. Balkus Jr.,- ,
Timo Thonhauser - ,
Jing Li *- , and
Yves J. Chabal *
Separation of hydrocarbons is one of the most energy demanding processes. The need to develop materials for the selective adsorption of hydrocarbons, under reasonable conditions, is therefore of paramount importance. This work unveils unexpected hydrocarbon selectivity in a flexible Metal–Organic Framework (MOF), based on differences in their gate opening pressure. We show selectivity dependence on both chain length and specific framework–gas interaction. By combining Raman spectroscopy and theoretical van der Waals Density Functional (vdW-DF) calculations, the separation mechanisms governing this unexpected gate-opening behavior are revealed.

Interrogating Near-Infrared Electrogenerated Chemiluminescence of Au25(SC2H4Ph)18+ Clusters
Kalen N. Swanick - ,
Mahdi Hesari - ,
Mark S. Workentin - , and
Zhifeng Ding *
The electrochemistry, near-infrared photoluminescence (NIR-PL) spectroscopy, and electrogenerated chemiluminescence (ECL) of Au25(SC2H4Ph)18+C6F5CO2– (Au25+) clusters were investigated. For the first time, NIR-ECL emission was observed in both annihilation and coreactant paths. Our newly developed spooling spectroscopy was employed during the ECL evolution and devolution processes along with explicit NIR-PL spectroscopy to elucidate light generation mechanisms. It was discovered that the electronic relaxation of the Au25– excited state to the ground state plays a key role in giving off ECL at 893 nm, while intermediate, strong, and weak NIR-PL emissions at 719/820, 857, and 1080 nm can be attributed to the excited states higher than the HOMO-LUMO gap, across the HOMO-LUMO gap, and of semi-rings, respectively.

Control of Protein-Binding Kinetics on Synthetic Polymer Nanoparticles by Tuning Flexibility and Inducing Conformation Changes of Polymer Chains
Yu Hoshino *- ,
Masahiko Nakamoto - , and
Yoshiko Miura *
Although a number of procedures to create synthetic polymer nanoparticles (NPs) with an intrinsic affinity to target biomacromolecules have been published, little has been reported on strategies to control the binding kinetics of target recognition. Here, we report an enzyme-mimic strategy to control binding/dissociation rate constants of NPs, which bind proteins through multipoint interactions, by taking advantage of the temperature-responsive coil–globule phase transition of poly-N-isopropylacrylamide (PNIPAm)-based NPs. PNIPAm NPs with a “flexible” random-coil conformation had a faster binding rate than NPs with a “rigid” globule conformation; however, the dissociation rate constant remained unchanged, resulting in stronger affinity. The dissociation rate of the “flexible” NPs was decelerated by the “induced-fit”-type conformation change of polymers around the coil–globule phase transition temperature, resulting in the formation of the most stable NP–protein complexes. These results provide a guide for designing plastic antibodies with tailor-made binding kinetics and equilibrium constants.

Monothiol Glutaredoxins Function in Storing and Transporting [Fe2S2] Clusters Assembled on IscU Scaffold Proteins
Priyanka Shakamuri - ,
Bo Zhang - , and
Michael K. Johnson *
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In the bacterial ISC system for iron–sulfur cluster assembly, IscU acts as a primary scaffold protein, and the molecular co-chaperones HscA and HscB specifically interact with IscU to facilitate ATP-driven cluster transfer. In this work, cluster transfer from Azotobacter vinelandii [Fe2S2]2+ cluster-bound IscU to apo-Grx5, a general purpose monothiol glutaredoxin in A. vinelandii, was monitored by circular dichroism spectroscopy, in the absence and in the presence of HscA/HscB/Mg-ATP. The results indicate a 700-fold enhancement in the rate of [Fe2S2]2+ cluster transfer in the presence of the co-chaperones and Mg-ATP, yielding a second-order rate constant of 20 000 M–1 min–1 at 23 °C. Thus, HscA and HscB are required for efficient ATP-dependent [Fe2S2]2+ cluster transfer from IscU to Grx5. The results support a role for monothiol Grx’s in storing and transporting [Fe2S2]2+ clusters assembled on IscU and illustrate the limitations of interpreting in vitro cluster transfer studies involving [Fe2S2]-IscU in the absence of the dedicated HscA/HscB co-chaperone system.

Self-Propelled Carbohydrate-Sensitive Microtransporters with Built-In Boronic Acid Recognition for Isolating Sugars and Cells
Filiz Kuralay - ,
Sirilak Sattayasamitsathit - ,
Wei Gao - ,
Aysegul Uygun - ,
Adlai Katzenberg - , and
Joseph Wang *
A new nanomotor-based target isolation strategy, based on a “built-in” recognition capability, is presented. The concept relies on a poly(3-aminophenylboronic acid) (PAPBA)/Ni/Pt microtube engine coupling the selective monosaccharide recognition of the boronic acid-based outer polymeric layer with the catalytic function of the inner platinum layer. The PAPBA-based microrocket is prepared by membrane-templated electropolymerization of 3-aminophenylboronic acid monomer. The resulting boronic acid-based microengine itself provides the target recognition without the need for additional external functionalization. “On-the-fly” binding and transport of yeast cells (containing sugar residues on their wall) and glucose are illustrated. The use of the recognition polymeric layer does not hinder the efficient propulsion of the microengine in aqueous and physiological media. Release of the captured yeast cells is triggered via a competitive sugar binding involving addition of fructose. No such capture and transport are observed in control experiments involving other cells or microengines. Selective isolation of monosaccharides is illustrated using polystyrene particles loaded with different sugars. Such self-propelled nanomachines with a built-in recognition capability hold considerable promise for diverse applications.

Visible Light Switching of a BF2-Coordinated Azo Compound
Yin Yang - ,
Russell P. Hughes - , and
Ivan Aprahamian *
Here we report the synthesis and characterization of a BF2–azo complex that can be induced to isomerize without the need of deleterious UV light. The complexation of the azo group with BF2, coupled with the extended conjugation of the N═N π-electrons, increases the energy of the n−π* transitions and introduces new π-nonbonding (πnb) to π* transitions that dominate the visible region. The well separated πnb–π* transitions of the trans and cis isomers enable the efficient switching of the system by using only visible light. The complexation also leads to a slow cis → trans thermal relaxation rate (t1/2 = 12.5 h). Theoretical calculations indicate that the absorption bands in the visible range can be tuned using different Lewis acids, opening the way to a conceptually new strategy for the manipulation of azo compounds using only visible light.

Quantitation of Affinity, Avidity, and Binding Kinetics of Protein Analytes with a Dynamically Switchable Biosurface
Jelena Knezevic - ,
Andreas Langer - ,
Paul A. Hampel - ,
Wolfgang Kaiser - ,
Ralf Strasser - , and
Ulrich Rant *
A label-free method for the analysis of interactions of proteins with surface-tethered ligands is introduced. Short DNA levers are electrically actuated on microelectrodes by ac potentials, and their switching dynamics are measured in real-time by fluorescence energy transfer. Binding of proteins to ligands attached to the top of the DNA levers is detected by time-resolved measurements of the levers’ dynamic motion. We demonstrate the quantitation of binding kinetics (kon, koff rate constants), dissociation constants (KD in the pM regime), and the influence of competitive binders (EC50 values). Moreover, the “switchSENSE” method reveals avidity effects and allows discriminating between analytes with one or more binding sites. In a comparative study, interactions of six hexa-histidine-tagged proteins with tris-nitrilotriacetic acid (NTA3) ligands are quantitated. Their binding kinetics and affinities are found to vary over up to 2 orders of magnitude, evidencing that the proteins’ individual chemical environments significantly influence the His6–NTA3 interaction.

Design of a “Turn-Off/Turn-On” Biosensor: Understanding Carbohydrate-Lectin Interactions for Use in Noncovalent Drug Delivery
Bala Kishan Gorityala - ,
Zhiqiang Lu - ,
Min Li Leow - ,
Jimei Ma - , and
Xue-Wei Liu *

Enantioselective Synthesis of α-Oxy Amides via Umpolung Amide Synthesis
Matthew W. Leighty - ,
Bo Shen - , and
Jeffrey N. Johnston *
α-Oxy amides are prepared through enantioselective synthesis using a sequence beginning with a Henry addition of bromonitromethane to aldehydes and finishing with Umpolung Amide Synthesis (UmAS). Key to high enantioselection is the finding that ortho-iodo benzoic acid salts of the chiral copper(II) bis(oxazoline) catalyst deliver both diastereomers of the Henry adduct with high enantiomeric excess, homochiral at the oxygen-bearing carbon. Overall, this approach to α-oxy amides provides an innovative complement to alternatives that focus almost entirely on the enantioselective synthesis of α-oxy carboxylic acids.

Synthesis of Alkaloid (−)-205B via Stereoselective Reductive Cross-Coupling and Intramolecular [3+2] Cycloaddition
Dexi Yang - and
Glenn C. Micalizio *
An asymmetric synthesis of alkaloid (−)-205B, a tricyclic member of the architecturally diverse family of natural products isolated from the skin of neotropical poison frogs, is described that proceeds through two recently developed stereoselective synthetic methods: (1) Ti-mediated allylic alcohol–imine reductive cross-coupling and (2) intramolecular [3+2] cycloaddition of a glyoxylate-based homoallylic nitrone. The utility of this latter cycloaddition process for the assembly of the stereochemically dense piperidine core of 205B is noteworthy, as this method enables direct [3+2] cycloaddition of an intermediate homoallylic (E)-nitrone via a pathway that is stereochemically unscathed by competitive [3,3]-sigmatropic rearrangement processes. Overall, the synthesis is asymmetric, concise, and highly stereoselective—features which point to the potential future utility of these chemical methods in natural product synthesis and medicinal chemistry.

Asymmetric Hydrogenation of Thiophenes and Benzothiophenes
Slawomir Urban - ,
Bernhard Beiring - ,
Nuria Ortega - ,
Daniel Paul - , and
Frank Glorius *
An efficient and highly asymmetric ruthenium-N-heterocyclic carbene-catalyzed hydrogenation of substituted thiophenes and benzothiophenes is described, providing a new strategy for the formation of valuable enantiomerically pure tetrahydrothiophenes and 2,3-dihydrobenzothiophenes.

Analysis of DNA-Guided Self-Assembly of Microspheres Using Imaging Flow Cytometry
Hao Tang - ,
Ryan Deschner - ,
Peter Allen - ,
Younjin Cho - ,
Patrick Sermas - ,
Alejandro Maurer - ,
Andrew D. Ellington - , and
C. Grant Willson *
Imaging flow cytometry was used to analyze the self-assembly of DNA-conjugated polystyrene microspheres. This technique enables quantitative analysis of the assembly process and thereby enables detailed analysis of the effect of structural and process variables on the assembly yield. In a demonstration of the potential of this technique, the influence of DNA strand base pair (bp) length was examined, and it was found that 50 bp was sufficient to drive the assembly of microspheres efficiently, forming not only dimers but also chainlike structures. The effect of stoichiometry on the yield was also examined. The analysis demonstrated that self-assembly of 50 bp microspheres can be driven nearly to completion by stoichiometric excess in a manner similar to Le Chatelier’s principle in common chemical equilibrium.

Iridium-Catalyzed Regioselective and Enantioselective Allylation of Trimethylsiloxyfuran
Wenyong Chen - and
John F. Hartwig *
We report the regio- and enantioselective allylation of an ester enolate, trimethylsiloxyfuran. This enolate reacts at the 3-position with linear aromatic allylic carbonates or aliphatic allylic benzoates to form the branched substitution products in the presence of a metallacyclic iridium catalyst. This process provides access to synthetically important 3-substituted butenolides in enantioenriched form. Stoichiometric reactions of the allyliridium intermediate suggest that the trimethylsiloxyfuran is activated by the carboxylate leaving group.

Computational Study of Anomalous Reduction Potentials for Hydrogen Evolution Catalyzed by Cobalt Dithiolene Complexes
Brian H. Solis - and
Sharon Hammes-Schiffer *
The design of efficient hydrogen-evolving catalysts based on earth-abundant materials is important for developing alternative renewable energy sources. A series of four hydrogen-evolving cobalt dithiolene complexes in acetonitrile–water solvent is studied with computational methods. Co(mnt)2 (mnt = maleonitrile-2,3-dithiolate) has been shown experimentally to be the least active electrocatalyst (i.e., to produce H2 at the most negative potential) in this series, even though it has the most strongly electron-withdrawing substituents and the least negative CoIII/II reduction potential. The calculations provide an explanation for this anomalous behavior in terms of protonation of the sulfur atoms on the dithiolene ligands after the initial CoIII/II reduction. One fewer sulfur atom is protonated in the CoII(mnt)2 complex than in the other three complexes in the series. As a result, the subsequent CoII/I reduction step occurs at the most negative potential for Co(mnt)2. According to the proposed mechanism, the resulting CoI complex undergoes intramolecular proton transfer to form a catalytically active CoIII-hydride that can further react to produce H2. Understanding the impact of ligand protonation on electrocatalytic activity is important for designing more effective electrocatalysts for solar devices.

Copper-Mediated Trifluoromethylation of α-Diazo Esters with TMSCF3: The Important Role of Water as a Promoter
Mingyou Hu - ,
Chuanfa Ni - , and
Jinbo Hu *
Copper-mediated trifluoromethylation of α-diazo esters with TMSCF3 reagent has been developed as a new method to prepare α-trifluoromethyl esters. This trifluoromethylation reaction represents the first example of fluoroalkylation of a non-fluorinated carbene precursor. Water plays an important role in promoting the reaction by activating the “CuCF3” species prepared from CuI/TMSCF3/CsF (1.0:1.1:1.1). The scope of this trifluoromethylation reaction is broad, and its efficiency is demonstrated in the synthesis of a variety of aryl-, benzyl-, and alkyl-substituted 3,3,3-trifluoropropanoates.

Characterization of a Paramagnetic, Mononuclear Pt(III)–Alkyl Complex Intermediate in Carbon–Halogen Bond Coupling Reactions
Orestes Rivada-Wheelaghan - ,
Manuel A. Ortuño - ,
Josefina Díez - ,
Sergio E. García-Garrido - ,
Celia Maya - ,
Agustí Lledós *- , and
Salvador Conejero *
Addition of Br2 or I2 to 14-electron, cationic Pt(II)-alkyl complexes led to the formation of the corresponding carbon–halogen Pt(II) coupling products. Low temperature experiments with Br2 allowed us to isolate and characterize crystallographically a very unusual mononuclear, paramagnetic Pt(III)-alkyl intermediate with a seesaw structure that can be further oxidized to a transient Pt(IV) species before reductive carbon–halogen coupling reaction takes place.

Molecular Engineering To Control the Magnetic Interaction between Single-Chain Magnets Assembled in a Two-Dimensional Network
Luminita M. Toma - ,
Catalina Ruiz-Pérez - ,
Jorge Pasán - ,
Wolfgang Wernsdorfer - ,
Francesc Lloret - , and
Miguel Julve *
Two two-dimensional (2D) systems having the formula [{FeIII(dmbpy)(CN)4}2CoIIL]n [L = pyetNO (1), tvpNO (2)] and consisting of single-chain magnets connected through organic ligands (L) have been prepared, and their magnetic properties have been investigated. The overall magnetic behavior depends on the capacity of the organic pillars to transmit long-range magnetic interactions. 1 is the first example of a 2D compound exhibiting double relaxation of the magnetization, whereas 2 behaves as a metamagnet.

Chromium(IV)–Peroxo Complex Formation and Its Nitric Oxide Dioxygenase Reactivity
Atsutoshi Yokoyama - ,
Jung Eun Han - ,
Jaeheung Cho - ,
Minoru Kubo - ,
Takashi Ogura - ,
Maxime A. Siegler - ,
Kenneth D. Karlin *- , and
Wonwoo Nam *
The O2 and NO reactivity of a Cr(II) complex bearing a 12-membered tetraazamacrocyclic N-tetramethylated cyclam (TMC) ligand, [CrII(12-TMC)(Cl)]+ (1), and the NO reactivity of its peroxo derivative, [CrIV(12-TMC)(O2)(Cl)]+ (2), are described. By contrast to the previously reported Cr(III)–superoxo complex, [CrIII(14-TMC)(O2)(Cl)]+, the Cr(IV)–peroxo complex 2 is formed in the reaction of 1 and O2. Full spectroscopic and X-ray analysis revealed that 2 possesses side-on η2-peroxo ligation. The quantitative reaction of 2 with NO affords a reduction in Cr oxidation state, producing a Cr(III)–nitrato complex, [CrIII(12-TMC)(NO3)(Cl)]+ (3). The latter is suggested to form via a Cr(III)–peroxynitrite intermediate. [CrII(12-TMC)(NO)(Cl)]+ (4), a Cr(II)–nitrosyl complex derived from 1 and NO, could also be synthesized; however, it does not react with O2.

Friction-Mediated Dynamic Disordering of Phospholipid Membrane by Mechanical Motions of Photoresponsive Molecular Glue: Activation of Ion Permeation
Yushi Suzuki - ,
Kou Okuro *- ,
Tadashi Takeuchi - , and
Takuzo Aida *
A water-soluble photoresponsive molecular glue, Azo-18Glue, consisting of a photochromic azobenzene core and two adhesive dendritic wedges with a total of 18 peripheral guanidinium ion (Gu+) pendants tightly adheres to the surface of a phospholipid membrane, even in buffer, via a multivalent salt-bridge formation with phosphate anions. A photomechanical motion of adhering Azo-18Glue possibly gives rise to dynamic structural disordering of the phospholipid membrane and activates transmembrane ion permeation. In sharp contrast, no activation of ion permeation results when poorly adhesive Azo-6Glue carrying only six Gu+ pendants is used in place of Azo-18Glue.

Photoswitchable Metal Coordinating Tweezers Operated by Light-Harvesting Dendrimers
Enrico Marchi - ,
Massimo Baroncini - ,
Giacomo Bergamini - ,
Jeroen Van Heyst - ,
Fritz Vögtle - , and
Paola Ceroni *
A dendrimer bearing two cyclam units linked by an azobenzene moiety, and luminescent naphthalene units at the periphery performs three different functions (light-harvesting, photoisomerization and coordination of metal ions) which can cooperate or interfere depending on the nature of the metal ion. It is thus an example of light controlled molecular tweezers in which Zn(II) coordination allows 100% efficient photosensitization of azobenzene switching, while Cu(II) shuts down azobenzene isomerization.

Reductive Elimination of Alkylamines from Low-Valent, Alkylpalladium(II) Amido Complexes
Patrick S. Hanley - ,
Seth L. Marquard - ,
Thomas R. Cundari - , and
John F. Hartwig *
A series of three-coordinate norbornylpalladium amido complexes ligated by bulky N-heterocyclic carbene (NHC) ligands were prepared that undergo reductive eliminations to form the alkyl–nitrogen bond of alkylamine products. The rates of reductive elimination reveal that complexes containing more-electron-donating amido groups react faster than those with less-electron-donating amido groups, and complexes containing more-sterically bulky amido groups undergo reductive elimination more slowly than complexes containing less-sterically bulky amido groups. Complexes ligated by more-electron-donating ancillary NHC ligands undergo reductive elimination faster than complexes ligated by less-electron-donating NHC ligands. In contrast to the reductive elimination of benzylamines from bisphosphine-ligated palladium amides, these reactions occur with retention of configuration at the alkyl group, indicating that these reductive eliminations proceed by a concerted pathway. The experimentally determined free energy barrier of 26 kcal/mol is close to the computed free energy barrier of 23.9 kcal/mol (363 K) for a concerted reductive elimination from the isolated, three-coordinate NHC-ligated palladium anilido complex.

Isolation of Lomaiviticins C–E, Transformation of Lomaiviticin C to Lomaiviticin A, Complete Structure Elucidation of Lomaiviticin A, and Structure–Activity Analyses
Christina M. Woo - ,
Nina E. Beizer - ,
Jeffrey E. Janso - , and
Seth B. Herzon *
We describe the isolation of (–)-lomaiviticins C–E (6–8), elucidation of the complete absolute and relative stereochemistry of (–)-lomaiviticin A (1), the synthetic conversion of (–)-lomaiviticin C (6) to (–)-lomaiviticin A (1), and the first evidence that the dimeric diazofluorene of (–)-lomaiviticin A (1) plays a defining and critical role in antiproliferative activity.

Reversible Control over the Valency of a Nanoparticle-Based Supramolecular System
Grégory Pieters - ,
Cristian Pezzato - , and
Leonard J. Prins *
The reversible “catch-and-release” of small molecules from the surface of monolayer-protected gold nanoparticles is described. The valency of the system (i.e., the number of molecules bound to the surface) can be controlled through the addition and removal of metal ions from the monolayer. Both the change in valency and the release rate of the molecules are strongly pH-dependent. The release rate can be regulated by altering the ratio of metal ions in the monolayer.

A Cooperative Role for the Counteranion in the PCl5-Initiated Living, Cationic Chain Growth Polycondensation of the Phosphoranimine Cl3P═NSiMe3
Vivienne Blackstone - ,
Stefan Pfirrmann - ,
Holger Helten - ,
Anne Staubitz - ,
Alejandro Presa Soto - ,
George R. Whittell - , and
Ian Manners *
The counteranion associated with the cationic initiator [Cl3P═N═PCl3]+ ([4]+) generated during the PCl5-initiated living, cationic chain growth polycondensation of the N-silylphosphoranimine Cl3P═NSiMe3 (3) to give poly(dichlorophosphazene), [N═PCl2]n (2), has been found to have a dramatic effect on the polymerization. When the counteranion of [4]+ was changed from PCl6– or Cl– to the weakly coordinating anions [BAr*F4]− and [BArF4]− (Ar*F = 3,5-{CF3}2C6H3, ArF = C6F5) instead of the polymerization of 3 being complete in 4–6 h, no reaction was observed after 24 h. Remarkably, the polymerization of 3 may be initiated by Cl– anions even in the absence of an active cation such as [4]+. However, in the presence of [4]+, the reaction proceeded significantly faster and allowed for molecular weight control. These results reveal that the currently accepted mechanism for the PCl5-initiated living polymerization of 3 needs to be revised to reflect the key role of the counteranion present.

Palladium-Catalyzed Alkyne Insertion/Suzuki Reaction of Alkyl Iodides
Brendan M. Monks - and
Silas P. Cook *
A palladium-catalyzed alkyne insertion/Suzuki reaction with unactivated alkyl iodides is described. Under the reaction conditions, selective migratory insertion of alkynes avoids β-hydride elimination and provides a facile synthesis of stereodefined, tetrasubstituted olefins. The transformation offers broad substrate scope for both the alkyl iodide and boron nucleophile. Mechanistic studies have revealed inversion of the stereocenter for the carbon bearing the iodide.

Selective Gas and Vapor Sorption and Magnetic Sensing by an Isoreticular Mixed-Metal–Organic Framework
Jesús Ferrando-Soria - ,
Pablo Serra-Crespo - ,
Martijn de Lange - ,
Jorge Gascon *- ,
Freek Kapteijn - ,
Miguel Julve - ,
Joan Cano - ,
Francesc Lloret *- ,
Jorge Pasán - ,
Catalina Ruiz-Pérez - ,
Yves Journaux - , and
Emilio Pardo *
A novel isoreticular oxamato-based manganese(II)–copper(II) open metal–organic framework H2O@iso1 featuring a pillared square/octagonal layer structure with alternating open and closed octagonal pores has been rationally prepared. The open-framework topology is responsible for a large selectivity in the separation of small gas (CO2 over CH4) and vapor molecules (CH3OH over CH3CN and CH3CH2OH). H2O@iso1 displays a long-range three-dimensional ferromagnetic ordering with a drastic variation of the critical temperature as a function of the guest molecule [TC < 2.0 K (CO2@iso1 and CH4@iso1) and TC = 6.5 (CH3OH@iso1) and 21.0 K (H2O@iso1)].

Redox-Neutral α-Cyanation of Amines
Longle Ma - ,
Weijie Chen - , and
Daniel Seidel *

Stibonium Ions for the Fluorescence Turn-On Sensing of F– in Drinking Water at Parts per Million Concentrations
Iou-Sheng Ke - ,
Mykhaylo Myahkostupov - ,
Felix N. Castellano *- , and
François P. Gabbaï *
The 9-anthryltriphenylstibonium cation, [1]+, has been synthesized and used as a sensor for the toxic fluoride anion in water. This stibonium cation complexes fluoride ions to afford the corresponding fluorostiborane 1-F. This reaction, which occurs at fluoride concentrations in the parts per million range, is accompanied by a drastic fluorescence turn-on response. It is also highly selective and can be used in plain tap water or bottled water to test fluoridation levels.
Articles

Guided Self-Assembly of Metal Atoms on Silicon Using Organic-Molecule Templating
Daniel R. Belcher - ,
Marian W. Radny *- ,
Steven R. Schofield - ,
Phillip V. Smith - , and
Oliver Warschkow
Assembling molecular components into low-dimensional structures offers new opportunities for nanoscale device applications. Here we describe the self-assembly of indium atoms into metallic chains on the silicon (001) surface using adsorbed benzonitrile molecules as nucleation and termination sites. Critically, individual benzonitrile adsorbates can be manipulated using scanning tunneling microscopy. This affords control over the position and orientation of the molecular adsorbates, which in turn determine the origin, direction, and length of the self-assembled metallic chains.

Release of High-Energy Water as an Essential Driving Force for the High-Affinity Binding of Cucurbit[n]urils
Frank Biedermann - ,
Vanya D. Uzunova - ,
Oren A. Scherman - ,
Werner M. Nau - , and
Alfonso De Simone *
Molecular dynamics simulations and isothermal titration calorimetry (ITC) experiments with neutral guests illustrate that the release of high-energy water from the cavity of cucurbit[n]uril (CBn) macrocycles is a major determinant for guest binding in aqueous solutions. The energy of the individual encapsulated water molecules decreases with increasing cavity size, because larger cavities allow for the formation of more stable H-bonded networks. Conversely, the total energy of internal water increases with the cavity size because the absolute number of water molecules increases. For CB7, which has emerged as an ultrahigh affinity binder, these counteracting effects result in a maximum energy gain through a complete removal of water molecules from the cavity. A new design criterion for aqueous synthetic receptors has therefore emerged, which is the optimization of the size of cavities and binding pockets with respect to the energy and number of residing water molecules.

Human Telomere Sequence DNA in Water-Free and High-Viscosity Solvents: G-Quadruplex Folding Governed by Kramers Rate Theory
Ford M. Lannan - ,
Irena Mamajanov - , and
Nicholas V. Hud *
Structures formed by human telomere sequence (HTS) DNA are of interest due to the implication of telomeres in the aging process and cancer. We present studies of HTS DNA folding in an anhydrous, high viscosity deep eutectic solvent (DES) comprised of choline choride and urea. In this solvent, the HTS DNA forms a G-quadruplex with the parallel-stranded (“propeller”) fold, consistent with observations that reduced water activity favors the parallel fold, whereas alternative folds are favored at high water activity. Surprisingly, adoption of the parallel structure by HTS DNA in the DES, after thermal denaturation and quick cooling to room temperature, requires several months, as opposed to less than 2 min in an aqueous solution. This extended folding time in the DES is, in part, due to HTS DNA becoming kinetically trapped in a folded state that is apparently not accessed in lower viscosity solvents. A comparison of times required for the G-quadruplex to convert from its aqueous-preferred folded state to its parallel fold also reveals a dependence on solvent viscosity that is consistent with Kramers rate theory, which predicts that diffusion-controlled transitions will slow proportionally with solvent friction. These results provide an enhanced view of a G-quadruplex folding funnel and highlight the necessity to consider solvent viscosity in studies of G-quadruplex formation in vitro and in vivo. Additionally, the solvents and analyses presented here should prove valuable for understanding the folding of many other nucleic acids and potentially have applications in DNA-based nanotechnology where time-dependent structures are desired.

One-Point Binding Ligands for Asymmetric Gold Catalysis: Phosphoramidites with a TADDOL-Related but Acyclic Backbone
Henrik Teller - ,
Matthieu Corbet - ,
Luca Mantilli - ,
Gopinadhanpillai Gopakumar - ,
Richard Goddard - ,
Walter Thiel - , and
Alois Fürstner *
Readily available phosphoramidites incorporating TADDOL-related diols with an acyclic backbone turned out to be excellent ligands for asymmetric gold catalysis, allowing a number of mechanistically different transformations to be performed with good to outstanding enantioselectivities. This includes [2 + 2] and [4 + 2] cycloadditions of ene-allenes, cycloisomerizations of enynes, hydroarylation reactions with formation of indolines, as well as intramolecular hydroaminations and hydroalkoxylations of allenes. Their preparative relevance is underscored by an application to an efficient synthesis of the antidepressive drug candidate (−)-GSK 1360707. The distinctive design element of the new ligands is their acyclic dimethyl ether backbone in lieu of the (isopropylidene) acetal moiety characteristic for traditional TADDOL’s. Crystallographic data in combination with computational studies allow the efficiency of the gold complexes endowed with such one-point binding ligands to be rationalized.

Probing Dynamic Conformations of the High-Molecular-Weight αB-Crystallin Heat Shock Protein Ensemble by NMR Spectroscopy
Andrew J. Baldwin *- ,
Patrick Walsh - ,
D. Flemming Hansen - ,
Gillian R. Hilton - ,
Justin L. P. Benesch - ,
Simon Sharpe - , and
Lewis E. Kay *
Solution- and solid-state nuclear magnetic resonance (NMR) spectroscopy are highly complementary techniques for studying supra-molecular structure. Here they are employed for investigating the molecular chaperone αB-crystallin, a polydisperse ensemble of between 10 and 40 identical subunits with an average molecular mass of approximately 600 kDa. An IxI motif in the C-terminal region of each of the subunits is thought to play a critical role in regulating the size distribution of oligomers and in controlling the kinetics of subunit exchange between them. Previously published solid-state NMR and X-ray results are consistent with a bound IxI conformation, while solution NMR studies provide strong support for a highly dynamic state. Here we demonstrate through FROSTY (freezing rotational diffusion of protein solutions at low temperature and high viscosity) MAS (magic angle spinning) NMR that both populations are present at low temperatures (<0 °C), while at higher temperatures only the mobile state is observed. Solution NMR relaxation dispersion experiments performed under physiologically relevant conditions establish that the motif interchanges between flexible (highly populated) and bound (sparsely populated) states. This work emphasizes the importance of using multiple methods in studies of supra-molecules, especially for highly dynamic ensembles where sample conditions can potentially affect the conformational properties observed.

Synthesis and Optical Properties of Phenylene-Containing Oligoacenes
Rebecca R. Parkhurst - and
Timothy M. Swager *
Synthesis of a new class of fully unsaturated ladder structures, phenylene-containing oligoacenes (POAs), using 3,4-bis(methylene)cyclobutene as a building block for sequential Diels–Alder reactions is described. The geometric effects of strain and energetic cost of antiaromaticity can be observed via the optical and electrochemical properties of the reported compounds. The resulting shape-persistant ladder structures contain neighboring chromophores that are partially electronically isolated from one another while still undergoing a reduction in the band gap of the material.

Price To Be Paid for Two-Metal Catalysis: Magnesium Ions That Accelerate Chemistry Unavoidably Limit Product Release from a Protein Kinase
Douglas M. Jacobsen - ,
Zhao-Qin Bao - ,
Patrick O’Brien - ,
Charles L. Brooks III,- , and
Matthew A. Young *
This publication is Open Access under the license indicated. Learn More
Incorporation of divalent metal ions into an active site is a fundamental catalytic tool used by diverse enzymes. Divalent cations are used by protein kinases to both stabilize ATP binding and accelerate chemistry. Kinetic analysis establishes that Cyclin-dependent kinase 2 (CDK2) requires simultaneous binding of two Mg2+ ions for catalysis of phosphoryl transfer. This tool, however, comes with a price: the rate-acceleration effects are opposed by an unavoidable rate-limiting consequence of the use of two Mg2+ ions by CDK2. The essential metal ions stabilize ADP product binding and limit the overall rate of the reaction. We demonstrate that product release is rate limiting for activated CDK2 and evaluate the effects of the two catalytically essential Mg2+ ions on the stability of the ADP product within the active site. We present two new crystal structures of CDK2 bound to ADP showing how the phosphate groups can be coordinated by either one or two Mg2+ ions, with the occupancy of one site in a weaker equilibrium. Molecular dynamics simulations indicate that ADP phosphate mobility is more restricted when ADP is coordinated by two Mg2+ ions compared to one. The structural similarity between the rigid ADP·2Mg product and the cooperatively assembled transition state provides a mechanistic rational for the rate-limiting ADP release that is observed. We demonstrate that although the simultaneous binding of two Mg2+ ions is essential for efficient phosphoryl transfer, the presence of both Mg2+ ions in the active site also cooperatively increases ADP affinity and opposes its release. Evolution of protein kinases must have involved careful tuning of the affinity for the second Mg2+ ion in order to balance the needs to stabilize the chemical transition state and allow timely product release. The link between Mg2+ site affinity and activity presents a chemical handle that may be used by regulatory factors as well as explain some mutational effects.

Studies of the Di-iron(VI) Intermediate in Ferrate-Dependent Oxygen Evolution from Water
Rupam Sarma - ,
Alfredo M. Angeles-Boza - ,
David W. Brinkley - , and
Justine P. Roth *
Molecular oxygen is produced from water via the following reaction of potassium ferrate (K2FeO4) in acidic solution: 4[H3FeVIO4]+ + 8H3O+ → 4Fe3+ + 3O2 + 18H2O. This study focuses upon the mechanism by which the O–O bond is formed. Stopped-flow kinetics at variable acidities in H2O and D2O are used to complement the analysis of competitive oxygen-18 kinetic isotope effects (18O KIEs) upon consumption of natural abundance water. The derived 18O KIEs provide insights concerning the identity of the transition state. Water attack (WA) and oxo-coupling (OC) transition states were evaluated for various reactions of monomeric and dimeric ferrates using a calibrated density functional theory protocol. Vibrational frequencies from optimized isotopic structures are used here to predict 18O KIEs for comparison to experimental values determined using an established competitive isotope-fractionation method. The high level of agreement between experimental and theoretic isotope effects points to an intramolecular OC mechanism within a di-iron(VI) intermediate, consistent with the analysis of the reaction kinetics. Alternative mechanisms are excluded based on insurmountably high free energy barriers and disagreement with calculated 18O KIEs.

High-Capacity Micrometer-Sized Li2S Particles as Cathode Materials for Advanced Rechargeable Lithium-Ion Batteries
Yuan Yang - ,
Guangyuan Zheng - ,
Sumohan Misra - ,
Johanna Nelson - ,
Michael F. Toney - , and
Yi Cui *
Li2S is a high-capacity cathode material for lithium metal-free rechargeable batteries. It has a theoretical capacity of 1166 mAh/g, which is nearly 1 order of magnitude higher than traditional metal oxides/phosphates cathodes. However, Li2S is usually considered to be electrochemically inactive due to its high electronic resistivity and low lithium-ion diffusivity. In this paper, we discover that a large potential barrier (∼1 V) exists at the beginning of charging for Li2S. By applying a higher voltage cutoff, this barrier can be overcome and Li2S becomes active. Moreover, this barrier does not appear again in the following cycling. Subsequent cycling shows that the material behaves similar to common sulfur cathodes with high energy efficiency. The initial discharge capacity is greater than 800 mAh/g for even 10 μm Li2S particles. Moreover, after 10 cycles, the capacity is stabilized around 500–550 mAh/g with a capacity decay rate of only ∼0.25% per cycle. The origin of the initial barrier is found to be the phase nucleation of polysulfides, but the amplitude of barrier is mainly due to two factors: (a) charge transfer directly between Li2S and electrolyte without polysulfide and (b) lithium-ion diffusion in Li2S. These results demonstrate a simple and scalable approach to utilizing Li2S as the cathode material for rechargeable lithium-ion batteries with high specific energy.

Highly-Efficient Gating of Solid-State Nanochannels by DNA Supersandwich Structure Containing ATP Aptamers: A Nanofluidic IMPLICATION Logic Device
Yanan Jiang - ,
Nannan Liu - ,
Wei Guo *- ,
Fan Xia *- , and
Lei Jiang
Integrating biological components into artificial devices establishes an interface to understand and imitate the superior functionalities of the living systems. One challenge in developing biohybrid nanosystems mimicking the gating function of the biological ion channels is to enhance the gating efficiency of the man-made systems. Herein, we demonstrate a DNA supersandwich and ATP gated nanofluidic device that exhibits high ON–OFF ratios (up to 106) and a perfect electric seal at its closed state (∼GΩ). The ON–OFF ratio is distinctly higher than existing chemically modified nanofluidic gating systems. The gigaohm seal is comparable with that required in ion channel electrophysiological recording and some lipid bilayer-coated nanopore sensors. The gating function is implemented by self-assembling DNA supersandwich structures into solid-state nanochannels (open-to-closed) and their disassembly through ATP–DNA binding interactions (closed-to-open). On the basis of the reversible and all-or-none electrochemical switching properties, we further achieve the IMPLICATION logic operations within the nanofluidic structures. The present biohybrid nanofluidic device translates molecular events into electrical signals and indicates a built-in signal amplification mechanism for future nanofluidic biosensing and modular DNA computing on solid-state substrates.

Efficient Greenish Blue Electrochemiluminescence from Fluorene and Spirobifluorene Derivatives
Federico Polo *- ,
Fabio Rizzo *- ,
Manoel Veiga-Gutierrez - ,
Luisa De Cola - , and
Silvio Quici
The spectroscopic and electrochemical behavior as well as electrogenerated chemiluminescence (ECL) of a series of donor−π–donor derivatives bearing triphenylamine groups as donor connected to a fluorene, 2,7-bis-(4-(N,N-diphenylamino)phen-1-yl)-9,9′-dimethylfluorene (1), or spirobifluorene core, 2,7-bis-(4-(N,N-diphenylamino)phen-1-yl)-9,9′-spirobifluorene (2) and 2,2′,7,7′-tetrakis(4-(N,N-diphenylamino)phen-1-yl)-9,9′-spirobifluorene (3), were investigated. Besides a high photoluminescence (PL) quantum yield in solution (between 81 and 87%), an efficient radical ions annihilation process induces intense greenish blue ECL emission that could be seen with the naked eye. Only the tetrasubstituted spirobifluorene derivative (compound 3) shows weak ECL obtained by a direct annihilation mechanism. Because the energy of the annihilation reaction is higher than the energy required to form the singlet excited state, the S-route could be considered the pathway followed by the ECL process in these molecules. The ECL emissions recorded by direct ion–ion annihilation show two bands compared to the single structureless PL band. The ECL spectra obtained by a coreactant approach using benzoylperoxide as a coreagent show no differences relative to that produced by annihilation, except for an increasing of ECL intensity for all compounds.

Gene Carrier Showing All-or-None Response to Cancer Cell Signaling
Riki Toita - ,
Jeong-Hun Kang - ,
Tetsuro Tomiyama - ,
Chan Woo Kim - ,
Shujiro Shiosaki - ,
Takuro Niidome - ,
Takeshi Mori *- , and
Yoshiki Katayama *
In this work we designed a novel nano carrier, a linear polyethylenimine (LPEI)-peptide conjugate, for cancer-specific expression of transgenes. The conjugate was easily synthesized by using a click chemistry scheme orthogonal to the reactive side groups of the peptide, which is the substrate of protein kinase Cα (PKCα). Polyplexes of the conjugates with plasmid DNA (pDNA) were intact and stably dispersed even in the presence of cell lysate. Despite this stability, the polyplexes readily dissociated upon phosphorylation of the grafted peptides by PKCα. Because of its endosomal escape ability and adequate susceptibility to PKCα, the polyplexes showed an all-or-none type response to PKCα activity in transgene expression in vitro. The polyplexes achieved cancer tissue-specific transgene expression even for a tumor with a relatively low PKCα activity. Thus the LPEI–peptide conjugate has high potential as a nanocarrier for cancer-targeted gene therapy.

Catalytic Mechanism in Artificial Metalloenzyme: QM/MM Study of Phenylacetylene Polymerization by Rhodium Complex Encapsulated in apo-Ferritin
Zhuofeng Ke - ,
Satoshi Abe - ,
Takafumi Ueno - , and
Keiji Morokuma *
Artificial metalloenzyme, composed of metal complex(es) and a host protein, is a promising way to mimic enzyme catalytic functions or develop novel enzyme-like catalysis. However, it is highly challenging to unveil the active site and exact reaction mechanism inside artificial metalloenzyme, which is the bottleneck in its rational design. We present a QM/MM study of the complicated reaction mechanism for the recently developed artificial metalloenzyme system, (Rh(nbd)·apo-Fr) (nbd = norbornadiene), which is composed of a rhodium complex [Rh(nbd)Cl]2 and the recombinant horse L-chain apo-Ferritin. We found that binding sites suggested by the X-ray crystal structure, i.e., sites A, B, and C, are only precursors/intermediates, not true active sites for polymerization of phenylacetylene (PA). A new hydrophobic site, which we name D, is suggested to be the most plausible active site for polymerization. Active site D is generated after coordination of first monomer PA by extrusion of the RhI(PA) complex to a hydrophobic pocket near site B. Polymerization occurs in site D via a RhI-insertion mechanism. A specific “hydrophobic region” composed by the hydrophobic active site D, the nonpolar 4-fold channel, and other hydrophobic residues nearby is found to facilitate accumulation, coordination, and insertion of PA for polymerization. Our results also demonstrate that the hydrophobic active site D can retain the native regio- and stereoselectivity of the Rh-catalyzed polymerization of PA without protein. This study highlights the importance of theoretical study in mechanistic elucidation and rational design of artificial metalloenzymes, indicating that even with X-ray crystal structures at hand we may still be far from fully understanding the active site and catalytic mechanism of artificial metalloenzymes.

Experimental and Theoretical Investigations of Energy Transfer and Hydrogen-Bond Breaking in the Water Dimer
Lee C. Ch’ng - ,
Amit K. Samanta - ,
Gábor Czakó - ,
Joel M. Bowman *- , and
Hanna Reisler *
The hydrogen bonding in water is dominated by pairwise dimer interactions, and the predissociation of the water dimer following vibrational excitation is reported here. Velocity map imaging was used for an experimental determination of the dissociation energy (D0) of (D2O)2. The value obtained, 1244 ± 10 cm–1 (14.88 ± 0.12 kJ/mol), is in excellent agreement with the calculated value of 1244 ± 5 cm–1 (14.88 ± 0.06 kJ/mol). This agreement between theory and experiment is as good as the one obtained recently for (H2O)2. In addition, pair-correlated water fragment rovibrational state distributions following vibrational predissociation of (H2O)2 and (D2O)2 were obtained upon excitation of the hydrogen-bonded OH and OD stretch fundamentals, respectively. Quasi-classical trajectory calculations, using an accurate full-dimensional potential energy surface, are in accord with and help to elucidate experiment. Experiment and theory find predominant excitation of the fragment bending mode upon hydrogen bond breaking. A minor channel is also observed in which both fragments are in the ground vibrational state and are highly rotationally excited. The theoretical calculations reveal equal probability of bending excitation in the donor and acceptor subunits, which is a result of interchange of donor and acceptor roles. The rotational distributions associated with the major channel, in which one water fragment has one quantum of bend, and the minor channel with both water fragments in the ground vibrational state are calculated and are in agreement with experiment.

Indirect C–H Azidation of Heterocycles via Copper-Catalyzed Regioselective Fragmentation of Unsymmetrical λ3-Iodanes
Dmitrijs Lubriks - ,
Igors Sokolovs - , and
Edgars Suna *
A C–H bond of electron-rich heterocycles is transformed into a C–N bond in a reaction sequence comprising the formation of heteroaryl(phenyl)iodonium azides and their in situ regioselective fragmentation to heteroaryl azides. A Cu(I) catalyst ensures complete regiocontrol in the fragmentation step and catalyzes the subsequent 1,3-dipolar cycloaddition of the formed azido heterocycles with acetylenes. The heteroaryl azides can also be conveniently reduced to heteroarylamines by aqueous ammonium sulfide. The overall C–H to C–N transformation is a mild and operationally simple one-pot sequential multistep process.

P–N/P–P Bond Metathesis for the Synthesis of Complex Polyphosphanes
Kai-Oliver Feldmann - and
Jan J. Weigand *
A unique hexaphosphane featuring a 2,2′-bi(1,2,3-triphosphacyclopentane) moiety (19) and an ethylene-bridged bis-isotetraphosphane (27c,m) were both selectively prepared in efficient one-pot syntheses from easily accessible tris(3,5-dimethyl-1-pyrazolyl)phosphane (14) and 1,2-bis(phenylphosphanyl)ethane (18c,m). The formation of 27c,m is an example of a highly efficient P–P bond formation via protolysis. In contrast, the formation of 19 comprises P–N/P–P bond metathesis steps. This constitutes a novel synthetic approach toward the preparation of complex polyphosphanes. Detailed spectroscopic investigations form the basis for a mechanistic understanding of this unprecedented methodology. Furthermore, the preparation of a unique dinuclear iron–carbonyl complex which features hexaphosphane 19 as a bridging ligand illustrates the potential use of complex polyphosphanes such as 19 as ligands in transition metal chemistry.

Squaring the Circle in Peptide Assembly: From Fibers to Discrete Nanostructures by de Novo Design
Aimee L. Boyle - ,
Elizabeth H. C. Bromley *- ,
Gail J. Bartlett - ,
Richard B. Sessions - ,
Thomas H. Sharp - ,
Claire L. Williams - ,
Paul M. G. Curmi - ,
Nancy R. Forde - ,
Heiner Linke - , and
Derek N. Woolfson *
The design of bioinspired nanostructures and materials of defined size and shape is challenging as it pushes our understanding of biomolecular assembly to its limits. In such endeavors, DNA is the current building block of choice because of its predictable and programmable self-assembly. The use of peptide- and protein-based systems, however, has potential advantages due to their more-varied chemistries, structures and functions, and the prospects for recombinant production through gene synthesis and expression. Here, we present the design and characterization of two complementary peptides programmed to form a parallel heterodimeric coiled coil, which we use as the building blocks for larger, supramolecular assemblies. To achieve the latter, the two peptides are joined via peptidic linkers of variable lengths to produce a range of assemblies, from flexible fibers of indefinite length, through large colloidal-scale assemblies, down to closed and discrete nanoscale objects of defined stoichiometry. We posit that the different modes of assembly reflect the interplay between steric constraints imposed by short linkers and the bulk of the helices, and entropic factors that favor the formation of many smaller objects as the linker length is increased. This approach, and the resulting linear and proteinogenic polypeptides, represents a new route for constructing complex peptide-based assemblies and biomaterials.

A Complete Family of Terminal Uranium Chalcogenides, [U(E)(N{SiMe3}2)3]− (E = O, S, Se, Te)
Jessie L. Brown - ,
Skye Fortier - ,
Richard A. Lewis - ,
Guang Wu - , and
Trevor W. Hayton *
Addition of 1 equiv of E (E = 0.125 S8, Se, Te) to U(H2C═PPh3)(NR2)3 (R = SiMe3) (1) in Et2O results in generation of the terminal chalcogenide complexes, [Ph3PCH3][U(E)(NR2)3] (E = S, 2; Se, 3; Te, 4; R = SiMe3), in modest yield. Complexes 2–4 represent extremely rare examples of terminal uranium monochalcogenides. Synthesis of the oxo analogue, [Cp*2Co][U(O)(NR2)3] (5), was achieved by reduction of [U(O)(NR2)3] with Cp*2Co. All complexes were fully characterized, including analysis by X-ray crystallography. In the solid state, complexes 2–5 feature short U–E bond lengths, suggestive of actinide–ligand multiple bonding.

Direct Calculation of Li-Ion Transport in the Solid Electrolyte Interphase
Siqi Shi - ,
Peng Lu - ,
Zhongyi Liu - ,
Yue Qi *- ,
Louis G. Hector Jr.,- ,
Hong Li - , and
Stephen J. Harris
The mechanism of Li+ transport through the solid electrolyte interphase (SEI), a passivating film on electrode surfaces, has never been clearly elucidated despite its overwhelming importance to Li-ion battery operation and lifetime. The present paper develops a multiscale theoretical methodology to reveal the mechanism of Li+ transport in a SEI film. The methodology incorporates the boundary conditions of the first direct diffusion measurements on a model SEI consisting of porous (outer) organic and dense (inner) inorganic layers (similar to typical SEI films). New experimental evidence confirms that the inner layer in the ∼20 nm thick model SEI is primarily crystalline Li2CO3. Using density functional theory, we first determined that the dominant diffusion carrier in Li2CO3 below the voltage range of SEI formation is excess interstitial Li+. This diffuses via a knock-off mechanism to maintain higher O-coordination, rather than direct-hopping through empty spaces in the Li2CO3 lattice. Mesoscale diffusion equations were then formulated upon a new two-layer/two-mechanism model: pore diffusion in the outer layer and knock-off diffusion in the inner layer. This diffusion model predicted the unusual isotope ratio 6Li+/7Li+ profile measured by TOF-SIMS, which increases from the SEI/electrolyte surface and peaks at a depth of 5 nm, and then gradually decreases within the dense layer. With no fitting parameters, our approach is applicable to model general transport properties, such as ionic conductivity, for SEI films on the surface of other electrodes, from the atomic scale to the mesoscale, as well as aging phenomenon.

On the Origins of Faster Oxo Exchange for Uranyl(V) versus Plutonyl(V)
Daniel Rios - ,
Maria del Carmen Michelini *- ,
Ana F. Lucena - ,
Joaquim Marçalo - , and
John K. Gibson *
Activation of uranyl(V) oxo bonds in the gas phase is demonstrated by reaction of U16O2+ with H218O to produce U16O18O+ and U18O2+. In contrast, neptunyl(V) and plutonyl(V) are comparatively inert toward exchange. Computed potential energy profiles (PEPs) reveal a lower yl oxo exchange transition state for uranyl(V)/water as compared with neptunyl(V)/water and plutonyl(V)/water. A correspondence between oxo exchange rates in gas phase and acid solutions is apparent; the contrasting oxo exchange rates of UO2+ and PuO2+ are considered in the context of covalent bonding in actinyls. Hydroxo exchange of U16O2(16OH)+ with H218O to give U16O2(18OH)+ proceeded much faster than oxo exchange, in accord with a lower computed transition state for OH exchange. The PEP for the addition of H2O to UO2+ suggests that both UO2+·(H2O) and UO(OH)2+ should be considered as potential products.

Scope and Mechanistic Analysis of the Enantioselective Synthesis of Allenes by Rhodium-Catalyzed Tandem Ylide Formation/[2,3]-Sigmatropic Rearrangement between Donor/Acceptor Carbenoids and Propargylic Alcohols
Zhanjie Li - ,
Vyacheslav Boyarskikh - ,
Jørn H. Hansen - ,
Jochen Autschbach - ,
Djamaladdin G. Musaev - , and
Huw M. L. Davies *
Rhodium-catalyzed reactions of tertiary propargylic alcohols with methyl aryl- and styryldiazoacetates result in tandem reactions, consisting of oxonium ylide formation followed by [2,3]-sigmatropic rearrangement. This process competes favorably with the standard O–H insertion reaction of carbenoids. The resulting allenes are produced with high enantioselectivity (88–98% ee) when the reaction is catalyzed by the dirhodium tetraprolinate complex, Rh2(S-DOSP)4. Kinetic resolution is possible when racemic tertiary propargylic alcohols are used as substrates. Under the kinetic resolution conditions, the allenes are formed with good diastereoselectivity and enantioselectivity (up to 6.1:1 dr, 88–93% ee), and the unreacted alcohols are enantioenriched to 65–95% ee. Computational studies reveal that the high asymmetric induction is obtained via an organized transition state involving a two-point attachment: ylide formation between the alcohol oxygen and the carbenoid and hydrogen bonding of the alcohol to a carboxylate ligand. The 2,3-sigmatropic rearrangement proceeds through initial cleavage of the O–H bond to generate an intermediate with close-lying open-shell singlet, triplet, and closed-shell singlet electronic states. This intermediate would have significant diradical character, which is consistent with the observation that the 2,3-sigmatropic rearrangement is favored with donor/acceptor carbenoids and more highly functionalized propargylic alcohols.

Metal-Free Iodine(III)-Promoted Direct Intermolecular C–H Amination Reactions of Acetylenes
José A. Souto - ,
Peter Becker - ,
Álvaro Iglesias - , and
Kilian Muñiz *
A direct metal-free amination of arylalkynes has been developed, which proceeds by reaction of the terminal alkyne with the hypervalent iodine reagent PhI(OAc)NTs2 within a single-step operation. This unprecedented intermolecular C–H to C–N bond conversion provides rapid access to the important class of ynamides. In addition to the title reaction, the related transformation between alkylated alkynes and the iodine(III) reagent is also discussed.

Mechanisms by which Alkynes React with CpCr(CO)3H. Application to Radical Cyclization
Deven P. Estes - ,
Jack R. Norton *- ,
Steffen Jockusch - , and
Wesley Sattler
The reaction of CpCr(CO)3H with activated alkynes in benzene has been examined. The kinetics of these reactions have been studied with various alkynes, along with the stereochemistry with which the alkynes are hydrogenated. The hydrogenation of phenyl acetylene and diphenyl acetylene with CpCr(CO)3H has been shown to occur by a hydrogen atom transfer (HAT) mechanism. The reaction of CpCr(CO)3H with dimethyl acetylenedicarboxylate (DMAD) produced hydrogenated products as well as phenyl substitution from reaction with solvent. On the basis of kinetic data, it is thought that the reaction of DMAD may proceed via a single electron transfer (SET) as the rate-determining step. The radical anion of dimethylfumarate was observed by EPR spectroscopy during the course of the reaction, supporting this claim. The aromatic 1,6 eneyne (8) gave cyclized products in 78% yield under catalytic conditions (35 psi H2), presumably by the 5-exo-trig cyclization of the vinyl radical arising from H• transfer. Using a cobaloxime catalyst (12) hydrogenation was completely eliminated to yield 100% cyclized products.

Nonequilibrium Synthesis of Silica-Supported Magnetite Tubes and Mechanical Control of Their Magnetic Properties
Rabih Makki - and
Oliver Steinbock *
Materials synthesis far from thermodynamic equilibrium can yield hierarchical order that spans from molecular to macroscopic length scales. Here we report the nonequilibrium formation of millimeter-scale iron oxide–silica tubes in experiments that tightly control the tube radius and growth speed. The experiments involve the hydrodynamic injection of an iron (II,III) solution into a large volume of solution containing sodium silicate and ammonium hydroxide. The forming tubes are pinned to a motorized glass rod that moves at a predetermined speed. X-ray diffraction and electron microscopy, as well as Raman and Mössbauer spectroscopy, reveal magnetite nanoparticles in the range of 5–15 nm. Optical data suggest that the magnetite particles follow first-order nucleation–growth kinetics. The hollow tubes exhibit superparamagnetic behavior at room temperature, with a transition to a blocked state at TB = 95 K for an applied field of 200 Oe. Heat capacity measurements yield evidence for the Verwey transition at 20 K. Finally, we show a remarkable dependence of the tubes’ magnetic properties on the speed of the pinning rod and the injection rate employed during synthesis.

Efficient Long-Range Stereochemical Communication and Cooperative Effects in Self-Assembled Fe4L6 Cages
Naoki Ousaka - ,
Sergio Grunder - ,
Ana M. Castilla - ,
Adam C. Whalley - ,
J. Fraser Stoddart *- , and
Jonathan R. Nitschke *
A series of large, optically active Fe4L6 cages was prepared from linear 5,5′-bis(2-formylpyridines) incorporating varying numbers (n = 0–3) of oligo-p-xylene spacers, chiral amines, and FeII. When a cage was constructed from the ligand bridged by one p-xylene spacer (n = 1) and a bulky chiral amine, both a homochiral Fe2L3 helicate and Fe4L6 cage were observed to coexist in solution due to a delicate balance between steric factors. In contrast, when a less bulky chiral amine was used, only the Fe4L6 cage was observed. In the case of larger cages (n = 2, 3), long-range (>2 nm) stereochemical coupling between metal centers was observed, which was minimally diminished as the ligands were lengthened. This communication was mediated by the ligands’ geometries and rigidity, as opposed to gearing effects between xylene methyl groups: the metal-centered stereochemistry was not observed to affect the axial stereochemistry of the ligands.

Manganese Nitride Complexes in Oxidation States III, IV, and V: Synthesis and Electronic Structure
Henning Kropp - ,
Amanda E. King - ,
Marat M. Khusniyarov - ,
Frank W. Heinemann - ,
Kyle M. Lancaster - ,
Serena DeBeer - ,
Eckhard Bill - , and
Karsten Meyer *
The synthesis and characterization of a series of manganese nitrides in a tripodal chelating tris(carbene) ligand framework is described. Photolysis of [(TIMENxyl)Mn(N3)]+ (where TIMENxyl = tris[2-(3-xylylimidazol-2-ylidene)ethyl]amine) yields the isolable molecular MnIV nitride, [(TIMENxyl)Mn(N)]+. Spectroscopic and DFT studies indicate that this MnIV d3 complex has a doublet electronic ground state. The metal-centered one-electron oxidation of this MnIV species results in formation of the pentavalent MnV nitride, [(TIMENxyl)Mn(N)]2+. Unlike previously reported, tetragonal MnV nitrides with a d2, nonmagnetic S = 0 ground state, this trigonal bipyramidal complex has a triplet ground state S = 1. One-electron reduction of [(TIMENxyl)Mn(N)]+ produces the neutral, nonmagnetic trivalent [(TIMENxyl)Mn(N)] species with a d4 low-spin, S = 0, ground state.

Circularly Polarized Luminescence of Curium: A New Characterization of the 5f Actinide Complexes
Ga-Lai Law - ,
Christopher M. Andolina - ,
Jide Xu - ,
Vinh Luu - ,
Philip X. Rutkowski - ,
Gilles Muller - ,
David K. Shuh - ,
John K. Gibson - , and
Kenneth N. Raymond *
A key distinction between the lanthanide (4f) and the actinide (5f) transition elements is the increased role of f-orbital covalent bonding in the latter. Circularly polarized luminescence (CPL) is an uncommon but powerful spectroscopy which probes the electronic structure of chiral, luminescent complexes or molecules. While there are many examples of CPL spectra for the lanthanides, this report is the first for an actinide. Two chiral, octadentate chelating ligands based on orthoamide phenol (IAM) were used to complex curium(III). While the radioactivity kept the amount of material limited to micromole amounts, spectra of the highly luminescent complexes showed significant emission peak shifts between the different complexes, consistent with ligand field effects previously observed in luminescence spectra.

Exceptional Chemical Properties of Sc@C2v(9)–C82 Probed with Adamantylidene Carbene
Makoto Hachiya - ,
Hidefumi Nikawa - ,
Naomi Mizorogi - ,
Takahiro Tsuchiya - ,
Xing Lu *- , and
Takeshi Akasaka *
It has been an interesting finding that reactions of M@C2v(9)–C82 (M = Y, La, Ce, Gd) with diazirine adamantylidene (AdN2, 1) gave rise to only two monoadduct isomers, indicating that the cage reactivity of monometallofullerenes is not dependent on the type of the internal metal. However, we found here that Sc@C2v(9)–C82 shows an exceptional chemical reactivity toward the electrophile 1, affording four monoadduct isomers (2a–d). Single-crystal X-ray diffraction crystallographic results of the most abundant isomer (2a) confirm that the addition takes place at a [6,6]-bond junction which is very close to the internal metal ion. Theoretical calculations reveal that 2 out of the 24 nonequivalent cage carbons of Sc@C2v(9)–C82 are highly reactive toward 1, but only one cage carbon of the other M@C2v–C82 (M = Y, La, Ce, Gd) is sufficiently reactive. The exceptional chemical property of Sc@C2v(9)–C82 is associated with the small ionic radius of Sc3+, which allows stronger metal–cage interactions and makes back-donation of charge from the cage to the metal more pronounced. Our results have provided new insights into the art of altering the chemical properties of fullerene molecules at the atomic level, which would be useful in the future in utilizing EMFs in quantum computing systems.

Endolysins of Bacillus anthracis Bacteriophages Recognize Unique Carbohydrate Epitopes of Vegetative Cell Wall Polysaccharides with High Affinity and Selectivity
Kai-For Mo - ,
Xiuru Li - ,
Huiqing Li - ,
Lieh Yoon Low - ,
Conrad P. Quinn - , and
Geert-Jan Boons *
Bacteriophages express endolysins which are the enzymes that hydrolyze peptidoglycan resulting in cell lysis and release of bacteriophages. Endolysins have acquired stringent substrate specificities, which have been attributed to cell wall binding domains (CBD). Although it has been realized that CBDs of bacteriophages that infect Gram-positive bacteria target cell wall carbohydrate structures, molecular mechanisms that confer selectivity are not understood. A range of oligosaccharides, derived from the secondary cell wall polysaccharides of Bacillus anthracis, has been chemically synthesized. The compounds contain an α-d-GlcNAc-(1→4)-β-d-ManNAc-(1→4)-β-d-GlcNAc backbone that is modified by various patterns of α-d-Gal and β-d-Gal branching points. The library of compounds could readily be prepared by employing a core trisaccharide modified by the orthogonal protecting groups Nα-9-fluorenylmethyloxycarbonate (Fmoc), 2-methylnaphthyl ether (Nap), levulinoyl ester (Lev) and dimethylthexylsilyl ether (TDS) at key branching points. Dissociation constants for the binding the cell wall binding domains of the endolysins PlyL and PlyG were determined by surface plasmon resonance (SPR). It was found that the pattern of galactosylation greatly influenced binding affinities, and in particular a compound having a galactosyl moiety at C-4 of the nonreducing GlcNAc moiety bound in the low micromolar range. It is known that secondary cell wall polysaccharides of various bacilli may have both common and variable structural features and in particular differences in the pattern of galactosylation have been noted. Therefore, it is proposed that specificity of endolysins for specific bacilli is achieved by selective binding to a uniquely galactosylated core structure.

Substrate-Assisted Catalytic Mechanism of O-GlcNAc Transferase Discovered by Quantum Mechanics/Molecular Mechanics Investigation
Igor Tvaroška *- ,
Stanislav Kozmon - ,
Michaela Wimmerová - , and
Jaroslav Koča
In higher eukaryotes, a variety of proteins are post-translationally modified by adding O-linked N-acetylglucosamine (GlcNAc) residue to serine or threonine residues. Misregulation of O-GlcNAcylation is linked to a wide variety of diseases, such as diabetes, cancer, and neurodegenerative diseases, including Alzheimer’s disease. GlcNAc transfer is catalyzed by an inverting glycosyltransferase O-GlcNAc transferase (uridine diphospho-N-acetylglucosamine:polypeptide β-N-acetylaminyltransferase, OGT) that belongs to the GT-B superfamily. The catalytic mechanism of this metal-independent glycosyltransferase is of primary importance and is investigated here using QM(DFT)/MM methods. The structural model of the reaction site used in this paper is based on the crystal structures of OGT. The entire enzyme–substrate system was partitioned into two different subsystems: the QM subsystem containing 198 atoms, and the MM region containing 11 326 atoms. The catalytic mechanism was monitored by means of three two-dimensional potential energy maps calculated as a function of three predefined reaction coordinates at different levels of theory. These potential energy surfaces revealed the existence of a concerted SN2-like mechanism, in which a nucleophilic attack by OSer, facilitated by proton transfer to the catalytic base, and the dissociation of the leaving group occur almost simultaneously. The transition state for the proposed reaction mechanism at the MPW1K level was located at C1–OSer = 1.92 Å and C1–O1 = 3.11 Å. The activation energy for this passage was estimated to be ∼20 kcal mol–1. These calculations also identified, for the first time for glycosyltransferases, the substrate-assisted mechanism in which the N-acetamino group of the donor participates in the catalytic mechanism.

Development of an Intramolecular Aryne Ene Reaction and Application to the Formal Synthesis of (±)-Crinine
David A. Candito - ,
Dennis Dobrovolsky - , and
Mark Lautens *
A general and high yielding annulation strategy for the synthesis of various carbo- and heterocycles, based on an intramolecular aryne ene reaction is described. It was found that the geometry of the olefin is crucial to the success of the reaction, with exclusive migration of the trans-allylic-H taking place. Furthermore, the electronic nature of the aryne was found to be important to the success of the reaction. Deuterium labeling studies and DFT calculations provided insight into the reaction mechanism. The data suggests a concerted asynchronous transition state, resembling a nucleophilic attack on the aryne. This strategy was successfully applied to the formal synthesis of the ethanophenanthridine alkaloid (±)-crinine.

EPR Spectroscopic Studies of the Fe–S Clusters in the O2-Tolerant [NiFe]-Hydrogenase Hyd-1 from Escherichia coli and Characterization of the Unique [4Fe–3S] Cluster by HYSCORE
Maxie M. Roessler - ,
Rhiannon M. Evans - ,
Rosalind A. Davies - ,
Jeffrey Harmer *- , and
Fraser A. Armstrong *
The unusual [4Fe–3S] cluster proximal to the active site plays a crucial role in allowing a class of [NiFe]-hydrogenases to function in the presence of O2 through its unique ability to undergo two rapid, consecutive one-electron transfers. This property helps to neutralize reactive oxygen species. Mechanistic details and the role of the medial and distal clusters remain unresolved. To probe the Fe–S relay, continuous wave and pulse electron paramagnetic resonance (EPR) studies were conducted on the O2-tolerant hydrogenase from Escherichia coli (Hyd-1) and three variants with point mutations at the proximal and/or medial clusters. Reduction potentials of the proximal ([4Fe–3S]5+/4+/3+) and medial ([3Fe–4S]+/0) clusters were determined by potentiometry. The medial [3Fe–4S]+/0 reduction potential is exceptionally high, implicating a mechanistic role in O2-tolerance. Numerous experiments establish that the distal cluster has a ground state S > 1/2 in all three variants and indicate that this is also the case for native Hyd-1. Concurrent with the Hyd-1 crystal structure, EPR data for the ‘superoxidized’ P242C variant, in which the medial cluster is ‘magnetically silenced’, reveal two conformations of the proximal [4Fe–3S]5+ cluster, and X-band HYSCORE spectroscopy shows two 14N hyperfine couplings attributed to one conformer. The largest, A(14N) = [11.5,11.5,16.0] ± 1.5 MHz, characterizes the unusual bond between one Fe (Fe4) and the backbone amide-N of cysteine-20. The second, A(14N) = [2.8,4.6,3.5] ± 0.3 MHz, is assigned to NC19. The 14N hyperfine couplings are conclusive evidence that Fe4 is a valence-localized Fe3+ in the superoxidized state, whose formation permits an additional electron to be transferred rapidly back to the active site during O2 attack.

Revelation of a Catalytic Calcium-Binding Site Elucidates Unusual Metal Dependence of a Human Apyrase
David W. Rooklin - ,
Min Lu - , and
Yingkai Zhang *
Human soluble calcium-activated nucleotidase 1 (hSCAN-1) represents a new family of apyrase enzymes that catalyze the hydrolysis of nucleotide di- and triphosphates, thereby modulating extracellular purinergic and pyrimidinergic signaling. Among well-characterized phosphoryl transfer enzymes, hSCAN-1 is unique not only in its unusual calcium-dependent activation, but also in its novel phosphate-binding motif. Its catalytic site does not utilize backbone amide groups to bind phosphate, as in the common P-loop, but contains a large cluster of acidic ionizable side chains. By employing a state-of-the-art computational approach, we have revealed a previously uncharacterized catalytic calcium-binding site in hSCAN-1, which elucidates the unusual calcium-dependence of its apyrase activity. In a high-order coordination shell, the newly identified calcium ion organizes the active site residues to mediate nucleotide binding, to orient the nucleophilic water, and to facilitate the phosphoryl transfer reaction. From ab initio QM/MM molecular dynamics simulations with umbrella sampling, we have characterized a reverse protonation catalytic mechanism for hSCAN-1 and determined its free energy reaction profile. Our results are consistent with available experimental studies and provide new detailed insight into the structure–function relationship of this novel calcium-activated phosphoryl transfer enzyme.
Additions and Corrections
Correction to Stereoselective Alkene Isomerization Over One Position
Casey R. Larsen - and
Douglas B. Grotjahn
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