▪ Abstract Proton-coupled electron transfer (PCET) is an important mechanism for charge transfer in a wide variety of systems including biology- and materials-oriented venues. We review several are...

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Proton-Coupled Electron Transfer

: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

6.1.2. Aqueous V(III) Chemistry 877 6.1.3. Oxidation State of Vanadium in Tunicates 878 6.1.4. Uptake of Vanadate into Tunicates 879 6.1.5. Vanadium Binding Proteins: Vanabins 879 6.1.6. Model Complexes and Their Chemistry 880 6.1.7. Catechol-Based Model Chemistry 880 6.1.8. Vanadium Sulfate Complexes 881 6.2. Fan Worm Pseudopotamilla occelata 883 7. Vanadium Nitrogenase 883 7.1. Nitrogenases 883 7.2. Biochemistry of Nitrogenase 884 7.3. Clusters in Nitrogenase and Model Systems: Structure and Reactivity 885

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The chemistry and biochemistry of vanadium and the biological activities exerted by vanadium compounds.

Based on its generally accessible I/II redox couple and bioavailability, copper plays a wide variety of roles in nature that mostly involve electron transfer (ET), O2 binding, activation and reduction, NO2− and N2O reduction and substrate activation. Copper sites that perform ET are the mononuclear blue Cu site that has a highly covalent CuII-S(Cys) bond and the binuclear CuA site that has a Cu2S(Cys)2 core with a Cu-Cu bond that keeps the site delocalized (Cu(1.5)2) in its oxidized state. In contrast to inorganic Cu complexes, these metalloprotein sites transfer electrons rapidly often over long distances, as has been previously reviewed.1–4 Blue Cu and CuA sites will only be considered here in their relation to intramolecular ET in multi-center enzymes. The focus of this review is on the Cu enzymes (Figure 1). Many are involved in O2 activation and reduction, which has mostly been thought to involve at least two electrons to overcome spin forbiddenness and the low potential of the one electron reduction to superoxide (Figure 2).5,6 Since the Cu(III) redox state has not been observed in biology, this requires either more than one Cu center or one copper and an additional redox active organic cofactor. The latter is formed in a biogenesis reaction of a residue (Tyr) that is also Cu catalyzed in the first turnover of the protein. Recently, however, there have been a number of enzymes suggested to utilize one Cu to activate O2 by 1e− reduction to form a Cu(II)-O2•− intermediate (an innersphere redox process) and it is important to understand the active site requirements to drive this reaction. The oxidases that catalyze the 4e−reduction of O2 to H2O are unique in that they effectively perform this reaction in one step indicating that the free energy barrier for the second two-electron reduction of the peroxide product of the first two-electron step is very low. In nature this requires either a trinuclear Cu cluster (in the multicopper oxidases) or a Cu/Tyr/Heme Fe cluster (in the cytochrome oxidases). The former accomplishes this with almost no overpotential maximizing its ability to oxidize substrates and its utility in biofuel cells, while the latter class of enzymes uses the excess energy to pump protons for ATP synthesis. In bacterial denitrification, a mononuclear Cu center catalyzes the 1e- reduction of nitrite to NO while a unique µ4S2−Cu4 cluster catalyzes the reduction of N2O to N2 and H2O, a 2e− process yet requiring 4Cu’s. Finally there are now several classes of enzymes that utilize an oxidized Cu(II) center to activate a covalently bound substrate to react with O2.



Figure 1

Copper active sites in biology.





Figure 2

Latimer Diagram for Oxygen Reduction at pH = 7.0 Adapted from References 5 and 6.



This review presents in depth discussions of all these classes of Cu enzymes and the correlations within and among these classes. For each class we review our present understanding of the enzymology, kinetics, geometric structures, electronic structures and the reaction mechanisms these have elucidated. While the emphasis here is on the enzymology, model studies have significantly contributed to our understanding of O2 activation by a number of Cu enzymes and are included in appropriate subsections of this review. In general we will consider how the covalency of a Cu(II)–substrate bond can activate the substrate for its spin forbidden reaction with O2, how in binuclear Cu enzymes the exchange coupling between Cu’s overcomes the spin forbiddenness of O2 binding and controls electron transfer to O2 to direct catalysis either to perform two e− electrophilic aromatic substitution or 1e− H-atom abstraction, the type of oxygen intermediate that is required for H-atom abstraction from the strong C-H bond of methane (104 kcal/mol) and how the trinuclear Cu cluster and the Cu/Tyr/Heme Fe cluster achieve their very low barriers for the reductive cleavage of the O-O bond.

Much of the insight available into these mechanisms in Cu biochemistry has come from the application of a wide range of spectroscopies and the correlation of spectroscopic results to electronic structure calculations. Thus we start with a tutorial on the different spectroscopic methods utilized to study mononuclear and multinuclear Cu enzymes and their correlations to different levels of electronic structure calculations.

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Copper Active Sites in Biology

Prediction of molecular properties and molecular spectroscopy with density functional theory: From fundamental theory to exchange-coupling

Molybdenum enzymes catalyze a wide variety of reactions that involve the net transfer of an oxygen atom either to or from the substrate with the metal cycling between the oxidation states Mo and Mo. The periplasmic dimethyl sulfoxide reductases (DMSORs) of the photosynthetic bacteria Rhodobacter capsulatus and Rhodobacter sphaeroides function in a respiratory chain with DMSO as the terminal electron acceptor. The DMSORs catalyze the environmentally important reaction (1) 3] that involves the direct transfer of an oxygen atom from DMSO to Mo producing dimethyl sulfide (DMS).

In Vivo Oxo Transfer: Reactions of Native and W‐Substituted Dimethyl Sulfoxide Reductase Monitored by 1H NMR Spectroscopy

A radical pathway to synthesise Mo and W dithiolene complexes

The structure of the title compound, C24H20P+·Cl−·2H2O, comprises hydrogen-bonded chains of water mol­ecules and chloride anions separated by hydro­phobic regions of tetra­phenyl­phospho­nium cations. Within the hydrogen-bonded chains the chloride anions are bridged by pairs of water mol­ecules. Each water mol­ecule donates two hydrogen bonds, while each chloride accepts four.

Tetraphenylphosphonium chloride dihydrate

The goal of this Advanced Series in Physical Chemistry is to provide texts to support the teaching of graduate courses that are focused on modern topics and new developments in experimental and theoretical physical chemistry. The reviews presented are set in the context of the applications of synchrotron light sources over the past 50 years, to the present, so-called third-generation, sources that are versatile and powerful radiation sources from the below-red to the X-ray regions. Given the wide applicability of synchrotron light and the potential signi®cance of the new investigations that are possible, it is timely to examine the impact that synchrotron light has made and will continue to make on chemical research, in its widest sense. With this objective in mind, the editor of this two-part volume invited contributions from practitioners who are at the forefront of particular aspects of this ®eld of research. The authors and editor are congratulated on achieving their individual and collective aims. Thus, the individual chapters summarize most of the signi®cant developments in the last decade in chemical and related research that have been achieved using synchrotron light sources. The utilization of the radiation as a probe as well as an energy source is emphasized. This book is organized in order of increasing photon energy. Part I deals with the applications of low-energy photons and covers areas such as gas-phase photodissociation reactions and dynamics, soft X-ray ̄uorescence, IR and photoemission analysis of surfaces, spectroscopy of organic and polymeric materials, catalysts, electronic and magnetic materials, and spectromicroscopy. Part II encompasses applications using soft to hard X-rays, including spectroscopy of surface and thin ®lms, XAFS, diffraction and scattering, and several technological applications, namely the microprobe, photoetching and tribology. The material is presented in a clear, concise and authoritative manner and each chapter contains ®gures that complement the text, together with a comprehensive list of relevant references. This text is essential for library purchase and, for those engaged in this ®eld of research and/or teaching graduate courses that cover the chemical applications of synchrotron radiation, personal purchase is recommended.

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Chemical Applications of Synchrotron Radiation, Vol. 12, Advanced Series in Physical Chemistry. Part I: Dynamics and VUV Spectroscopy. Part II: X-ray Applications. Edited by Tsun-Kong Sham. Pp. 1304. Singapore: World Scientific, 2002. Price USD 198, GBP 132, ISBN 981-02-4480-0 (set).

C. David Garner

Papers

In Vivo Oxo Transfer: Reactions of Native and W‐Substituted Dimethyl Sulfoxide Reductase Monitored by 1H NMR Spectroscopy

A radical pathway to synthesise Mo and W dithiolene complexes

Tetraphenylphosphonium chloride dihydrate

Chemical Applications of Synchrotron Radiation, Vol. 12, Advanced Series in Physical Chemistry. Part I: Dynamics and VUV Spectroscopy. Part II: X-ray Applications. Edited by Tsun-Kong Sham. Pp. 1304. Singapore: World Scientific, 2002. Price USD 198, GBP 132, ISBN 981-02-4480-0 (set).