Internal Electron Transfer Reactions

Dithiete can be used as an oxidant for the thiometalates and oxythiometalates, and thus induced internal electron-transfer reactions provide an efficient method for synthesizing selected metal dithiolene and oxo-containing... 

Speculation about the precise roles of active-site sulfur is tempered by an appreciation of the redox versatihty and interplay of sulfur and molybdenum. This is evident from synthetic systems, where the catenation of sulfur (with attendant redox and/or ligand elaboration) and induced internal electron-transfer reactions are frequently observed. The redox interplay of Mo and S, reflected in undesirable synthetic outcomes, may prove crucial to a fifll description of enzyme behavior. see also Sulfur Inorganic Chemistry)... 

Modification of larger proteins with ruthenium complexes, while possible, have proven difficult. Fortunately, a ruthenium labeled partner such as Cc can be used to rapidly inject or remove an electron from the large protein complex, and thus study internal electron-transfer reactions. In a complementary approach, Nilsson found that the excited state of Ru(bpy)3 + can inject an electron into CcO. The ruthenium complex binds electrostatically to the protein in a location similar to that occupied by Cc. The initial site of electron transfer is the Cua site, as it is when Cc is the electron donor. The simplicity of this technique makes it very attractive since it eliminates any modification of the protein which might alter the structure. Sadoski etal. showed that significant improvements in the yield of electron transfer could be obtained with ruthenium complexes of higher charge. One specific complex used was the dinuclear complex [(bpy)2Ru(qpy)Ru(bpy)2] + (Figure 10). ... 

The subunits of CODH/ACS have been isolated (see earlier discussion). The isolated a subunit contains one Ni and four Fe and has spectroscopic properties (186) similar to those of Cluster A, the active site of acetyl-CoA synthesis (212). Unfortunately, it has no ACS activity. Therefore, ACS activity may reside in the a subunit or it may require both the a and the fi subunits. If Clusters B and/or C of the B subunit are involved in acetyl-CoA synthesis, one possible role could be in electron transfer. Although acetyl-CoA synthesis and the CO/ exchange reactions do not involve net electron transfer, both of these reactions are stimulated by ferredoxin, indicating that internal electron transfer within CODH/ACS may be required during the reaction (121). Further studies with the isolated subunits and the reconstitu-... 

The forward and reverse rate constants are thus equal at zero standard free energy. However, this will be difficult to check in practice, for both reactions are very slow, since a bond-breaking/bond-forming process endowed with a quite large internal reorganization is involved. The result is that dissociative electron transfer reactions are usually carried out with electron donors that have a standard potential largely negative to the dissociative standard potential. The reoxidation of the R, X- system is thus possible only with electron acceptors, D +, that are different from the D,+ produced in the reduction process (they are more powerful oxidants). There is no reason then that the oxidation mechanism be the reverse of the... 

M. Fabian and co-workers have studied the protein s role in internal electron transfer to the catalytic center of cytochrome c oxidase using stopped-flow kinetics. Mitochondrial cytochrome c oxidase, CcO, an enzyme that catalyzes the oxidation of ferrocytochrome c by dioxygen, is discussed more fully in Section 7.8. In the overall process, O2 is reduced to water, requiring the addition of four electrons and four protons to the enzyme s catalytic center. Electrons enter CcO from the cytosolic side, while protons enter from the matrix side of the inner mitochondrial membrane. This redox reaction. 

Finally, some diradicals can be made in situ by an internal hydrogen-transfer reaction from a suitable hydrogen donor to a carbene or nitrene. In benzene derivatives, this is a well-tested route to o-quinoid compounds, which are not biradicals, although a biradical valence structure probably makes a significant contribution to their electronic structure. However, if the donor and the carbene or nitrene are... 

The conclusions described in the previous section are inferred from a relatively small number of observations of spin-equilibrium dynamics. Nevertheless, they are internally self-consistent and also compatible with a much wider set of observations derived from studies of electron transfer reactions of metal complexes. For these reasons there is hope that they possess some generality and can be applied to other systems. 

J. C. Holland, Ph.D. Thesis, Purdue University, W. Lafayette, IN, Computer Controlled Internal Reflection Spectroelectrochemical Measurements of Fast Electron Transfer Reactions, University Microfilms, No. 7926386, Ann Arbor, MI, 1979. 

Although such reactions have been known for a long time, it appears a somewhat neglected area of study. Most attention has been on cysteine and its oxidation to the disulfide which is catalyzed by metal ions, in particular CuI[ (see Section 20.2.2.2.2) and FeI,[.81 The likely intermediates in these reactions are metal-cysteine complexes which undergo internal electron transfer. As noted earlier (Section 20.2.2.2.2), penicillamine differs from cysteine in its reactivity and gives rise to mixed valence species. More recently Mn11 has also been found to catalyze the oxidations of Cys and Pen. 

Shown in Figures 5-7 are the redox pathways for xanthine oxidase, sulfite oxidase, and nitrate reductase (assimilatory and respiratory), respectively. These schemes address the electron and proton (hydron) flows. The action of the molyb-doenzymes is conceptually similar to that of electrochemical cells in which half reactions occur at different electrodes. In the enzymes, the half reactions occur at different prosthetic groups and intraprotein (internal) electron transfer allows the reactions to be coupled (i.e., the circuit to be completed). In essence, this is the modus operandi of these enzymes, which must be determined before intimate mechanistic considerations are seriously addressed. 

The rate of electron transfer that occurs to/from the metal center is high. Structure based modeling of the active site of human MnSOD [40], which includes calculating the energies of both the oxidized and reduced states with either water or hydroxide bound to the metal, suggests the rate of this internal electron transfer is enhanced by electron-relaxation effects. In addition, a 0.17 V redox potential is calculated, a value that is low compared with the experimental values of 0.31 V fori . coli and 0.26 V for B. stearothermophilus, respectively. A potential of —0.30 V seems to be optimal as it lies midway between the redox potentials of the two half reactions of the dismutation process [41],... 

Complexes I 20) and II (68) are unstable with respect to internal electron transfer, and the rates of the thermal reactions have been measured complex III (2e) is stable with respect to electron transfer. Complex IV is symmetrical and presumably subject to rapid internal transfer (103). Optical charge transfer has not been detected in any of these systems. In the case of the two cobalt(III) complexes, comparison with the data of Table III suggests that the bands should be... 

Marcus, R. A., 1993, Electron-transfer reactions in chemistryotheory and experiment (Nobel lecture). Angewandte Chemie-International Edition in English, 32 llllnll21. 

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