Complex long-range electron transfer

Hoffman BM, Natan MJ, Nocek JM, Wallin SA (1991) Long-Range Electron Transfer Within Metal-Substituted Protein Complexes. 75 85-108 Hoffmann BM, see Ibers JA (1982) 50 1-55... 

Electron transfer processes induce variations in the occupancy and/or the nature of orbitals which are essentially localized at the redox centers. However, these centers are embedded in a complex dielectric medium whose geometry and polarization depend on the redox state of the system. In addition, a finite delocalization of the centers orbitals through the medium is essential to-promote long-range electron transfers. The electron transfer process must therefore be viewed as a transition between two states of the whole system. The expression of the probability per unit time of this transition may be calculated by the general formahsm of Quantum Mechanics. 

Long-Range Electron Transfer Within Metal-Substituted Protein Complexes... 

Metal-substituted hemoglobin hybrids, [MP, Fe " (H20)P] are particularly attractive for the study of long-range electron transfer within protein complexes. Both photoinitiated and thermally activated electron transfer can be studied by flash excitation of Zn- or Mg-substituted complexes. Direct spectroscopic observation of the charge-separated intermediate, [(MP), Fe " P], unambiguously demonstrates photoinitiated ET, and the time course of this ET process indicates the presence of thermal ET. Replacement of the coordinated H2O in the protein containing the ferric heme with anionic ligands (CN , F , Nj ) dramatically lowers the photoinitiated rate constant, k(, but has a relatively minor effect on the thermal rate, kg. 

Reviews of kinetics and mechanisms deal with long-range electron transfer and with the uptake and release of dioxygen species, especially uptake from organic peroxides. " " The photochemistry of iron porphyrin complexes has been documented." ... 

In suggesting an increased effort on the experimental study of reaction rates, it is to be hoped that the systems studied will be those whose properties are rather better defined than many have been. By far and away more information is known about the rate of reactions of the solvated electron in various solvents from hydrocarbons to water. Yet of all reactants, few can be so poorly understood. The radius and solvent structure are certainly not well known, and even its energetics are imprecisely known. The mobility and importance of long-range electron transfer are not always well characterised, either. Iodine atom recombination is probably the next most frequently studied reaction. Not only are the excited states and electronic relaxation processes of iodine molecules complex [266, 293], but also the vibrational relaxation rate of vibrationally excited recombined iodine molecules may be at least as slow as the recombination rate [57], Again, the iodine atom recombination process is hardly ideal. 

Long range electron-transfer has also been demonstrated within the complex between zinc-substituted cytochrome c peroxidase and cyt c 59). The kinetics of intramolecular electron-transfer from Ru(II) to Fe(III) in ruthenium modified cyt c has also been investigated 58). 

Examples of photoredox processes belonging to each of the above groups are given in Table 4. The first three types of photoredox processes (a, b, c) are mostly intracomplex monomolecular reactions (photoredox additions are bimolecular processes) which usually do not occur when a complex is in its ground state. Photoinduced long-range electron transfer reactions can be understood as a boundary between outer-sphere processes (due to a great distance between the reaction sites) and inner-sphere ones (they are, in fact, realized in one molecule). 

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