Thermodynamics of electron transfer

Noviandri I, Brown KN, Fleming DS, Gulyas PT, Lay PA, Masters AF, Phillips L (1999) The decamethylferrocenium/decamethylferrocene redox couple a superior redox standard to the ferrocenium/ferrocene redox couple for studying solvent effects on the thermodynamics of electron transfer. J Phys Chem B 103 6713-6722... 

It is tempting to relate the thermodynamics of electron-transfer between metal atoms or ions and organic substrates directly to the relevant ionization potentials and electron affinities. These quantities certainly play a role in ET-thermo-dynamics but the dominant factor in inner sphere processes in which the product of electron transfer is an ion pair is the electrostatic interaction between the product ions. Model calculations on the reduction of ethylene by alkali metal atoms, for instance [69], showed that the energy difference between the M C2H4 ground state and the electron-transfer state can be... 

Electron transfer reactions have been characterized with much more rigor in inorganic chemistry than with organic molecules. Marcus has provided the principal description relating the kinetics and thermodynamics of electron transfer between metal complexes (1). The Marcus theory, a computationally simple approach with good predictive power, is an empirical treatment which uses thermodynamic parameters and spectroscopic measurements to calculate kinetic data. It assumes that bimolecular electron transfer reactions occur in three stages as shown in Scheme 1 (1) formation of the precursor complex, (2) electron transfer, and (3) solvation of the redox pair. 

Boichenko, V.A., Hou, J-M., and Mauzerall, D. (2001) Thermodynamics of electron transfer in oxigen photosynthetic reaction centers volume change, enthalpy and entropy of electron-transfer reactions in intact cells of the cyanobacterium Synechocystis PCC 6803, Biochemistry 40, 7126-7132. 

The thermodynamics of electron transfer can influence CT quenching only inasmuch as electron transfer contributes to the excited CT complex(exdplex). In valence bond terms, the value of lc2 describes the percentage of electron... 

Woodbury, N. W., and Allen, J. P., 1995, The pathway, kinetics and thermodynamics of electron transfer in wild type and mutant hacterial reaction centers of purple nonsulfur hacteria. In Anoxygenic Photosynthetic Bacteria, (R. E. Blankenship, M. T. Madigan, and C. E. Bauer, eds.) pp. 5279557, Kluwer Academic Publishers, Dordrecht, The Netherlands. 

T. W. Hambley, Inorg. Chem., 27, 2496 (1988). Steric Contributions to the Thermodynamics of Electron Transfer in Cobalt(III) Hexaamine Comple.xes. 

Lastly, electron transfer in D—[H]—A assemblies is not a perquisite of the excited states of metal complexes. Organic ensembles 38 and 39 (R = SiMe2 Bu), containing a dimethylaniline-anthracene redox pair, have been synthesized recently [124]. Preliminary time-resolved and steady-state fluorescence experiments indicate the occurrence of photoinduced electron transfer. In work related to Watson Crick base-paired systems, the excited state of the fluorescent pyrene derivative 40 is efficiently quenched (94-99 %) by 2 -deoxyguanosine (dG), 2 -deoxycytidine (dC), or 2 -deoxythymidine (dT) in aqueous solution [125]. A PCET mechanism is thought to be responsible for this process, as the thermodynamics of electron transfer are unfavorable unless coupled to a rapid proton-transfer step. The quenched lifetime of 40 in the presence of dC and dT in H2O is significantly extended by a factor of 1.5-2.0 in D2O this isotope effect is similar to that observed in the kinetics studies of 1 [70]. The invoked PCET reaction mechanism also accounts for the inability of dC and dT to quench the fluorescence of 40 in the aprotic organic solvent DMSO. 

The redox potentials of organic cofactors are directly responsible for controlling the equilibrium behavior of the corresponding cofactor-mediated electron transfer processes. The relative redox potentials of the cofactor and its redox partner are also intimately related to the rate of adiabatic electron transfer Ret through the classical Marcus equation [13, 14], and nonadiabatic electron transfer through the semi-classical Marcus equation [15, 16]. The direct dependence of both the kinetics and thermodynamics of electron transfer processes on the cofactor redox potential makes the control of these potentials a key determinant of the activity of redox proteins. 

Thermodynamics of electron transfer. It should become possible to specifically alter reduction potentials by suitable modification of proteins and/or redox groups. The shifts in E° arising from these modifications should also be accurately predictable from theoretical calculations. Reasonable agreement between observation and theory has been already reported for the effects of certain electrostatic modifications on E° (49, 53, 54, 59). The more difficult problem of ab initio calculations of E° values will take longer to accomplish, although encouraging results have been described for quinones (194). 

The energetics of solvation play an important role in the thermodynamics of electron transfer in solution. The thermodynamics of electron transfer can be envisioned as electron transfer from one Fermi level to another, with the Fermi level of solution determined by the Nernst equation, and the determined by both LUMO energy and solvation changes. 

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