Rudolph Marcus

Electron Transfer in Chemistry and Biology


Abstract

The field of electron transfers has expanded greatly since its modern inception in the middle 1940s. We first review some of the key developments and then comment on recent progress [1-3]. During the late 1940s and 1950s simple electron transfers, selfexchange reactions, were studied, such as reaction (1) in aqueous solution

Mn*O4- + MnO42- --> Mn*O42- + MnO4-(1)

where the asterisk denotes a radioactive tracer. They form the simplest class of reactions in all of chemistry. Their study led to theoretical concepts such as the applicability of the Franck-Condon principle to these reactions, and the resulting realization that solvent and vibrational reorganization prior to electron transfer are normally needed to satisfy both this principle and energy conservation during the electron transfer act.
During the period of the late 1940s and 1950s fast electronic techniques were introduced into electrochemistry and permitted the study of simple electron transfers at metal electrodes. Further introduction of fast techniques for homogeneous reactions in the millisecond regime permitted the study of "cross-reactions", such as reaction (2) in aqueous solution.

Fe(CN)64- + MnO4- --> Fe(CN)63- + MnO42-(2)

i.e., reactions between two different redox systems. Their study provided a test of one of the predictions of the theory of the rate of cross-reactions in terms of the rates of the two relevant self-exchange reactions and the equilibrium constant for the crossreaction. Another prediction which was tested was the relation between the rate of a self-exchange reaction in solution and the corresponding electrochemical reaction at a metal electrode.
With the introduction of still faster techniques in the picosecond and later the femtosecond regimes, the scope of available electron transfer reactions which could be studied was expanded further, and stimulated the investigation of solvent dynamics effects on electron transfer reaction rates: When a reaction is intrinsically very fast, the rate-controlling step can be the "sluggishness" of the solvent (or other environment) itself, which, in the case of electron transfers, is the slowness of the dielectric relaxation of the environment. A skillful combination of organic synthesis and fast methods also permitted in the mid-1980s the confirmation of a long-standing prediction of the electron transfer theory, "the inverted effect," in which very "downhill" reactions (very negative standard free energy of reaction) were slower the more downhill the reaction was. The confirmation of this last remaining and unusual prediction of the electron transfer theory, the inverted effect, some twenty-five years after the prediction itself, had other' consequences. It has been applied to solar energy conversion and to the early steps in the reaction center in photosynthesis. Synthesis of various series of related compounds play a major role in systematic studies of the distance dependence of the electron transfer rate. This work has been continued in modified proteins and in materials such as DNA. An extension of electron transfer theory was made to other kinds of transfer reactions, such as methyl transfers, proton transfers, and hydride transfers, and tested experimentally.


Recent reviews:

1.R. A. Marcus, Electron Transfer Reactions in Chemistry. Theory and Experiment. In Protein Electron Transfer, D. S. Bendall, eds., Bios Scientific, Oxford, 1996, Chapter 10

2.R. A. Marcus, Interaction of Theory and Experiment in Reaction Kinetics, In Comprehensive Chemical Kinetics, G. Hancock and R. Compton, eds. (Elsevier, Amsterdam) vol. 37, 1 (1999)

3.Articles in Electron Transfer - From Isolated Molecules to Biomolecules, Parts I and II, J. Jortner and M. Bixon, eds. Adv. Chem. Phys. 106 and 107 (1999)


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