Metalloproteins, electron transfer theory

The ability to exist in more than one oxidation state allows transition-metal complexes to serve as the active site of enzymes whose function is to transfer electrons (39). A great deal of effort has been directed at understanding the mechanisms of electron transfer in metalloproteins, such as cytochromes and blue copper proteins (40). Of particular interest is the mechanism by which an electron can tunnel from a metal center that is imbedded in a protein matrix to a site on the outer surface of the protein (7). A discussion of current theories is given in this volume. 

An important application of Marcus-Hush theory is in bioinorganic electron-transfer systems. For example, cytochrome c is an electron-transfer metalloprotein (see Section 29.4) and contains haem-iron as either Fe(II) or Fe(III). Electron transfer from one Fe centre to another is long range, the electron tunnelling through the protein. 

Model systems have been devised to investigate electron transfer between cytochrome c and molecular complexes such as [Ru(NH3)6], and kinetic data are consistent with Marcus theory, indicating outer-sphere processes. For electron transfer in both metalloproteins and the model systems, the distance between the metal centres is significantly greater than for transfer between two simple metal complexes, e.g. up to 2500pm. The rate of electron transfer decreases exponentially with increasing distance, r, between the two metal centres (eq. 26.65, where p is a parameter which depends on the molecular environment). 

In this section I have tried to follow a pattern comparable to the one used in the relevant chapter of the Specialist Periodical Reports series though minor adjustments have been made where necessary. The major developments in theory are followed by an outline of the outer-sphere and inner-sphere electron transfer literature. Photoinduced electron transfer, which remains fashionable, and bioinorganic studies covering mainly the interactions of metalloproteins with small inorganic reagents complete the chapter. Coverage is as comprehensive as space will allow and I have continued the practice of tabulating all the relevant rate data at the end of the chapter. 

---