Electron transfer in proteins

Abstract

Thermal electron transfer in protein structures is discussed on the basis of a model first proposed by R. A. Marcus. The electronic matrix element H₁₂ is obtained in a many-electron theory for a number of typical structures of interest in proteins. Repetitive structures often permit resonance transfer over long distances provided the absolute value of the coupling matrix element between neighbouring atomic orbitals is larger than a critical value. The crucial energy separation Δ is obtained as a product of the transfer ‘pathway’ multiplied by coupling matrix elements at the gaps. It is shown that empty gaps are responsible for non-adiabatic behaviour and that saturated as well as aromatic parts of the protein permit electron transfer over larger distances than so far recognized. The Salemme model of the cytochrome c–cytochrome b₅ interaction may permit electron transfer if the 8.4 Å gap between the haem edges is filled by matter, for instance one of the propionate groups at the haem edge. Finally, the cytochrome c–cytochrome c peroxidase model of Poulos and Kraut is discussed and some alternative pathways suggested.