ABSTRACT
This chapter deals with single electron transfer processes. In most electron transfer reactions of interest, electrons flow between redox active cofactors in protein: blue copper centres, haems, iron–sulphur clusters, chlorophylls, etc. To describe the electron tunnelling, we need to consider the strength and the nature of the electronic interactions between donor and acceptor imbedded in the protein as well as the vibronic coupling. Electrons tunnel in biological systems because thermal energies are much smaller than the energies needed to promote an electron from the donor onto the bridge or from the bridge onto the acceptor. Two types of virtual (i.e. energetically forbidden) intermediates facilitate tunnelling. These are species with extra electron(s) on the protein or electron(s) removed from the bridge. Much of physical organic chemistry is based upon orbital symmetry arguments and much of electron transfer theory can be summarized with a few orbital interaction concepts.
