Electron transfer in living matter

Electron transfer processes are abundant in living matter. The electrons jump, stepwise, between metal ions or molecules. With the help of the Marcus model we have understood why, for example, iron ions in aqueous solution exchange electrons slowly. The high reorganization energy explains this phenomenon. Consequently Nature has chosen to keep the metal ions inside the proteins where they are protected from the surrounding water to speed up electron transfer processes. The ligating atoms are often sulphur atoms.
    Electron transfer in proteins often occur over large distances. In the expression for the reaction rate a factor called the transmission co-efficient has to be included. This factor decreases with increasing distance between the metal ions. In the efficient biological processes the distance between the metal ions usually varies between 5 and 15 Å (1Å = 10^-10 metres).

Azurin, an electron transporting protein with part of its peptide chain drawn. The dark blue copper ion is bonded in a cavity between two imidazole groups (the five-membered rings) and two yellow sulphur atoms.