Review: Probing protein electron transfer mechanisms from the molecular to the cellular length scales.

The mechanisms of bridge-mediated electron transfer (ET) reactions vary from coherent deep tunneling to thermally activated hopping. This short review focuses on some developments in the study of protein ET mechanisms at the molecular and at the cellular levels. It explains experimental and theoretical work on the influence of electronic-energy and electronic-coupling fluctuations on ET rates and on the switch from the tunneling to the hopping regimes. It also describes recent work on extracellular ET, in particular on bacterial nanowires which support ET over micron length scales. Future directions in these research areas are discussed.