Electron tunneling in biological molecules

Electron transfers in photosynthesis and respiration commonly occur between protein-bound prosthetic groups that are separated by large molecular distances (often greater than 10 Angstrom). Although the electron donors and accepters are expected to be weakly coupled, the reactions are remarkably fast and proceed with high specificity. Tunneling timetables based on analyses of Fe2+/Cu+ to Ru3+ electron-transfer rates for Ru-modified heme and copper proteins reveal that the structure of the intervening polypeptide can control these distant donor-acceptor couplings. Multistep tunneling can account for the relatively rapid Cu+ to Re2+ electron transfer observed in Re-modified azurin.