Electron tunneling in biological molecules

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