Electron tunneling in cytochrome c

An upper limit for the extinction coefficient of the charge transfer band in the ferrohexacyanide-ferricytochrome c redox couple has been determined from 800 to 1800 nm. We obtain ε≤0.003 M^-1 cm^-1 between 800-1400 nm, ε≤0.02 M^-1 cm^-1 between 1400-1800 nm. Prompt transfer of charge due to excitation in the heme absorption bands occurs with a quantum yield of approximately 10^-4. Simple vibronically assisted tunneling theories and appropriate choice of highly localized wave functions have been used to show that tunneling theory can give a reasonable fit to all aspects of charge transfer between cytochrome c and iron hexacyanide, including the small value of the charge transfer band extinction coefficient.