Experimental test of the vibronically coupled tunneling description of biological electron transfer.

Evidence for the constructs central to vibronically coupled electron transfer has been obtained. Our experiments show the existence of a weak (f congruent to 10(-6)) charge-transfer absorption band in the near infrared for the bound donor-acceptor complex, cytochrome c-Fe(CN)6. Such a charge-transfer band had been predicted from the theory of such transfers. The experimental method, using a form of excitation modulation spectroscopy, measures only the optical absorption that induces charge transfer between the donor and the acceptor (and does not detect other absorptions) and allows the study of charge-transfer bands whose absorbances are small compared to the sample absorbance. The energy position and oscillator strength of the band agree with the general predictions of this vibronically coupled tunneling theory. We suggest that, in this system at room temperature, the electron transfer can be described by this tunneling theory. This model system result gives credence to the short electron transfer distances the theory has predicted for biological electron transfers.