THE DISTRIBUTION OF STANDARD RATE CONSTANTS FOR ELECTRON-TRANSFER BETWEEN THIOL-MODIFIED GOLD ELECTRODES AND ADSORBED CYTOCHROME-C

The magnitude of the total rate constant of an electrochemical reactio n at small overpotentials is shown to depend on the exponentials of th e distance between the electron donor and acceptor, the reorganization energy, and the formal potential of the reaction. When a population d istribution of one or more of these three parameters exists, the effec t on the measured standard rate constant can be illuminated by using s mall-amplitude a.c. electrochemical impedance spectroscopy. Assuming a Gaussian distribution of the logarithm of the standard rate constants , the results from the system of cytochrome c molecules adsorbed on th iol-modified gold electrodes yield a standard deviation of approximate ly 0.5 for log k degrees/k(avg)degrees. The peak broadening of reversi ble voltammograms is related to a comparable distribution of formal po tentials.