Long-range electron-transfer reaction rates to cytochrome c across long- and short-chain alkanethiol self-assembled monolayers: Electroreflectance studies

The kinetics of electron transfer (ET) between cytochrome c and a gold (111) electrode through self-assembled monolayers of alkanethiols with terminal carboxylic acid groups, COOH(CH₂)_nSH, have been studied for n = 2-11 using an ac potential-modulated UV-VIS reflectance spectroscopic technique (electroreflectance spectroscopy, ER). For 9 ≤ n ≤ 11, the standard ET rate constant, k_app, depends exponentially on the chain lengths and the exponential decay factor is 1.09 per methylene group; for n < 9, however, k_app deviates from the exponential plot. The ET reaction through short-chain alkanethiol monolayers is controlled by the preceding chemical reaction. The rate-controlling step is very likely to be the reorganization of cytochrome c to the favourable conformation for the ET reaction. The ET reaction rate constant from cytochrome c in the favourable conformation to the electrode surface obeys Marcus theory for long-range ET. The ET reaction through long-chain alkanethiol monolayers is controlled by the ET rate through alkanethiols.