Dynamics and mechanism of the electron transfer process of cytochrome c probed by resonance Raman and surface enhanced resonance Raman spectroscopy

Abstract

Stationary and time-resolved surface enhanced resonance Raman (SERR) spectroscopic techniques were employed to probe the mechanism and dynamics of the redox process of cytochrome c (Cyt- c) adsorbed on silver electrodes. On the bare silver electrode, the electron transfer steps were found to be coupled with conformational transitions between the native state B1 and a structurally altered state B2 that exhibits a drastic downshift of the redox potential. State B2, that was characterised by comparison with resonance Raman spectra of various Cyt- c states in solution, differs from state B1 by the lack of the axial methionine-80 ligand, which in a fraction of these species is most likely replaced by a histidine. For Cyt- c adsorbed on the bare electrode, the rate constants of the conformational transitions are similar to the formal heterogeneous electron transfer rate constants which for all species are between 2 and 4 s −1. Much faster electron transfer rates were found for Cyt- c adsorbed on coated electrodes covered with self-assembled monolayers (SAM) of ω-carboxyl alkanethiols. For such electrodes, state B2 could only be detected for SAMs with chain lengths shorter than 19 Å, confirming the view that the formation of this state results from electrostatic interactions. The implications of these findings for the biological electron transfer process are discussed.