Electric-field effects on the interfacial electron transfer and protein dynamics of cytochrome c

Time-resolved surface enhanced resonance Raman and surface enhanced infrared absorption spectroscopy have been employed to study the interfacial redox process of cytochrome c (Cyt-c) immobilised on various metal electrodes coated with self-assembled monolayers (SAMs) of carboxyl-terminated mercaptanes. The experiments, carried out with Ag, Au and layered Au–SAM–Ag electrodes, afford apparent heterogeneous electron transfer constants (krelax) that reflect the interplay between electron tunnelling, redox-linked protein structural changes, protein re-orientation, and hydrogen bond re-arrangements in the protein and in the protein/SAM interface. It is shown that the individual processes are affected by the interfacial electric field strength that increases with decreasing thickness of the SAM and increasing difference between the actual potential and the potential of zero-charge. At thick SAMs of mercaptanes including 15 methylene groups, electron tunnelling (kET) is the rate-limiting step. Pronounced differences for kET and its overpotential-dependence are observed for the three metal electrodes and can be attributed to the different electric-field effects on the free-energy term controlling the tunnelling rate. With decreasing SAM thickness, electron tunnelling increases whereas protein dynamics is slowed down such that for SAMs including less than 10 methylene groups, protein re-orientation becomes rate-limiting, as reflected by the viscosity dependence of krelax. Upon decreasing the SAM thickness from 5 to 1 methylene group, an additional H/D kinetic isotope effect is detected indicating that at very high electric fields re-arrangements of the interfacial or intra-protein hydrogen bond networks limit the rate of the overall redox process.