Electric Field Effects on the Interfacial Electron Transfer and Protein Dynamics of Cytochrome c

LY, H. K.; WISITRUANGSAKUL, N.; SEZER, M.; FENG, J.J.; KRANICH, A.; WEIDINGER, I. M.; ZEBGER, I.; MURGIDA, D.H.; HILDEBRANDT , P.

JOURNAL OF ELECTROANALYTICAL CHEMISTRY - (Print)

ELSEVIER SCIENCE SA

Año: 2011 vol. 660 p. 367 - 376

1572-6657

Time-resolved surface enhanced resonance Raman and surface enhanced infrared absorptionspectroscopy 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 reorientation, 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 15methylene groups, electron tunnelling (kET) is the rate-limiting step. Pronounced differences forkET and its overpotential dependence are observed for the three metal electrodes and can beattributed to the different electric field effects on the free-energy term controlling the tunnellingrate. With decreasing SAM thickness, electron tunnelling increases whereas protein dynamics isslowed down such that for SAMs including less than 10 methylene groups, protein re-orientationbecomes rate-limiting, as reflected by the viscosity dependence of krelax. Upon decreasing theSAM thickness from 5 to 1 methylene group, an additional H/D kinetic isotope effect is detectedindicating that at very high electric fields rearrangements of the interfacial or intra-protein hydrogen bond networks limit the rate of the overall redox process.