Heterogeneous Electron Transfer of Cytochrome c on Coated Silver Electrodes. Electric Field Effects on Structure and Redox Potential

Abstract

Cytochrome c (Cyt-c) was electrostatically bound to self-assembled monolayers (SAM) of ω-carboxylalkanethiols that were covalently attached to Ag electrodes. Employing surface-enhanced resonance Raman (SERR) spectroscopy, the redox equilibria and the structural changes of the adsorbed Cyt-c were analyzed quantitatively for SAMs of different chain lengths ranging from 2-mercaptoacetic acid (C2-SAM) to 16-mercaptohexadecanoic acid (C16-SAM). In the presence of Cyt-c in the bulk solution, the SERR spectra of the adsorbed Cyt-c display the characteristic vibrational band pattern of the native protein conformation denoted as state B1. The enhancement of the SERR signals decreases with increasing chain length, but even at distances as large as 24 A (C16-SAM), SERR spectra of high quality could be obtained. Conversely, no SERR signals could be detected for SAMs including hydroxyl instead of carboxylate headgroups, implying that Cyt-c is adsorbed via electrostatic interactions. On the basis of potential-dependent...