Conformational and Redox Equilibria and Dynamics of Cytochrome <i>c</i> Immobilized on Electrodes via Hydrophobic Interactions

Abstract

Surface enhanced resonance Raman (SERR) spectroscopy was employed to study structure and dynamics of cytochrome c (Cyt-c) immobilized on Ag electrodes that were coated with self-assembled monolayers (SAM) of n-alkanethiols HS−(CH2)n−CH3. Cyt-c is bound to these electrodes by hydrophobic interactions most likely via the peptide segment 81−85 that may partially penetrate into the monolayers. The immobilized proteins are partially converted to the conformational state B2 that exhibits a different heme pocket structure and coordination configuration than the native state B1. The B2/B1 conformational equilibrium, which can also be induced by binding to electrodes coated with ω-carboxyl alkanethiols (Murgida, D. H.; Hildebrandt, P. J. Phys. Chem. B 2001, 105, 1578), is potential-dependent with the B1 state prevailing at potentials < −0.3 V. On hydrophobic coatings, the equilibrium constant is largely independent of the thickness of the SAM except for HS−CH2CH3 coatings, which do not allow integration of the apolar peptide segment. Using time-resolved SERR spectroscopy, the dynamics of the conversion from the ferric B2 state to the ferrous B1 state were analyzed. The underlying conformational transitions are potential-dependent and involve a fast step (ca. 104−105 s-1) that may either be the formation of the ferric B2 state or the back-conversion to the ferrous B1 state. The conformational transitions are substantially faster than those observed for electrostatically bound Cyt-c where rearrangements of the hydrogen-bonding network in the protein are associated with high electric field induced activation barriers. For the hydrophobically bound Cyt-c, the fast conformational transitions proceed on the same time scale as the intermolecular electron transfer between Cyt-c and its natural partner cytochrome c oxidase and may play a role in the biological process.