Surface-enhanced resonance Raman spectroscopic and electrochemical study of cytochrome c bound on electrodes through coordination with pyridinyl-terminated self-assembled monolayers

Cytochrome c (Cyt-c) is immobilized on Ag and Au electrodes coated with self-assembled monolayers (SAM), comprised of pyridine-terminated alkanethiols and a shorter chain diluent thiol. Surface-enhanced resonance Raman (SERR) spectroscopy of coated Ag electrodes reveals that the adsorbed Cyt-c forms a potential-dependent coordination equilibrium with a predominant five-coordinated high-spin (5cHS) state in the reduced form and six-coordinated low-spin (6cLS) state prevailing in the oxidized form. In the oxidized species, the native Met-80 ligand of the heme is replaced by a pyridinyl residue of the bifunctional thiols that according to earlier scanning tunneling microscopy form islands in the hydrophobic monolayer. The redox potentials derived from the SERR band intensities are estimated to be -0.24 and -0.18 V (vs AgCl) for the 6cLS and 5cHS states, respectively, and lie in the range of the midpoint potential determined for Cyt-c on coated Au electrodes by cyclic voltammetry (CV). Whereas in the latter case, a nearly ideal Nernstian behavior for a one-electron couple was observed, the SERR spectroscopic analysis yields about 0.4 for the number of transferred electrons for each spin state. This discrepancy is mainly attributed to a distribution of substates of the immobilized protein in both the 6cLS and 5cHS forms, as indicated by substantial band broadening in the SERR spectra. These substates may arise from different orientations and heme pocket structures and exhibit different redox properties. Whereas SERR spectroscopy probes all adsorbed Cyt-c species including those that are largely redox inactive, CV measurements reflect only the substates that are electrochemically active.