Redox and conformational equilibria of cytochrome c₅₅₂ from Thermus thermophilus adsorbed on a chemically modified silver electrode probed by surface-enhanced resonance Raman spectroscopy

Surface-enhanced resonance Raman spectroscopy (SERRS) was employed to study the potential-dependent processes of the electron transferring heme protein cytochrome c₅₅₂ (Cyt-c₅₂₂) from Thermus thermophilus. Cyt-c₅₅₂ was adsorbed on Ag electrodes coated with functionalized self-assembled monolayers (SAMs) of alkanethiols, regarded as a model of its redox partner ba₃-oxidase. By a quantitative analysis of the SERR spectra recorded at different potentials, the redox potential of Cyt-c₅₅₂ and the number of transferred electrons were determined. On pure hydrophobic alkanethiols, the Cyt-c₅₅₂ heme structure is greatly modified in a non-electroactive way with the appearance of a 5cHS species. When Cyt-c₅₅₂ is adsorbed on mixed surfaces of hydroxyl- and methyl-terminated alkanethiols, the electron transfer is effective (n = 1) and the structure of the heme is not modified, as is assumed when Cyt-c ₅₅₂ interacts with its natural redox partner ba₃-oxidase. When the chain length of the mixed SAMs is increased, the defects on the surface are decreased and the electron transfer becomes less efficient. The presence of defects in the organization of the short chain of five carbons seems to be relevant for having a good surface model of the redox partner.