Effect of surface modifiers on the electrode reactions and conformation of cytochrome c3 adsorbed on a silver electrode

Surface-enhanced resonance Raman scattering and electroreflectance voltammetry were used to investigate the effect of electrode surface modification on the structure and redox properties of cytochrome c₃ immobilized on Ag surfaces. It is shown that the redox reactions of cytochrome c₃ are more reversible at an 11-mercaptoundecanoic acid modified Ag electrode as compared to a bare metal surface. The heme of cytochrome c₃ is in a mixed low and high spin state when adsorbed at the bare electrode, whereas only the low spin form is present on the 11-mercaptoundecanoic acid modified electrode, suggesting that the native conformation is maintained in the latter case. The reduction potential is close to that of the most positive macroscopic potential as determined by electroreflectance spectroscopy. In contrast, the reduction potential as determined by SERRS undergoes a large positive shift in the presence of 4,4′-bipyridine, the magnitude of which is dependent upon the concentration of 4,4′-bipyridine. These results indicate that the effect of the cytochrome c₃ interaction with the 4,4′-bipyridine-modified surface is significantly different as compared to its interaction with the 11-mercaptoundecaodoic acid modified surface. Moreover, the results emphasize that electrode modifiers can have dramatically different effects on the redox properties of different proteins. It is well known that 4,4′-bipyridine acts as a redox promoter in the case of cytochrome c, whereas no electrochemical or electroreflectance response was observed in the case of cytochrome c₃. © 1998 John Wiley & Sons, Inc. Biospectroscopy 4: 161–170, 1998