Influence of the molecular structure of carboxyl-terminated self-assembled monolayer on the electron transfer of cytochrome c adsorbed on an an electrode: In situ observation by surface-enhanced infrared absorption spectroscopy

Surface-enhanced infrared adsorption spectroscopy (SEIRAS) was employed for the in situ observation of structural changes that occur in a self-assembled monolayer (SAM) of 11-mercaptoundecanoic acid (MUA) bound on a gold surface. The observed SEIRA spectra reveal a deprotonation of the carboxyl head group of the MUA-SAM layer after adsorption. An analysis of the vibrational spectra suggests that the deprotonation process occurs when the adsorbed MUA molecules reach a critical mutual distance. MUA-SAMs promote direct electron transfer between the metal electrode and cytochrome c, the electron mediator between the integral membrane protein complexes of the respiratory chain. The results show that the coverage of cytochrome c increases with the coverage of deprotonated MUA on the surface. On the other hand, the electron transfer of cytochrome c is optimized only when a moderate amount of the carboxyl head group is deprotonated. The electron transfer of cytochrome c is suppressed with a further increase of the deprotonated MUA. The relationship between the surface structure of the MUA layer and the electron transfer of cytochrome c is discussed on the basis of the spectroscopic data.