Redox processes of cytochrome c immobilized on solid supported polyelectrolyte multilayers.
The heme protein cytochrome c (Cyt-c), immobilized on polyelectrolyte multilayers on a silver electrode, was studied by stationary and time-resolved surface-enhanced resonance Raman (SERR) spectroscopy to probe the redox site structure and the mechanism and dynamics of the potential-dependent interfacial processes. The layers were built up by sequential adsorption of polycations (poly[ethylene imine] (PEI); polyallylamine hydrochloride (PAH)) and polyanions (poly[styrene sulfonate] (PSS)). All multilayers terminated by PSS electrostatically bind Cyt-c. On PEI/PSS coatings, Cyt-c is peripherally bound and fully redox-active. Due to the interfacial potential drop, the apparent redox potential is lowered by 40 mV compared to that in solution. The rate constant for the heterogeneous electron transfer (ET) of ca. 0.1 s(-1) is consistent with electron tunneling through largely ordered PEI/PSS layers. ET is coupled to a reversible conformational transition of Cyt-c that involves a change of the coordination pattern of the heme. Additional (PAH/PSS) double layers cause a broadening of the redox transition and a drastic negative shift of the redox potential, which is attributed to the formation of PSS/Cyt-c complexes. It is concluded that Cyt-c can effectively compete with PAH for binding of PSS, resulting in a rearrangement of the layered structure and a penetration of the PSS-bound Cyt-c into the PAH/PSS double layers. This conclusion is consistent with SERR intensity and quartz microbalance measurements. ET was found to be overpotential-independent and faster than that for PEI/PSS coatings, which is interpreted in terms of specific PSS/Cyt-c complexes serving as gates for the heterogeneous ET.