Proton-coupled electron transfer of cytochrome c

Cytochrome c (Cyt-c) was electrostatically bound to self-assembled monolaye rs (SAM) on an Ag electrode. which are formed by omega -carboxyl alkanethio ls of different chain lengths (C,). The dynamics of the electron-transfer ( ET) reaction of the adsorbed heme protein, initiated by a rapid potential j ump to the redox potential, was monitored by time-resolved surface enhanced resonance Raman (SERR) spectroscopy. Under conditions of the present exper iments, only the reduced and oxidized forms of the native protein state con tribute to the SERR spectra. Thus, the data obtained from the spectra were described by a one-step relaxation process yielding the rate constants of t he ET between the adsorbed Cyt-e and the electrode for a driving force of z ero electronvolts. For C-16- and C-11-SAMs, the respective rate constants o f 0.073 and 43 s(-1) correspond to an exponential distance dependence of th e ET (beta = 1.28 Angstrom (-1)), very similar to that observed for long-ra nge intramolecular ET of redox proteins. Upon further decreasing the chain length, the rate constant only slightly increases to 134 s(-1) at C-6- and remains essentially unchanged at C-3- and C-2-SAMs. The onset of the nonexp onential distance dependence is paralleled by a kinetic H/D effect that inc reases from 1.2 at C-6- to 4.0 at Ct-coatings, indicating a coupling of the redox reaction with proton-transfer (PT) steps. These PT processes are att ributed to the rearrangement of the hydrogen-bonding network of the protein associated with the transition between the oxidized and reduced state of C yt-c. Since this unusual kinetic behavior has not been observed for electro n-transferring proteins in solution, it is concluded that at the Ag/SAM int erface the energy barrier for the PT processes of the adsorbed Cyt-c is rai sed by the electric field. This effect increases upon reducing the distance to the electrode, until nuclear tunneling becomes the rate-limiting step o f the redox process. The electric field dependence of the proton-coupled ET may represent a possible mechanism for controlling biological redox reacti ons via changes of the transmembrane potential.