Electron transfer dynamics of cytochrome c bound to self-assembled monolayers on silver electrodes

Cytochrome c (Cyt-c) was electrostatically immobilised on Ag electrodes coated with self-assembled monolayers (SAM) that are formed by ω-carboxyl alkanethiols with different alkyl chain lengths (Cx). Surface enhanced resonance Raman (SERR) spectroscopy demonstrated that electrostatic binding does not lead to conformational changes of the heme protein under the conditions of the present experiments. Employing time-resolved SERR spectroscopy, the rate constants of the heterogeneous electron transfer (ET) between the adsorbed Cyt-c and the Ag electrode were determined for a driving force of zero electronvolts. For SAMs with long alkyl chains (C16, C11), the rate constants display a normal exponential distance dependence, whereas for shorter chain lengths (C6, C3, C3), the ET rate constant approaches a constant value (ca. 130 s−1). The onset of the non-exponential distance-dependence is paralleled by an increasing kinetic H/D effect, indicating a coupling of the redox reaction with proton transfer (PT) steps. This unusual kinetic behaviour is attributed to the effect of the electric field at the Ag/SAM interface that increasingly raises the energy barrier for the PT processes with decreasing distance of the adsorbed Cyt-c from the electrode. The distance-dependence of the electric field strength is estimated on the basis of a simple electrostatic model that can consistently describe the redox potential shifts of Cyt-c as determined by stationary SERR spectroscopy for the various SAMs. At low electric fields, PT is sufficiently fast so that rate constants, determined as a function of the driving force, yield the reorganisation energy (0.217 electronvolts) of the heterogeneous ET.