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

Abstract

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 (C x ). 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 (C 16, C 11), the rate constants display a normal exponential distance dependence, whereas for shorter chain lengths (C 6, C 3, C 3), 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.