Kinetics of the electron transfer reaction of Cytochrome c552 adsorbed on biomimetic electrode studied by time-resolved surface-enhanced resonance Raman spectroscopy and electrochemistry

Cytochrome c(552) (Cyt-c(552)) and its redox partner ba(3) -oxidase from Thermus thermophilus possess structural differences compared with Horse heart cytochrome c (cyt-c)/cytochrome c oxidase (CcO) system, where the recognition between partners and the electron transfer (ET) process is initiated via electrostatic interactions. We demonstrated in a previous study by surface-enhanced resonance Raman (SERR) spectroscopy that roughened silver electrodes coated with uncharged mixed self-assembled monolayers HS-(CH2) (n)-CH3/HS-(CH2) (n + 1)-OH 50/50, n = 5, 10 or 15, was a good model to mimic the Cyt-c(552) redox partner. All the adsorbed molecules are well oriented on such biomimetic electrodes and transfer one electron during the redox process. The present work focuses on the kinetic part of the heterogeneous ET process of Cyt-c(552) adsorbed onto electrodes coated with such mixed SAMs of different alkyl chain length. For that purpose, two complementary methods were combined. Firstly cyclic voltammetry shows that the ET between the adsorbed Cyt-c(552) and the biomimetic electrode is direct and reversible. Furthermore, it allows the estimation of both the density surface coverage of adsorbed Cyt-c(552) and the kinetic constants values. Secondly, time-resolved SERR (TR-SERR) spectroscopy showed that the ET process occurs without conformational change of the Cyt-c(552) heme group and allows the determination of kinetic constants. Results show that the kinetic constant values obtained by TR-SERR spectroscopy could be compared to those obtained from cyclic voltammetry. They are estimated at 200, 150 and 40 s(-1) for the ET of Cyt-c(552) adsorbed onto electrodes coated with mixed SAMs HS-(CH2) (n) -CH3/HS-(CH2) (n + 1)-OH 50/50, n = 5, 10 or 15, respectively.