Impact of Self-Assembly Composition on the Alternate Interfacial Electron Transfer for Electrostatically Immobilized Cytochrome C

We report on the effects of self-assembled monolayer (SAM) dilution and thickness on the electron transfer (ET) event for cytochrome c (CytC) electrostatically immobilized on carboxyl terminated groups. We observed biphasic kinetic behavior for a logarithmic dependence of the rate constant on the SAM carbon number (ET distance) within the series of mixed SAMs of C5COOH/ C2OH, C10COOH/C6OH, and C15COOH/C11OH that is in overall similar to that found earlier for the undiluted SAM assemblies. However, in the case of C15COOH/C11OH and C10COOH/C6OH mixed SAMs a notable increase of the ET standard rate constant was observed, in comparison with the corresponding unicomponent (x–COOH) SAMs. In the case of the C5COOH/C2OH composite SAM a decrease of the rate constant versus the unicomponent analogue was observed. The value of the reorganization free energy deduced through the Marcuslike data analysis did not change throughout the series;this fact along with the other observations indicates uncomplicated rate-determining unimolecular ET in all cases. Our results are consistent with a model that considers a changeover between the alternate, tunneling and adiabatic intrinsic ET mechanisms. The physical mechanism behind the observed fine kinetic effects in terms of the protein-rigidifying x–COOH/CytC interactions arising in the case of mixed SAMs are also discussed.