Abstract
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 C~5~COOH/C~2~OH, C~10~COOH/C~6~OH, and C~15~COOH/C~11~OH that is in overall similar to that found earlier for the undiluted SAM assemblies. However, in the case of C~15~COOH/C~11~OH and C~10~COOH/C~6~OH mixed SAMs a notable increase of the ET standard rate constant was observed, in comparison with the corresponding unicomponent (ω–COOH) SAMs. In the case of the C~5~COOH/C~2~OH composite SAM a decrease of the rate constant versus the unicomponent analogue was observed. The value of the reorganization free energy deduced through the Marcus‐like 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 ω–COOH/CytC interactions arising in the case of mixed SAMs are also discussed. © 2007 Wiley Periodicals, Inc. Biopolymers 87: 68–73, 2007.
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