Electron-Transfer Kinetics of Covalently Attached Cytochrome c/SAM/Au Electrode Assemblies

The rotational motion of cytochrome c has been restricted by cross-linking it to mixed self-assembled monolayers (SAMs) with the compositions S−(CH 2 ) m COOH/S−(CH 2 ) n OH on gold electrodes via the formation of amide bonds between lysine residues on the protein and terminal carboxylate groups of the SAM. The effect of SAM thickness on the electron-transfer rate has been studied, and two main observations are drawn. First, the electron-transfer rate displays the same qualitative dependence on SAM thickness that was previously reported for electrostatically adsorbed and pyridine-ligated assemblies, suggesting a tunneling mechanism at long distance and some other rate-limiting process at short distance. Second, a significant effect on the rate is observed for mixed SAMs having a hydroxyl-terminated alkanethiol diluent when the diluent is more than one methylene group shorter than the carboxylic acid alkanethiol. These conclusions suggest that large-amplitude protein motion (i.e., gating) is not rate-limiting at short distance, though smaller-amplitude motions cannot be ruled out.