An electrochemical approach to investigate gated electron transfer using a physiological model system:: Cytochrome c immobilized on carboxylic acid-terminated alkanethiol self-assembled monolayers on gold electrodes

The electron transfer (ET) scheme of cytochrome c (cyt. c) coupled to carboxylic acid-terminated alkanethiol self-assembled monolayers (SAMs) on well-defined gold (111) electrodes is a simplified model system to investigate both long range and intermolecular ET processes. The advantages of an electrochemical approach to investigate the ET mechanism are that one can both regulate the ET path length by using alkanethiol SAMs of varying chain lengths and deconvolute the intermolecular ET event at the interface from the intramolecular ET event. It has been shown that the interactions between cyt, c and the carboxylate termini are electrostatic in nature, analogous to those between cyt. c and negatively charged proteins such as cytochrome c peroxidase. In the present work, the effects of alkanethiol chain length, ionic strength, pH, and viscosity of supporting electrolyte on the ET kinetics were studied. The ET rates through long alkanethiol chains were observed to be slow because electron tunneling through the alkyl chain was the rate-limiting step in the process. On the other hand, the ET rate through shorter chain alkanethiols appeared to be independent of chain length, and the effect of ionic strength and pH on the observed ET rates was insignificant. It is proposed that the rate-limiting ET step through short alkyl chains results from a configurational rearrangement process preceding the ET event, and that its rate is 2.6 x 10(3) s(-1). This "gating" process arises from a rearrangement of the cyt. c from a stable binding form (binding complex) on the carboxylic acid terminus to a configuration (ET complex) which facilitates the most efficient ET pathway. The rate of the configurational rearrangement reaction that precedes the ET reaction was found to be markedly influenced by solution viscosity, but its equilibrium constant was independent of solution viscosity. The change in the configurational rearrangement reaction rate with solution viscosity follows a modified Kramers equation.