An electrochemical approach to investigate gated electron transfer using aphysiological 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 carbo xylic acid-terminated alkanethiol self-assembled monolayers (SAMs) on well- defined gold (111) electrodes is a simplified model system to investigate b oth long range and intermolecular ET processes. The advantages of an electr ochemical approach to investigate the ET mechanism are that one can both re gulate the ET path length by using alkanethiol SAMs of varying chain length s and deconvolute the intermolecular ET event at the interface from the int ramolecular ET event. It has been shown that the interactions between cyt, c and the carboxylate termini are electrostatic in nature, analogous to tho se between cyt. c and negatively charged proteins such as cytochrome c pero xidase. In the present work, the effects of alkanethiol chain length, ionic strength, pH, and viscosity of supporting electrolyte on the ET kinetics w ere studied. The ET rates through long alkanethiol chains were observed to be slow because electron tunneling through the alkyl chain was the rate-lim iting step in the process. On the other hand, the ET rate through shorter c hain alkanethiols appeared to be independent of chain length, and the effec t of ionic strength and pH on the observed ET rates was insignificant. It i s proposed that the rate-limiting ET step through short alkyl chains result s 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 rea rrangement of the cyt. c from a stable binding form (binding complex) on th e carboxylic acid terminus to a configuration (ET complex) which facilitate s the most efficient ET pathway. The rate of the configurational rearrangem ent reaction that precedes the ET reaction was found to be markedly influen ced by solution viscosity, but its equilibrium constant was independent of solution viscosity. The change in the configurational rearrangement reactio n rate with solution viscosity follows a modified Kramers equation.