INTERFACIAL ELECTROCHEMISTRY OF CYTOCHROME-C AT A LIPID BILAYER MODIFIED ELECTRODE - EFFECT OF INCORPORATION OF NEGATIVE CHARGES INTO THE BILAYER ON CYCLIC VOLTAMMETRIC PARAMETERS

The electrochemistry of cytochrome c at a gold electrode modified with a self-assembled lipid bilayer membrane has been investigated at different ionic strengths, using the electrically neutral lipid egg phosphatidylcholine (PC), either alone or in combination with various amounts of the negatively charged surfactant dihexadecylphosphate (DHP-). The results demonstrate that even electrically neutral PC membranes at low lipid concentrations and a low ionic strength of the electrolyte, interact with oxidized cytochrome c strongly enough to support an electron transfer reaction between the protein and the electrode. The addition of negatively charged molecules of DHP- to the lipid bilayer causes large changes in the shape of the cyclic voltammetry curves, and increases the values of the limiting currents. The observations support the electrostatic nature of the interaction between cytochrome c and the lipid bilayer, and also indicate that the adsorption of protein molecules at a bilayer surface influences the process of electron transfer as well. The experimental data have been interpreted using both the linear (macroscopic) and radial (microscopic) models of protein diffusion to the electrode surface, resulting in an electrochemical rate constant in the range of quasi-reversibility (10(-3) cm s-1) or reversibility (> 1 cm s-1), depending on the model used, and demonstrating that the pathway of cytochrome diffusion can be effectively controlled by the DHP- concentration.