ELECTROCHEMICAL MEASUREMENTS OF THE LATERAL DIFFUSION OF ELECTROACTIVE AMPHIPHILES IN SUPPORTED PHOSPHOLIPID MONOLAYERS

Chronocoulometry was used to characterize the fluidity and lateral dif fusion coefficient of supported phospholipid bilayer assemblies. The b ilayers were formed on the inner surfaces of the microporous template films of aluminum oxide on gold electrodes. The lipid monolayers were formed by adsorption and fusion of phospholipid vesicles on alkylated oxide surfaces. Octadecyltrichlorosilane (OTS) was used in the initial alkylation step. The surface concentration of the lipids in monolayer assemblies was measured by a radioactive assay method. Surface densit ies corresponding to 48 +/- 10 Angstrom(2)/molecule (DPPC) and 56 +/- 11 Angstrom(2)/molecule (DMPC) were obtained (for exposure times > 120 min) largely independent at the temperature of the vesicle's fusion ( below or above chain-melting transition). Octadecylviologen (C(18)MV(2 +)) was used as an electroactive probe species. Its limiting lateral d iffusion coefficient in DMPC monolayers was 5 x 10(-8) cm(2)/s, measur ed as C(18)MV(2+) mole fraction extrapolated to 0 decreasing linearly from 20 to below 1 mol %. Linear Arrhenius plots for C(18)MV(2+) diffu sion in DMPC monolayers were obtained with slopes of similar to 40 kJ/ mol between 18 and 45 degrees C, demonstrating homogeneity and fluidit y of the lipid monolayers. Chronocoulometry was also used to obtain la teral diffusion coefficient of ubiquinone in DMPC/OTS bilayers. A valu e of 1.9 x 10(-8) cm(2)/s at 30 degrees C was obtained.