The influence of sterols on pentachlorophenol-induced charge transfer across lipid bilayers studied by alternating current methods.

The frequency dependence of membrane admittance has been determined for a series of phosphatidylcholine/sterol/n-decane bilayers in the presence of an aqueous environment containing pentachlorophenol. Variations in the results among membranes can be related to differences in the kinetic parameters of a kinetic model of pentachlorophenol-induced charge transport by characterizing both measurements and model behavior in terms of a common equivalent circuit. The kinetic model assumes a three-layer structure for the membrane and immediate environment. Data from membranes formed with beta-hydroxysterols having a flat ring structure and an intact side-chain (cholestanol, cholesterol, 7-dehydrocholesterol), after correction for sterol-induced membrane thinning, suggest that these sterols affect charge translocation by altering both interior fluidity and surface dipolar fields. The effects almost cancel for the case of cholesterol. These sterols also affect interfacial processes, either by inhibiting proton exchange between the aqueous and lipid environments, or by suppressing the adsorption of pentachlorophenol anions. Stigmasterol, coprostanol and epicholesterol cause only minor alterations in both translocation and interfacial processes. None of the sterols investigated has a significant influence on the capacitance of the interfacial region.