Electric field effect on cholesterol-phospholipid complexes

Monolayer mixtures of dihydrocholesterol acid phospholipids at the air-wate r interface are used to model membranes containing cholesterol and phosphol ipids. Specific, stoichiometric: interactions between cholesterol and some but not all phospholipids have been proposed to lead to the formation of co ndensed complexes. It is reported here that an externally applied electric field of the appropriate sign can destabilize these complexes, resulting in their dissociation. This is demonstrated through the application of an ele ctric field gradient that leads to phase separations in otherwise homogeneo us monolayers, This is observed only when the monolayer composition is clos e to the stoichiometry of the complex. The electric field effect is analyze d with the same mean field thermodynamic model as that used previously to a ccount for pairs of upper miscibility critical points in these mixtures. Th e concentrations of dihydrocholesterol, phospholipid, and complex vary stro ngly and sometimes discontinuously in the monolayer membrane in the field g radient. The model is an approximation to a two-dimensional liquid in which molecules freely exchange between free and complexed form so that the chem ical potentials are constant throughout the membrane. The calculations are illustrated for a complex of about 15 molecules, composed of 5 cholesterol molecules and 10 phospholipid molecules.