SOLUTE EFFECTS ON THE COLLOIDAL AND PHASE-BEHAVIOR OF LIPID BILAYER-MEMBRANES - ETHANOL-DIPALMITOYLPHOSPHATIDYLCHOLINE MIXTURES

By means of the scanning differential calorimetry, x-ray diffractometr y, and the dynamic light scattering, we have systematically studied th e phase and packing properties of dipalmitoylphosphatidylcholine vesic les or multibilayers in the presence of ethanol. We have also determin ed the partial ternary phase diagram of such dipalmitoylphosphatidylch oline/water/ ethanol mixtures. The directly measured variability of th e structural bilayer parameters implies that ethanol binding to the ph ospholipid bilayers increases the lateral as well as the transverse re pulsion between the lipid molecules. This enlarges the hydrocarbon til t (by up to 23 degrees) and molecular area (by less than or equal to 4 0%). Ethanol-phospholid association also broadens the interface and, t hus, promotes lipid headgroup solvation. This results in excessive swe lling (by 130%) of the phosphatidylcholine bilayers in aqueous ethanol solutions. Lateral bilayer expansion, moreover, provokes a successive interdigitation of the hydrocarbon chains in the systems with bulk et hanol concentrations of 0.4-1.2 M. The hydrocarbon packing density as well as the propensity for the formation of lamellar gel phases simult aneously increase. The pretransition temperature of phosphatidylcholin e bilayers is more sensitive to the addition of alcohol (initial shift : Delta T-p = 22 degrees C/mol) than the subtransition temperature (De lta T-s = 5 degrees C/mol), whereas the chain-melting phase transition temperature is even less affected (Delta T-m = 1.8 degrees C/mol). Af ter an initial decrease of 3 degrees for the bulk ethanol concentratio ns below 1.2 M, the T-m value increases by 2.5 degrees above this limi ting concentration. The gel-phase phosphatidylcholine membranes below T-m are fully interdigitated above this limiting concentration. The ch ain tilt on the fringe of full chain interdigitation is zero and incre ases with higher ethanol concentrations. Above T-m, some of the lipid molecules are solubilized by the bound ethanol molecules. More highly concentrated ethanol solutions (>7 M) solubilize the phosphatidylcholi ne bilayers with fluid chains fully and result in the formation of mix ed lipid-alcohol micelles.