Solute effects on the colloidal and phase behavior of lipid bilayer membranes: ethanol-dipalmitoylphosphatidylcholine mixtures.

By means of the scanning differential calorimetry, x-ray diffractometry, and the dynamic light scattering, we have systematically studied the phase and packing properties of dipalmitoylphosphatidylcholine vesicles or multibilayers in the presence of ethanol. We have also determined the partial ternary phase diagram of such dipalmitoylphosphatidylcholine/water/ethanol mixtures. The directly measured variability of the structural bilayer parameters implies that ethanol binding to the phospholipid bilayers increases the lateral as well as the transverse repulsion between the lipid molecules. This enlarges the hydrocarbon tilt (by up to 23 degrees) and molecular area (by < or = 40%). Ethanol-phospholid association also broadens the interface and, thus, promotes lipid headgroup solvation. This results in excessive swelling (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 ethanol concentrations of 0.4-1.2 M. The hydrocarbon packing density as well as the propensity for the formation of lamellar gel phases simultaneously increase. The pretransition temperature of phosphatidylcholine bilayers is more sensitive to the addition of alcohol (initial shift: delta Tp = 22 degrees C/mol) than the subtransition temperature (delta Ts reversible 5 degrees C/mol), whereas the chain-melting phase transition temperature is even less affected (delta Tm = 1.8 degrees C/mol). After an initial decrease of 3 degrees for the bulk ethanol concentrations below 1.2 M, the Tm value increases by 2.5 degrees above this limiting concentration. The gel-phase phosphatidylcholine membranes below Tm are fully interdigitated above this limiting concentration. The chain tilt on the fringe of full chain interdigitation is zero and increases with higher ethanol concentrations. Above Tm, some of the lipid molecules are solubilized by the bound ethanol molecules. More highly concentrated ethanol solutions (> 7 M) solubilize the phosphatidylcholine bilayers with fluid chains fully and result in the formation of mixed lipid-alcohol micelles.