Surface charge markedly attenuates the nonlamellar phase-forming propensities of lipid bilayer membranes: Calorimetric and P-31-nuclear magnetic resonance studies of mixtures of cationic, anionic, and zwitterionic lipids

The lamellar/nonlamellar phase preferences of lipid model membranes compose d of mixtures of several cationic lipids with various zwitterionic and anio nic phospholipids were examined by a combination of differential scanning c alorimetry and P-31 NMR spectroscopy. Ail of the cationic lipids utilized i n this study form only lamellar phases in isolation. Mixtures of these cati onic lipids with zwitterionic strongly lamellar phase-preferring lipids suc h as phosphatidylcholine form only the lamellar liquid-crystalline phase ev en at high temperatures, as expected. Moreover, mixtures of these cationic lipids with strongly nonlamellar phase-preferring zwitterionic lipids such as phosphatidylethanolamine exhibit a markedly reduced propensity to form i nverted nonlamellar phases, again as expected. However, when mixed with ani onic lipids such as phosphatidylserine, phosphatidylglycerol, cardiolipin, or phosphatidic acid, a marked enhancement of nonlamellar phase-forming pro pensity occurs, despite the fact both components of the mixture are nominal ly lamellar phase-preferring. An examination of the lamellar/nonlamellar ph ase transition temperatures and the nature of the nonlamellar phases formed , as a function of temperature and of the composition of the mixture, indic ates that the propensity to form inverted nonlamellar phases is maximal in mixtures where the mean surface charge of the membrane surface approaches n eutrality and decreases markedly with increases in the density of positive or negative charge at the membrane surface. Moreover, the onset temperature s of the reversed hexagonal phase rise more steeply than do those of the in verted cubic phase as the ratio of cationic and anionic lipids is varied, s uggesting that the formation of inverted hexagonal phases is more sensitive to this surface charge effect. These results indicate that surface charge per se is a significant and effective modulator of the lamellar/nonlamellar phase preferences of membrane lipids and that charged group interactions a t membrane surfaces may have a major role in regulating this particular mem brane property.