INFLUENCE OF CA2+ AND ETHANOL ON THE AGGREGATION AND THERMAL PHASE-BEHAVIOR OF L-DIHEXADECYLPHOSPHATIDYLCHOLINE LIPOSOMES

The influence of Ca2+ and ethanol on vesicle aggregation and thermal phase behaviour of the diether lipid 1,2-dihexadecylphosphatidylcholine (DHPC) was studied by light absorbance and DSC. At temperatures below the pretransition the ethanol-injected vesicles Of L-DHPC were rapidly aggregated by Ca2+. Upon raising the cation concentration a biphasic increase in aggregation saturating at an approximate [Ca2+]/[lipid) ratio of 1.5:1 was observed. Further increase in [Ca2+] up to [Ca2+]/[lipid] stoichiometries exceeding 2.5:1 led to the loss of aggregation. Removal of ethanol by dialysis abolished Ca2+-induced aggregation. Ethanol-injected vesicles of the ester-linked L-dipalmitoylphosphatidylcholine (L-DPPC) or the racemic DL-DHPC were not aggregated by Ca2+ thus indicating the importance of the absence of ester carbonyls as well as the stereochemical configuration of the lipid in determining the mode of interaction of DHPC with Ca2+. Differential scanning calorimetry of multilamellar liposomes of L-DHPC showed an increase by 8-degrees in the pretransition temperature T(p) in the presence of 250 mM ethanol. Both with and without ethanol, increasing concentrations of Ca2+ corresponding to [Ca2+]/[lipid] ratios of 1:1 to 20:1 caused a gradual decrease in T(p) and finally the disappearance of the pretransition. Concomitantly a slight elevation in T(m) occurred. No principal differences were observed in the thermal phase behaviour of the L-isomer and racemic DL-DHPC.