Effect of the asymmetric Ca2+ distribution on the bilayer properties of phosphatidylcholine-sonicated vesicles

The incorporation of Ca2+ in the inner volume of egg phosphatidylcholine vesicles increases the fluorescence anisotropy of a diphenylhexatriene probe. This increase is higher than for Na+ at the same normality. An effect of the same magnitude is induced by Ca2+ when using binary lipid mixture (dioleoylphosphatidylcholine and dipalmitoylphosphatidylcholine) as long as the mixture is maintained below the phase-transition temperature of the saturated species. The influence of Ca2+ may be explained by an asymmetric distribution of the saturated and unsaturated lipids between the internal and the external monolayers.