Osmotic behavior of liposomes of phosphatidylcholine and phosphatidylsulfocholine as a function of lipid concentration

A relationship between the initial rate of liposome swelling, d(1/A)/dt and the reciprocal of the lipid concentration of the liposomes has been derived and then utilized to describe the osmotic swelling behavior of serially diluted liposomes and chloroplasts exposed to hypertonic urea solutions. The slopes of plots of d(1/A)/dt vs. the reciprocal of the lipid concentration of liposomes were not affected by differences in the initial absorbance of phosphatidylcholine-sterol bilayers, and were used to assess the ability of sterols to reduce the initial rates of urea permeation through dimyristoylphosphatidylcholine (DMPC) bilayers in the liquid-crystalline state. Multilamellar liposomes and sonicated vesicles were prepared from dimyristoylphosphatidylsulfocholine (DMPSC), in which the quaternary ammonium group of choline is replaced by -S⁺(CH₃)₂. Cholesterol reduced the initial rate of osmotic urea penetration into liposomes and the rate of 6-carboxyfluorescein efflux from vesicles at 35°C. The effect of cholesterol on bilayers of phosphatidylsulfocholine and phosphatidylcholine was very similar, suggesting that no strict structural requirements need be met in the choline moiety for lecithin-cholesterol interaction. The sulfonium analog could thus functionally replace phosphatidylcholine in natural membranes.