Stereodependent fusion and fission of vesicles: Calcium binding of synthetic gemini phospholipids containing two phosphate groups

Three different stereoisomers of a phosphatidic acid analog bearing two phosphate groups have been synthesized from tartaric acid and erythritol. The obtained diastereomeric gemini surfactants differ in their aggregation behavior due to the different spatial orientations of their functional groups. The most remarkable difference in aggregation behavior is encountered when calcium ions are added to vesicle suspensions of the respective isomers. The vesicles formed from the (S,S) and (R,R) isomer undergo fusion, whereas those of the (R,S) isomer show vesicle fission. This remarkable behavior can be explained by a change in the molecular packing of the lipid molecules upon the complexation with calcium ions. An analysis of physical data obtained prior to and after the addition of the calcium ions reveals that the head groups of the diastereomeric surfactants respond differently to these ions: those of the (S,S) isomer increase in size, whereas those of the (R,S) isomer decrease in size. This phenomenon accounts for occurrence of fusion and fission of the vesicles, respectively.