STEREODEPENDENT FUSION AND FISSION OF VESICLES - CALCIUM-BINDING OF SYNTHETIC GEMINI PHOSPHOLIPIDS CONTAINING 2 PHOSPHATE GROUPS

Three different stereoisomers of a phosphatidic acid analog bearing tw o phosphate groups have been synthesized from tartaric acid and erythr itol. The obtained diastereomeric gemini surfactants differ in their a ggregation behavior due to the different spatial orientations of their functional groups. The most remarkable difference in aggregation beha vior 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 vesic le fission. This remarkable behavior can be explained by a change in t he molecular packing of the lipid molecules upon the complexation with calcium ions. An analysis of physical data obtained prior to and afte r the addition of the calcium ions reveals that the head groups of the diastereomeric surfactants respond differently to these ions: those o f the (S,S) isomer increase in size, whereas those of the (R,S) isomer decrease in size. This phenomenon accounts for occurrence of fusion a nd fission of the vesicles, respectively.