THE WATER-ADSORPTION CHARACTERISTICS OF CHARGED PHOSPHOLIPIDS

The hydration isotherms of the negatively charged phospholipids, egg p hosphatidic acid, bovine heart cardiolipin and two phosphatidylserines as well as one positively charged phospholipid, 1,2-dioleoyl-sn-glyce ro-3-ethylphosphocholine, have been obtained gravimetrically. The pres ence of an electrical charge on these phospholipids does not, in itsel f, determine whether the water binding to the phospholipids is 'strong ' or 'weak'. Interestingly, hysteresis effects were present for certai n charged phospholipids suggesting some rearrangement of the lipid mol ecular organization upon hydration and dehydration, perhaps due to the presence of the ionizable moiety. The hydration isotherms of the char ged phospholipids have been analyzed by BET theory, although, of the c harged lipids studied, all may not be amenable to the application of B ET theory. The hydration isotherms and the resulting parameters obtain ed from the BET analysis are compared to those found previously for zw itterionic phospholipids, especially egg phosphatidylcholine. The wate r adsorption characteristics of phospholipids are found to depend main ly on the total head-group structure including the presence of hydroph obic groups as well as electrical charge on the head-group.