A monolayer and freeze-etching study of charged phospholipids. I. Effects of ions and pH on the ionic properties of phosphatidylglycerol and lysylphosphatidylglycerol

A comparative study of the ionic properties of phosphatidylglycerol (PG) and lysylphosphatidylglycerol (LPG) has been carried out using monolayer and freeze-etch techniques. It is shown that the ionization state of the PG monolayer is strongly dependent on the subphase ionic strength. Mg⁺⁺ and Ca⁺⁺ induced a marked condensing effect. With Ca⁺⁺ a typical cylindrical structure could be observed by freeze etching, this structure being assumed to be generated by Ca⁺⁺ binding to PG. These phenomena were observed with both didodecanoyl-PG and PG from S. aureus. With respect to LPG it has been shown that the anion MoO₄⁼ gave a strong film condensation of didodecanoyl-LPG monolayers at pH 6 corresponding to a change in the bulk morphology from a transparent gel to a lamellar liposomal structure. A similar decrease in the molecular packing of S. aureus LPG was induced by MoO₄⁼, without a change in the freeze etch morphology of the dispersion. Evidence is presented on the important role that the polar head groups of these charged phospholipids and their ionic environment have on the overall molecular packing. Differential scanning calorimetric measurements demonstrated that the liquid-crystalline to gel transition of didodecanoyl-PG is strongly dependent on the cations in the suspension. These phenomena may be relevant to the physical state of lipids in biological membranes.