Phase behavior of phosphatidylglycerol bilayers as a function of buffer composition: fluorescence studies using Laurdan probe

Abstract

The influence of solution composition in lipid vesicles composed of different charged and zwitterionic phospholipids was studied using a Laurdan fluorescent probe. Particular results were found for dipalmitoylphosphatidylglycerol (DPPG) bilayers, in which quantitation of phase state domains as well as the main phase transition temperature are strongly dependent on ionic composition. The Laurdan generalised polarisation (GP) parameter decreases significantly in alkaline cation solutions in the concentration range 1–20 mM, in the order K +>Na +>Li +. Gel phase domains at 44°C change from 77% in vesicles prepared in pure milliQ water to 8% in 10 mM KCl solutions. Simultaneously, the steady-state fluorescence anisotropy of diphenylhexatriene (DPH) placed in the alkyl chain region of the bilayer decreases. No effect of alkaline ions on Laurdan relaxation was noticed in other negatively charged phospholipids such as dimyristoylphosphatidic acid (DMPA) or brain phosphatidylserine (PS). The addition of Ca 2+ to phosphatidylglycerol membranes was also detected even at micromolar concentrations. For example, 50 μM calcium concentration increases Laurdan GP from 0.17 to 0.477 at 51.6°C. The generation of a gel like phase was dependent on temperature and the presence of other ions in the medium. These results stress the particular phase behaviour of dipalmitoylphosphatidylglycerol (DPPG) membranes. Regarding these features, a structural mechanism related to phosphatidylglycerol arrangement is discussed.