Thermal transitions of DMPG bilayers in aqueous solution: SAXS structural studies

Abstract

Dimyristoylphosphatidylglycerol (DMPG) has been extensively studied as a model for biological membranes, since phosphatidylglycerol is the most abundant anionic phospholipid in prokaryotic cells. At low ionic strengths, this lipid presents a peculiar thermal behavior, with two sharp changes in the light scattering profile, at temperatures named here T on m and T off m. Structural changes involved in the DMPG thermal transitions are here investigated by small angle X-ray scattering (SAXS), and compared to the results yielded by differential scanning calorimetry (DSC) and electron spin resonance (ESR). The SAXS results show a broad peak, indicating that DMPG is organized in single bilayers, for the range of temperature studied (10–45°C). SAXS intensity shows an unusual effect, starting to decrease at T on m, and presenting a sharp increase at T off m. The bilayer electron density profiles, obtained from modeling the SAXS curves, show a gradual decrease in electron density contrast (attributed to separation between charged head groups) and in bilayer thickness between T on m and T off m. Results yielded by SAXS, DSC and ESR indicate that a chain melting process starts at T on m, but a complete fluid phase exists only for temperatures above T off m, with structural changes occurring at the bilayer level in the intermediate region.