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

AbstractDimyristoylphosphatidylglycerol (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 Tonm and Toffm. 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 Tonm, and presenting a sharp increase at Toffm. 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 Tonm and Toffm. Results yielded by SAXS, DSC and ESR indicate that a chain melting process starts at Tonm, but a complete fluid phase exists only for temperatures above Toffm, with structural changes occurring at the bilayer level in the intermediate region