Chemically modified lipids — a suitable tool to study molecular interactions in model systems

The polymorphism of triple-chain glycerophosphoethanolamines differing in the degree of methylation of the head group has been investigated in both monolayers at the air/water interface and in multilayers of the lipid/water dispersions by means of film balance measurements, Grazing incidence X-ray diffraction (GIXD), differential scanning calorimetry (DSC) and simultaneous small- and wide-angle X-ray diffraction, respectively. Despite the larger area requirements of the three chains compared to the smaller head group, the change in the degree of methylation affects the thermodynamical data and structures formed in mono- and multilamellar systems. Increasing methylation leads to larger tilt angles of the hydrocarbon chains in monolayers at low lateral pressures and shifts the pressure for the tilting transition from NN tilted centered rectangular to untilted hexagonal phases towards larger values. In the lipid/water dispersions, the change in head group methylation is connected with a change of the phase sequence. For the phosphatidylethanolamines (PE) and the monomethyl derivative a stabilization of lamellar liquid-crystalline phases by interdigitation of adjacent head groups was found, whereas the introduction of a second methyl group results in the appearance of an HII phase.