Influence of head group methylation on the phase behavior of lipid monolayers

Monolayers of racemic 1-(2-tetradecylhexadecanoyl)-2-O-hexadecyl-phospholipids 1(2C14-16:0)-2H-PE(CH3)n, with n=0, 1, 2 and 3, at the air/water interface are studied by film balance measurements, Brewster angle microscopy (BAM) and grazing incidence X-ray diffraction (GIXD). Although the area per three tails exceeds that per head, an influence of the head group methylation on the monolayer structure is observed. The tilt angles at lower lateral pressures and the transition pressure to a hexagonal packing of upright oriented chains increase with increasing methylation degree. The transition from the NN tilted rectangular to this hexagonal phase is connected with a pressure region where the in-plane components Qxy of the two peaks coincide while the out-of-plane components Qz differ. This indicates an undistorted hexagonal in-plane lattice even for tilted chains. The area-pressure isotherm of the 1(2C16-18:0)-2H-PE monolayer shows at 20°C a plateau region which does not directly correspond to the transition from the NN tilted to the hexagonal phase with untilted molecules. BAM measurements show that the large differences between the molecular areas, derived from isotherm and X-ray measurements below 10 mN/m, can be explained by the formation of holes in the monolayer. Possibly the tilting of the triple-chain molecules leads to an orientational ordering of the head group dipoles and therefore to an electrostatic repulsion between condensed phase domains