Influence of side-chain length on phospholipid ordering in two dimensions

Phosphoethanolamines (PE) with a 2-branched hexadecanoyl chain (ethyl, butyl and tetradecyl) at the sn-1 position of the glycerol backbone and an unbranched hexadecyl residue at the sn-2 position have been synthesised. They form stable monolayers at the air/water interface, The thermodynamic properties are characterised using the classical film balance technique, The structure of the monolayer phase has been characterised by grazing incidence X-ray diffraction, The introduction of an ethyl side-chain changes the tilt direction in a rectangular unit cell from nearest neighbour (NN) to next-nearest neighbour (NNN). The elongation of the side-chain to butyl disturbs the monolayer packing and a phase with an undefined tilt direction is formed. At low lateral pressures the triple-chain PE forms slightly tilted phases. At higher pressures a hexagonal packing of upright oriented chains can be observed. Initial X-ray experiments of aqueous dispersions of the triple-chain PE point to an interdigitation of head groups from adjacent bilayers, (C) 1998 Elsevier Science Ireland Ltd. All rights reserved.