Sphingomyelin at the air–water interface

Abstract

X-ray reflectivity (XR) and grazing incident-angle x-ray diffraction (GIXD) reveal that sphingomyelin, although forming homogeneous monomolecular films at air–water interfaces, does not develop long-range crystalline in-plane order at any in-plane density, most notably at high surface pressures where many other membrane lipids order as two-dimensional crystals. Studies were carried out on the monodisperse synthetic C18-sphingomyelin where both hydrocarbon chains are saturated and on natural sphingomyelin which contains a distribution of chain lengths, with saturated and unsaturated hydrocarbon chains. The surface pressure versus molecular area (π–A) isotherm for the natural sphingomyelin is similar to that of the synthetic, but has a somewhat higher surface pressure at intermediate areas. The absence of in-plane crystalline order is attributed to competing interactions in the head group region: hydrogen bonding in the erythro region, electrostatic interactions among zwitterions of neighboring head groups, and van der Waals among acyl chains.