The structure of D-erythro-C18 ceramide at the air-water interface

X-ray reflectivity (XR) and diffraction at grazing angles of incidence (GID ) were conducted to determine the structure of synthetic D-erythro C18-cera mide films at the air-water interface at various surface pressures (pi). An alysis of the GID reveals that the monomolecular film, at the crystalline p hase (pi > 0 mN/m), is predominantly hexagonal. In this crystalline phase, the analysis of the reflectivity yields an electron density profile that co nsists of three distinct homogeneous slabs, one associated with the headgro up region and the other two with the hydrocarbon chains. At large molecular areas (pi approximate to 0), isolated crystalline domains coexist with two -dimensional gas phase. Within the crystalline domains, we find an orthorho mbic arrangement of the chains that coexists with the hexagonal symmetry. I t is argued that the two-dimensional orthorhombic crystals are induced by h ydrogen bonding between headgroups even at very low surface pressures. Alth ough their structure is incommensurate with the simple hexagonal arrangemen t, they act as nucleation centers for the conventional hexagonal phase whic h dominates at high pi.