Molecular arrangements in sphingolipids. The crystal structure of cerebroside

The single crystal analysis of a complex, membrane glycolipid is described. Cerebroside (β-D-galactosyl-N-(2-D-hydroxyoctadecanoyl)-D-dihydrosphingosine, $\text{C}{}_{42}\text{H}{}_{32}\text{O}{}_9\text{N}\text{·}\text{1}\text{2}\text{C}{}_2\text{H}{}_5\text{OH}$) — an important constituent of plasma membranes — crystallizes in the monoclinic spacegroup P2, with a = 11.202, b = 9.262, c = 46.46 A and β = 99.00^°. There are two independent molecules in the asymmetric unit partly related by a non-crystallographic symmetry. The structure was determined by direct methods and refined to R = 0.116.The molecules pack in a typical bilayer arrangement with adjacent double layers separated by ethanol molecules of crystallization. The planes of the sugar rings are turned almost parallel to the layer interface which gives the molecules the shape of a shovel. Together with the polar ceramide part, the galactose head groups form an extensive lateral network of hydrogen bonds within the polar region of each layer. The chains tilt by an angle of 49^° towards this polar boundary. A parallel stacking of the chains is achieved by a bend of the sphingosine chain as far up as carbon atom 5 and 6 in the two independent molecules. The lateral hydrocarbon chain packing is of an earlier unknown hybrid type (HS2). Chains with parallel zigzag planes are arranged in pleated shoets. These sheets contain alternatively fatty acid and sphingosine chains which have a mutually perpendicular chain plane orientation.