Interactions and space requirements of the phosphate head group in membrane lipids. The crystal structure of disodium lysophosphatidate dihydrate.

The packing arrangement and molecular conformation of lysophosphatidate (disodium 3-lauroyl-DL-glycero-1-phosphate dihydrate (LPA] was determined by single crystal analysis. The lipid crystallizes in the triclinic space group P1 with unit cell dimensions of a = 7.74, b = 5.54, c = 32.87 A and alpha = 92.6, beta = 99.2, gamma = 128.3 degrees. The molecules are arranged tail to tail in a bilayer structure. Thereby the D- and L-enantiomers pack separately in the opposite halves of the bilayer and are conformationally related by mirror symmetry. The hydrocarbon chains adopt the triclinic (T parallel) chain packing mode and are tilted by 55 degrees with respect to the layer normal. The phosphate groups are linked by one of the sodium ions to tightly packed rows. Between these rows the second sodium ion and the two water molecules of hydration are accommodated forming an extensive network of hydrogen, ionic and coordination bonds. The two sodium ions are positioned 1 A below and above the plane defined by the centres of gravity of the phosphate charges. The phosphate group and its 4 ligands thus pack effectively with a small cross-sectional area in the layer plane of 33.6 A2.