M. Lundmark, P. Nyholm, I. Pascher
Aug 1, 1984
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Journal
Chemistry and Physics of Lipids
Abstract
The erythro-2,3-dihydroxyoctadecanoic acid studied is a synthetic homologue of a natural occurring constituent of sphingolipids. The potassium salt of the acid crystallizes in the monoclinic space group P2, with the unit cell dimension: a = 5.39, b = 7.06, c = 26.26 A and β = 94.9°. The crystal structure was solved by direct methods and was refined to R = 0.062. The absolute configuration of the compound was determined by means of anomalous scattering effects, showing that the natural fatty acid has d-erythro configuration. The compound packs tail to tail in an unusual bilayer arrangement. The hydrocarbon chains have an extreme tilt of 60° and opposite inclination in the two halves of the bilayer. Laterally the hydrocarbon chains are arranged according to the monoclinic M∥ packing mode. The carbon chain makes a perpendicular bent at carbon atom 2. This places the 2-hydroxyl group in a preferred co-planar conformation towards the carboxylate group and at hydrogen bond distance to one of the carboxylate oxygens. The carboxylate group and the two hydroxyl groups are co-ordinated to K+ ions and together account for a large molecular cross-ection of 38 A2. Monolayer studies show that the acid forms a phase with this spacious molecular area also in contact with water. On compression above 10 mN m−1 transition to a more condensed state (S = 27 A2) takes place accompanied by marked changes of the surface potential.