Crystal structure of a mixed chain diacylglycerol, 1-stearoyl-3-oleyl-glycerol

Diacylglycerols composed of one saturated and one unsaturated chain make up the hydrophobic core of many biological membranes. We report here the first crystalline structure of such a mixed chain diacylglycerol. The mixed chain diacylglycerol, 1-stearoyl-3-oleyl-glycerol (1,3-SODG) was produced by solution isomerization of 1-stearoyl-2-oleyl-sn-glycerol. 1,3-SODG was isolated by flash chromatography and crystallized by slow evaporation in ethyl acetate at 4 degrees C. The melting point was 42.5 degrees C and the enthalpy was 18.0 kcal/mol. The crystal structure was determined to a final R factor of 0.127. Four molecules are present in the monoclinic unit cell: space group Cc, a = 9.362(2), b = 5.495(2), c = 77.92(3)A, beta = 91.46(2), V = 4007(4)A3, Z = 4, D = 1.032 g/cm3. The molecule forms an extended V-shaped conformation with the oleate and stearate chains coming off the two ends of the glycerol with an angle between their planes of 94 degrees. The two chains pack separately in individual layers and do not interact. The hydrogen bonds between the free hydroxyl group on the glycerol-2 position and the carbonyl oxygen on the oleyl chain of an adjacent molecule are 2.78 A in length and stabilize the glycerol layers. The stearoyl chain is roughly straight and packed in a triclinic parallel subcell. Both portions of the oleyl chain also pack in triclinic parallel packing. The torsion angle sequence along the double bond extending from the oleyl carbons C7 to C13 is tscsst (173 degrees, -152 degrees, -17 degrees, -157 degrees, -163 degrees, -178 degrees). This sequence is different from other monounsaturates. The torsion angle sequence around the glycerol region shows that the glycerol conformation is quite similar to the A conformer of racemic alpha monolaurin, and to 1,3-di-11-bromoundecanoyl glycerol, but completely unlike 1,2-diacyl-sn-glycerols. Thus the glycerol conformation appears to be driven by the hydrogen bond formation, which in turn determines whether chains interact or are segregated. In 1,3-diacylglycerols the two acyl chains point in different directions and are segregated. In 1,2-diacylglycerols the acyl chains lie side by side and must interact. When the two chains are quite different, then serious problems in packing occur, giving rise to disordered crystal packing. Probably as a result of the disordered chain packing in 1-stearoyl-2-oleyl-sn-glycerol (Di, L. and D.M. Small. 1993. J. Lipid Res. 34: 1611-1623) we were not successful in growing adequate crystals suitable for crystallographic structure determination of this 1,2 mixed chain diacylglycerol.