Structure and Polymorphism of Saturated Monoacid l,2-Diacyl-sn-glycerols

The 1,2-diacyl-sn-glycerols (1,2-DGs) are the predominant naturally occurring isomer found in cell membranes, lipid droplets, and lipoproteins. They are involved in the metabolism of monoacylglycerols, triacylglycerols, and phospholipids. The 1,2-DGs participate in the activation of protein kinase C, in phosphorylation of target proteins, and in transduction of extracellular signals into the cell. We have undertaken a study of the physical properties of a homologous series of synthetic optically active diacyl-glycerols. Stereospecific 1,2-diacyl-sn-glycerols were synthesized with saturated fatty acyl chains of 12, 16, 18, 22, and 24 carbons in length. Their polymorphic behavior was examined by differential scanning calorimetry and X-ray powder diffraction. The solvent-crystallized form for all the 1,2-DGs packs in the orthorhombic perpendicular subcell (β′) and melts with a single sharp endotherm to an isotropic liquid. On quenching, the C₁₂, C₁₆ and C₁₈ compounds pack in a hexagonal subcell (α), whereas the C₂₂ and C₂₄ pack in a pseudohexagonal subcell (sub-α). The sub-α phase reversibly converts to the α phase. The long spacings of these compounds in both the α and β′ phases increase with chain length. In the α and β′ phases, the acyl chain tilts were found to be 90° and 62° from the basal methyl plane. The polymorphic behavior of 1,2-diacyl-sn-glycerol is quite different from that of the corresponding monoacid saturated 1,3-diacylglycerols which form two β phases with triclinic parallel subcells.