Synthesis and polymorphism of 3-acyl-sn-glycerols

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Abstract
3-Acyl-sn-glycerols with even-numbered saturated fatty acyl chains from decanoate to lignocerate were synthesized. Successful hydrolysis of the long acyl chain intermediate 1,2-isopropylidene-3-acyl-sn-glycerols from stearate to lignocerate was accomplished by applying the compounds to silica gel and exposing them to HCl gas at -75.degree. C. The purity of the compounds were checked by boric acid impregnanted TLC, 13C NMR and reverse-phase high-pressure liquid chromatography. Differential scanning calorimetry and X-ray diffraction techniques were used to study the polymorphism of the compounds. In the .beta. phase obtained from solvent of crystallization, the acyl chain packing was in a 2-dimensional oblique lattice with specific chain-chain interactions with a tilt angle of 55.4.degree. from the bilayer plane. The thickness of the region containing 2 glycerol head groups was 12.7 .ANG.. The phase transition enthalpy of melting for the .beta. phase was 1.06 kcl/mol of CH2. On being cooled these compounds crystallized reversibly to an unsatble .alpha. phase, which on being further cooled underwent a 2nd crystallization to a .beta. or .beta.'' phase. The thermodynamic parameters and long spacings of these compounds in both .beta. and .alpha. phases were linear, indicating isostructural packing in each phase. The enthalpy of the melting transition of the .alpha. phases was 0.69 kcal/mol of CH2. In this phase, the chains were placed in a hexagonal lattice with non-specific chain-chain interactions. The thickness of the head-group region (12.2 .ANG.) and the tilt angle (55.degree.) of the acyl chains in the .alpha. phase were very similar to those in the .beta. phase.

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