Synthesis and Polymorphism of 3-Acyl-sn-glycerols

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 l,2-isopropylidene-3-acyl-sn-glycerols from stearate to lignocerate was accomplished by applying the compounds to silica gel and exposing them to hydrogen chloride gas at -75 °C. The purity of the compounds was checked by boric acid impregnated thin-layer chromatography, 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 β phase obtained from solvent of crystallization, the acyl chain packing was in a two-dimensional oblique lattice with specific chain-chain interactions with a tilt angle of 55.4° from the bilayer plane. The thickness of the region containing two glycerol head groups was 12.7 Å. The phase transition enthalpy of melting for the β phase was 1.06 kcal/mol of CH2. On being cooled these compounds crystallized reversibly to an unstable α phase, which on being further cooled underwent a second crystallization to a β or β' phase. The thermodynamic parameters and long spacings of these compounds in both β and α phases were linear, indicating isostructural packing in each phase. The enthalpy of the melting transition of the α phase was 0.69 kcal/mol of CH2. In this phase, the chains were packed in a hexagonal lattice with nonspecific chain-chain interactions. The thickness of the head-group region (12.2 Å) and the tilt angle (55°) of the acyl chains in the α phase were very similar to those in the β phase.