Polymorphic behavior of 1,2-dipalmitoyl-3-lauroyl(PP12)- and 3-myristoyl(PP14)-sn-glycerols.

The polymorphic behavior and molecular packing in different polymorphic forms of stereospecific triacylglycerols, 1,2-dipalmitoyl-3-lauroyl-sn-glycerol (PP12) and 1,2-dipalmitoyl-3-myristoyl-sn-glycerol (PP14) were examined by X-ray diffraction, differential scanning calorimetry, infrared and Raman spectroscopy techniques. The molecular packing and the polymorphic behavior of the metastable forms of these two compounds are very similar. In both compounds the isotropic liquid, on quenching, crystallizes into a hexagonally packed alpha-phase. The long spacing periodicity of the alpha-phase indicates a tilted bilayer structure to compensate the voids created by the short acyl chains. Upon heating, the alpha-phase converts into an orthorhombic perpendicular (O perpendicular) beta'2-phase. The beta' 2-phase, on further heating, exothermally converts to beta' 1-phase with slightly different O perpendicular subcell. On incubation of PP12 near the melting temperature of beta' 1-phase, there is a slow (hours) conversion to a beta-phase with triclinic parallel (T//) packing. However the beta' 1-phase of PP14 is the most stable structure and the beta-phase is absent. The thermodynamic parameters of the O perpendicular packings of these compounds compared to those of the higher homologue, tripalmitoylglycerol, suggest that the O perpendicular subcell is more stable in PP12 and PP14. The X-ray diffraction long spacings indicate that all the polymorphic forms of these compounds pack in a bilayer structure. The vibrational spectra confirm the lateral chain packing and inter- and intra-molecular order/disorder in the various polymorphic forms. The Raman and infrared spectra further indicate perturbation in the carbonyl and the end methyl plane regions of the beta'-phases.