Structure and polymorphism of 18-carbon fatty acyl triacylglycerols: Effect of unsaturation and substitution in the 2-position

The polymorphic behavior of symmetric diacid triacylglycerols (TGs), 1,3-dioleoyl-2-stearoyl (OSO), 2-elaidoyl (OEO), and 2-vaccinoyl (OVO) glycerols were studied by differential scanning colorimetry (DSC) and X-ray diffraction and compared with the corresponding monoacid TGs triolein (OOO), tristearin (SSS), trielaidin (EEE), and trivaccinin (VVV). The monoacid TGs formed a bilayered structure in all the polymorphic forms. On quenching from the melt, the diacid TGs OEO and OVO formed a bilayered (D = 45 Å) β'-phase with the exception of OSO, which formed a hexagonally packed bilayered (D = 52 Å) α-phase. At -7°C, the α-phase of OSO quickly transformed to a bilayered (D = 45 Å) β'-phase. Incubation at the β'-phase melting temperature transformed OVO, OEO, and OSO into a trilayered (D = 65 Å) β-phase, where the 1,3-dioleoyl chains are segregated from the vaccinoyl, elaidoyl, or stearoyl chains into alternating layers. In summary, when all the acyl chains in a TG are the same (saturated, cis or trans unsaturated), the stable β-phase packs into a bilayered structure. However, when the 1- and 3-acyl chains are cis unsaturated (bent) and the 2-acyl chain is either saturated or trans-unsaturated (straight), a bilayered β'-phase can form, but transforms to a stable trilayered β-phase, where the 2-acyl chains form a layer between two different layers of 1,3-oleoyl chains.