Structural study on polymorphism of cis-unsaturated triacylglycerol: Triolein

To clarify the influence of cis-unsaturation on solid-state structures of triacylglycerols (TAGs), the crystal structures of three crystalline phases (α, β′ and β) of triolein [C₃H ₅(OCOC₁₇H₃₃)₃] were investigated by powder X-ray diffractometry and IR and Raman spectroscopy. The influence on the structural change of the a phase in the course of cooling was also studied. With respect to the subcell structure and conformational order of hydrocarbon chains in the β and β′ phases, triolein resembles saturated TAGs; trans-zigzag hydrocarbon chains are adopted in the T_∥ subcell for the β phase and in the O_⊥ subcell for the β′ phase. The influence of cis-unsaturation was most obvious in the structure of the α phase and its temperature dependence. The α phase of triolein does not form the ordinary hexagonal subcell but a rather loose distorted subcell, which hardly changes in cooling, forming a striking contrast to the hexagonal → pseudohexagonal subcell transformation found in the a phase of saturated TAGs.