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 (alpha', beta' and beta) of triolein [C3H5(OCOC17H33)(3)] were investigated by powder X-ray diffractometry and IR and Raman spectroscopy. The influence on the structural change of the (x phase in the course of cooling was also studied. With respect to the subcell structure and conformational order of hydrocarbon chains in the beta and beta' phases, triolein resembles saturated TAGs; trans-zigzag hydrocarbon chains are adopted in the T-vertical bar vertical bar subcell for the beta phase and in the beta' subcell for the beta' phase. The influence of cis-unsaturation was most obvious in the structure of the alpha phase and its temperature dependence. The alpha 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.