Structures of mono-unsaturated triacylglycerols. IV. The highest melting beta'-2 polymorphs of trans-mono-unsaturated triacylglycerols and related saturated TAGs and their polymorphic stability.

The beta'(1)-2 crystal structures of a series of mixed-chain saturated and trans-mono-unsaturated triacylglycerols containing palmitoyl, stearoyl and elaidoyl acyl chains have been solved from high-resolution powder diffraction data, from synchrotron as well as laboratory X-ray sources. The structures crystallized in the space group I2 with two independent molecules forming a dimer in the asymmetric unit, and packed in double-chain length layers. Unlike the corresponding beta-2 structures the solved beta'(1)-2 structures have different molecular conformations for the symmetric and the asymmetric mixed triacylglycerols, both with the sn-2 chain in a leg position of the chair-shaped conformation. A transformation to the beta-2 structure with the sn-2 chain in the back position is complicated and unlikely to take place in the solid state. A novel beta'-2 polymorph of PSS has been crystallized and its structure has been solved. The melting point (239 K) of this so-called beta'(0)-2 polymorph is 2 K above that of the beta'(1)-2 polymorph and almost equal to that of the beta-2 polymorph of PSS. The difference in packing of the beta'(0)-2 versus beta'(1)-2 structure explains the slow beta'(1)-2 to beta'(0)-2 phase transition. The transition is strikingly similar to the beta(2)-3 to beta(1)-3 transition in cis-mono-unsaturated triacylglycerols.