Acta Crystallographica Section B

Structural Science

Volume 64, Part 2 (April 2008)

research papers

Acta Cryst. (2008). B64, 249-259 [ doi:10.1107/S0108768108004825 ]

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

J. B. van Mechelen, R. Peschar and H. Schenk

Abstract: The [beta] $_1^{\prime}$ -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] $_1^{\prime}$ -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] $_0^{\prime}$ -2 polymorph is 2 K above that of the [beta] $_1^{\prime}$ -2 polymorph and almost equal to that of the [beta] -2 polymorph of PSS. The difference in packing of the [beta] $_0^{\prime}$ -2 versus [beta] $_1^{\prime}$ -2 structure explains the slow [beta] $_1^{\prime}$ -2 to [beta] $_0^{\prime}$ -2 phase transition. The transition is strikingly similar to the [beta] ₂-3 to [beta] ₁-3 transition in cis-mono-unsaturated triacylglycerols.

Keywords: triacylglycerols; powder diffraction; ' polymorph.