CRYSTAL-STRUCTURES AND MELTING-POINTS OF SATURATED TRIACYLGLYCEROLS IN THE BETA-3 PHASE

A comprehensive packing analysis is presented of the crystal structures of a large number of saturated triacylglycerols in the β-3 phase. The triacylglycerols p.q.r have been grouped into four classes:r=p, p+2, p+4 and p+6. The length of the middle chain, q, dictates whether a β-2 or β-3 packing occurs. The latter packing arrangement is adopted when q differs at least 4 from p or r. The model-building approach starts from the known molecular conformation of β-2 10.10.10 and the T{norm of matrix} subcell packing mode of the hydrocarbon chains. Purely geometrical model-building allows a preliminary assignment of crystal structures. Crystal lattice energy calculations using the atom-atom potential method support these tentative assignments and clarify some details regarding the stacking of layers. In common with the β-2 phase crystal structures, the structure of the terrace-like arrangement of the terminal methyl groups plays a crucial role. Triacylglycerols with chains 1 and 3 differing 2 or 4 carbon atoms have a common arrangement of the end-methyl groups (type β-3 A). Those with a difference of 0 (i.e. symmetrical) or 6 carbon atoms have a different, less favorable methyl terrace (type β-3 B). The available experimental evidence (x-ray powder diffraction, unit-cell data and melting points) is entirely compatible with these proposals. © 1990 American Oil Chemists' Society.