Alternation of Melting Points in Odd- and Even-Numbered Monoacid Triacylglycerols

Abstract

Alternation of melting points in the series of odd- and even-numbered monoacid triacylglycerols has been known for a long time, but hitherto little experimental evidence has been available to support existing theories. In this paper the melting point alternation of β-CnCnCn-type triacylglycerols is discussed, based on the crystal structures of β-CnCnCn (n = number of C atoms per hydrocarbon chain = 10, 12, 14, 16, 18 and n = 13, described in this paper). The crystal structure of β-1,2,3-tris(tridecanoyl)glycerol (β-C13C13C13) has been determined from high-resolution synchrotron X-ray powder diffraction data and is presented here. Grid search and Rietveld refinement have been used to determine and refine the structure, respectively. Like the even-numbered monoacid triacylglycerols, β-C13C13C13 is crystallized in an asymmetric tuning-fork conformation and its acyl chains are laterally packed, resulting in a layered structure. Within a molecular layer, the odd- and even-numbered monoacid triacylglycerols are identically packed, but the packing of adjacent molecular layers is different, clarifying the alternation of melting points.