Structural models of metastable phases occurring during the crystallization process of saturated/unsaturated triacylglycerols

Abstract

Isothermal crystallizations of saturated and mixed saturated/unsaturated triacylglycerols (TAGs) have been studied by simultaneous time-resolved small-angle and wide-angle X-ray scattering measurements (SAXS/WAXS). The projection of the electron density profile along the layer normal derived from the lamellar peaks of the α2-phase can be assembled from two types of molecular dimers initially formed in the liquid state. One dimer corresponds to a typical two-chain packing of TAGs and the other is formed by opposing molecules overlapping by their two-acyl-chain sides. This structure occurs only in triacylglycerols containing both saturated and unsaturated acyl chains. The structural organization of the α2-phase is analogous to smectic A interdigitated phases. In the particular case of 1,3-distearoyl-2-oleoyl-sn-glycerol, StOSt, the α2-phase can evolve into a mixture of two phases (α1 and γ) with double and triple (2L and 3L) chain packing, respectively. A calculation of X-ray scattering patterns for simulated structures of randomly packed three-chain and two-chain layers of StOSt (2L + 3L) reproduced the three diffuse scattering peaks observed in the experimental SAXS patterns. © International Union of Crystallography 2007.