The packing of Triacylglycerols from SAXS measurements:: Application to the structure of 1,3-distearoyl-2-oleoyl-sn-glycerol crystal phases

An approach by which electron density profiles of triacylglycerols (TAGs) can be reconstructed from X-ray diffraction data has been applied to analyze lamellar structures formed by 1,3-distearoyl-2-oleoyl-sn-glycerol (StOSt). The method is analogous to one previously used to analyze lipid bilayer structures. Based on the structure of tristearin molecules crystallized in the beta phase, the projection of the electron density profile along the layer normal in structures with a lamellar packing of the TAG has been calculated for both tilted and nontilted arrangements of the molecules. This enables the position of glycerol groups to be identified in the electron density profile reconstructed from the first few 00l lamellar peaks measured by small-angle X-ray scattering (SAXS). It is demonstrated that structure amplitudes of both two-chain- and three-chain-length layers formed by the same equally tilted TAG molecules follow a unique curve on a plot of the structure amplitudes versus the scattering wave vector. The approach is applied to examine the complex polymorphism of StOSt during crystallization. Time-resolved SAXS/WAXS measurements have been performed during isothermal crystallization (from the liquid phase at 55 degreesC) at different temperatures (0-22.5 degreesC), followed by annealing at 30 degreesC. It is concluded that six phases (alpha(2), alpha(1), gamma, beta', beta(1), and beta(2)) and one metastable structure can be formed by StOSt molecules through polymorphic transformations. Fourier reconstructed projections of the electron density profile along the layer normal show that symmetry-related pairs of StOSt molecules adopt a two-chain packing motif in the alpha(1) phase and a three-chain packing motif in the more stable gamma, beta', and beta phases of StOSt. In accordance with the electron density profile, the unstable alpha(2) phase appearing at the initial stage of the crystallization can be ascribed to three-chain packing, which is probably the result of the twisted acyl chains of StOSt molecules. A combination of structural parameters obtained in the study, together with models for terrace-like arrangements of terminal methyl groups developed by de Jong et al., allowed the structure of gamma and beta phases to be clarified. The driving forces for the polymorphic transformations taking place during crystallization and a schematic diagram of phase transitions are discussed.