The Packing of Triacylglycerols from SAXS Measurements:  Application to the Structure of 1,3-Distearoyl-2-oleoyl-sn-glycerol Crystal Phases

Abstract

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 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-chainlength 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°C) at different temperatures (0-22.5°C), followed by annealing at 30°C. It is concluded that six phases (R 2 , R 1 , γ, ′, 1 , and 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 R 1 phase and a three-chain packing motif in the more stable γ, ′, and phases of StOSt. In accordance with the electron density profile, the unstable R 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 γ and phases to be clarified. The driving forces for the polymorphic transformations taking place during crystallization and a schematic diagram of phase transitions are discussed.

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

“…1 and 2, Table 1). The 2 -phase has been identified previously in phase transformations of StOSt (Mykhaylyk & Hamley, 2004). These observations and others (Kodali et al, 1987(Kodali et al, , 1990Lavigne et al, 1993;Lutton et al, 1948) suggest that the formation of the 2 -phase is totally dependent on the presence of both saturated and unsaturated acyl chains in TAG molecules.…”

“…Assuming this packing of StOSt molecules in the 2 -phase, the mass density of this phase should be about 1.41 g cm À3 (based on a lamellar period 54.3 Å and a hexagonal period of acyl chains in the lamellar a hex~4 .73 Å ), which is very large even in comparison with the most stable -phases of TAGs [ < 1.05 g cm À3 (Small, 1986)]. This unphysical estimate of density and the observation that the assigned structural amplitudes of the 2 -phase had a poor agreement with the unique curve of structural amplitudes established for the 1 -, -and 0 -phases of StOSt (Mykhaylyk & Hamley, 2004) suggests that the real projection of the electron density profile on the layer normal of the 2 -phase is different from that proposed previously.…”

“…3a) and a corresponding packing of StOSt molecules with a 'twisted' structure of acyl chains has been suggested (Mykhaylyk & Hamley, 2004). Assuming this packing of StOSt molecules in the 2 -phase, the mass density of this phase should be about 1.41 g cm À3 (based on a lamellar period 54.3 Å and a hexagonal period of acyl chains in the lamellar a hex~4 .73 Å ), which is very large even in comparison with the most stable -phases of TAGs [ < 1.05 g cm À3 (Small, 1986)].…”

“…where z is a coordinate along the layer normal, l are the Miller indices of the lamellar peaks, |F(l)| are the corresponding structural amplitudes obtained from SAXS patterns, L is the period of the lamellar structure and m(l) = F(l)/|F(l)| = AE1 are phase signs (Mykhaylyk & Hamley, 2004).…”

“…Thus, to understand polymorphic transformations taking place in TAGs, especially in natural fats, the structures of various unstable phases have to be determined. Some attempts have already been made to interpret these structures with little or no success (Loisel et al, 1998;Mykhaylyk & Hamley, 2004;Ueno et al, 1997). The work presented herein extends the application of smallangle X-ray scattering (SAXS) methods to the structural analysis of stable phases in saturated/unsaturated 1,3-distearoyl-2-oleoyl-snglycerol (StOSt) (Mykhaylyk & Hamley, 2004) and is dedicated to modelling the structural organization of metastable phases appearing in saturated/unsaturated TAGs at the very beginning of their crystallization from the melt.…”