Structures of mono-unsaturated triacylglycerols. I. The beta1 polymorph.

The crystal structures of the beta1 polymorphs of mono-unsaturated triacylglycerols have been solved from high-resolution laboratory and synchrotron powder diffraction data for five pure compounds, the 1,3-dimyristoyl-2-oleoylglycerol (beta1-MOM), 1,3-dipalmitoyl-2-oleoylglycerol (beta1-POP), 1,3-distearoyl-2-oleoylglycerol (beta1-SOS), 1-palmitoyl-2-oleoyl-3-stearoylglycerol (beta1-POS), 1-stearoyl-2-oleoyl-3-arachidoylglycerol (beta1-SOA) and three mixtures: the co-crystallized 1:1 molar mixture of SOS and POP [beta1-SOS/POP (1:1)] and two cocoa butters from Bahia and Ivory Coast, both in their beta-VI (=beta1) polymorph. All eight beta1 structures crystallized in the space group (P2(1)/n) and have two short cell axes (5.44-5.46 and 8.18-8.22 A), as well as a very long b axis (112-135 A). The dominant-zone problem in the indexing of the powder patterns was solved with the special brute-force indexing routine LSQDETC from the POWSIM program. Structures were solved using the direct-space parallel-tempering method FOX and refined with GSAS. Along the b axis, alternations of inversion-centre-related ;three-packs' can be discerned. Each ;three-pack' has a central oleic zone, with oleic acyl chains of the molecules being packed together, that is sandwiched between two saturated-chain zones. The conformation of the triacylglycerol molecules is relatively ;flat' because the least-square planes through the saturated chains and those through the saturated parts of the olein chain are parallel. The solution of the beta1 structures is a step forward towards understanding the mechanism of fat-bloom formation in dark chocolate and has led to a reexamination of the beta2 structural model [see van Mechelen et al. (2006). Acta Cryst. B62, 1131-1138].