Structures of mono-unsaturated triacylglycerols.: I.: The β1 polymorph

The crystal structures of the beta(1) polymorphs of monounsaturated triacylglycerols have been solved from high-resolution laboratory and synchrotron powder diffraction data for five pure compounds, the 1,3-dimyristoyl-2-oleoylglycerol (beta 1-MOM), 1,3-dipalmitoyl-2-oleoylglycerol (beta(1)-POP), 1,3-distearoyl-2-oleoylglycerol (beta(1)-SOS), 1-palmitoyl-2-oleoyl-3-stearoylglycerol (beta(1)-POS), 1-stearoyl-2-oleoyl-3-arachidoylglycerol (beta 1-SOA) and three mixtures: the co-crystallized 1:1 molar mixture of SOS and POP [beta(1)-SOS/POP (1:1)] and two cocoa butters from Bahia and Ivory Coast, both in their beta-VI (= beta 1) polymorph. All eight beta 1 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 beta 1 structures is a step forward towards understanding the mechanism of fat-bloom formation in dark chocolate and has led to a reexamination of the beta(2) structural model.