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

The crystal structures of the β₁ 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 (β₁-MOM), 1,3-dipalmitoyl-2-oleoylglycerol (β₁-POP), 1,3-distearoyl-2-oleoylglycerol (β₁-SOS), 1-palmitoyl-2-oleoyl-3-stearoylglycerol (β₁-POS), 1-stearoyl-2-oleoyl-3-arachidoylglycerol (β₁-SOA) and three mixtures: the co-crystallized 1:1 molar mixture of SOS and POP [β₁-SOS/POP (1:1)] and two cocoa butters from Bahia and Ivory Coast, both in their β-VI (= β₁) polymorph. All eight β₁ structures crystallized in the space group (P2₁/n) and have two short cell axes (5.44–5.46 and 8.18–8.22 Å), as well as a very long b axis (112–135 Å). 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 β₁ structures is a step forward towards understanding the mechanism of fat-bloom formation in dark chocolate and has led to a reexamination of the β₂ structural model [see van Mechelen et al. (2006). Acta Cryst. B62, 1131–1138].