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

Abstract

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].

Structures of mono-unsaturated triacylglycerols. III. The β-2 polymorphs of trans-mono-unsaturated triacylglycerols and related fully saturated triacylglycerols

“…As explained in a previous publication (van Mechelen et al, 2006a), in view of the (nonatomic) resolution of the data one should be aware of local minima, implying that the molecular conformations and types of packing found by FOX do not necessarily correspond to those of the absolute minimum. Problems with local minimum conformations have also been encountered for the -2 structures discussed here.…”

“…The indexing of powder patterns of TAGs is complicated by dominant zone problems and severe peak overlap, as already explained in a previous publication (van Mechelen et al, 2006a). The program McMaille (Le Bail, 2004) was used to index the diffraction patterns.…”

“…The restraint values for distances and angles were mean values taken from the Cambridge Structural Database (CSD; Allen, 2002). Displacement parameters have not been refined as tests with cis-mono-unsaturated TAGs (van Mechelen et al, 2006a) and data of similar low resolution showed that this approach yields inconclusive results.…”

“…The pair of strong diffraction maxima at $ 4.6 Å (19.5 2, Cu K 1 ) that correspond to the (110) and (111) reflections is characteristic for triclinic -2 structures of saturated TAGs and mono-unsaturated trans TAGs. The -3 structures of mono-unsaturated cisTAGs have only the (111) reflection at this position in the fingerprint area (van Mechelen et al, 2006aMechelen et al, ,b, 2007. The (11')-zone reflections are related to the average plane defined Overview of the -2 patterns of symmetric TAG molecules with 2 scale converted to Cu K 1 radiation.…”

Structures of mono-unsaturated triacylglycerols. II. The β2 polymorph. Corrigendum

“…B62, 1131-1138] is actually a duplicate of Fig. 3 from van Mechelen et al [(2006a), Acta Cryst. B62, 1121-1130.…”