Acta Crystallographica Section B

Structural Science

Volume 62, Part 6 (December 2006)

research papers

   

Structures of mono-unsaturated triacylglycerols. II. The ₂ polymorph

J. B. van Mechelen, R. Peschar and H. Schenk

Abstract: An improved crystal structure model has been established for the ₂ polymorph of the symmetric mono-unsaturated triacylglycerol 1,3-distearoyl-2-oleoylglycerol (SOS) and the equivalent -V polymorph of Ivory Coast cocoa butter. In addition, the crystal structures of the ₂ polymorphs are reported for the triacylglycerols 1,3-dipalmitoyl-2-oleoylglycerol (POP) and 1-palmitoyl-2-oleoyl-3-stearoylglycerol (POS), which are, together with SOS, the major components of cocoa butter, and that of 1-stearoyl-2-oleoyl-3-arachidoylglycerol (SOA). The existence of ₂-POS and ₂-SOA has not been previously reported in the literature. All structures have been solved from high-resolution laboratory or synchrotron powder diffraction data with the direct-space parallel-tempering program FOX and refined with the Rietveld module of GSAS. All compounds crystallize in similar monoclinic unit cells (Cc) with very long b axes (> 127 Å). The oleic chains are packed together and sandwiched between saturated chain layers, forming acyl-chain three-packs. An analysis of the ₂ polymorphs and ₁ polymorphs [van Mechelen et al. (2006). Acta Cryst. B62, 1121-1130] shows that they contain the same three-packs and differ only in the symmetry relation between the three-packs. The three-pack build-up provides an explanation of the mechanism of the phase transition that causes the formation of fat bloom on dark chocolate.

Keywords: powder diffraction; crystal structure; ₂ polymorph; triacylglycerol; cocoa butter.

	Rietveld powder data file (CIF format) (246.5 kbytes) [ doi:10.1107/S0108768106037086/dr5012SOS-B2-XPERT-ENAN2_publsup2.rtv ] Contains datablock SOS-B2-XPERT-ENAN2_publ
	Rietveld powder data file (CIF format) (284.8 kbytes) [ doi:10.1107/S0108768106037086/dr5012IV14_B5_280K-ENAN_publsup4.rtv ] Contains datablock IV14_B5_280K-ENAN_publ
	Rietveld powder data file (CIF format) (280.3 kbytes) [ doi:10.1107/S0108768106037086/dr5012POP-B2-CH1731-2_ENAN_publsup6.rtv ] Contains datablock POP-B2-CH1731-2_ENAN_publ
	Rietveld powder data file (CIF format) (173.8 kbytes) [ doi:10.1107/S0108768106037086/dr5012POS-B2-ENAN_publsup8.rtv ] Contains datablock POS-B2-ENAN_publ
	Rietveld powder data file (CIF format) (213.4 kbytes) [ doi:10.1107/S0108768106037086/dr5012SOA-B2CAP2-XPERT-EXS_publsup10.rtv ] Contains datablock SOA-B2CAP2-XPERT-EXS_publ

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