Journal of Lipid Research, Vol 28, 403-413, Copyright © 1987 by Lipid Research, Inc.
Structure and polymorphism of 18-carbon fatty acyl triacylglycerols: effect of unsaturation and substitution in the 2-position
DR Kodali, D Atkinson, TG Redgrave and DM Small
The polymorphic behavior of symmetric diacid triacylglycerols (TGs),
1,3-dioleoyl-2-stearoyl (OSO), 2-elaidoyl (OEO), and 2-vaccinoyl (OVO)
glycerols were studied by differential scanning colorimetry (DSC) and X-
ray diffraction and compared with the corresponding monoacid TGs triolein
(OOO), tristearin (SSS), trielaidin (EEE), and trivaccinin (VVV). The
monoacid TGs formed a bilayered structure in all the polymorphic forms. On
quenching from the melt, the diacid TGs OEO and OVO formed a bilayered (D =
45 A) beta'-phase with the exception of OSO, which formed a hexagonally
packed bilayered (D = 52 A) alpha- phase. At -7 degrees C, the alpha-phase
of OSO quickly transformed to a bilayered (D = 45 A) beta'-phase.
Incubation at the beta'-phase melting temperature transformed OVO, OEO, and
OSO into a trilayered (D = 65 A) beta-phase, where the 1,3-dioleoyl chains
are segregated from the vaccinoyl, elaidoyl, or stearoyl chains into
alternating layers. In summary, when all the acyl chains in a TG are the
same (saturated, cis or trans unsaturated), the stable beta-phase packs
into a bilayered structure. However, when the 1- and 3-acyl chains are cis
unsaturated (bent) and the 2-acyl chain is either saturated or
trans-unsaturated (straight), a bilayered beta'-phase can form, but
transforms to a stable trilayered beta-phase, where the 2-acyl chains form
a layer between two different layers of 1,3-oleoyl chains.