Structural analyses of polymorphic transitions of sn -1,3-distearoyl-2-oleoylglycerol (SOS) and sn -1,3-dioleoyl-2-stearoylglycerol (OSO): assessment on steric hindrance of unsaturated and saturated acyl chain interactions

Abstract

Polymorphic transformations in two saturated-unsaturated mixed acid triacylglycerols, SOS (sn-1,3-distearoyl-2-oleoylglycerol) and OSO (sn-1,3-dioleoyl-2-stearoylglycerol), have been studied by FT-IR spectroscopy using deuterated specimens in which stearoyl chains are fully deuterated. A reversible phase transition between sub α and α and a series of irreversible transitions (α→γ→β′→β (β2, β1) for SOS and α→β′→β for OSO) were studied with an emphasis on the conformational ordering process of stearoyl and oleoyl chains. The α→sub α reversible transition was due to the orientational change of stearoyl chains in the lateral directions from the hexagonal subcell to a perpendicularly packed one. As the first stage of the series of irreversible transitions from α to β, the conformational ordering of saturated chains took place in the α→γ transition of SOS and in the α→β′ transition of OSO; one stearoyl chain in SOS and OSO takes the all-trans conformation and the second stearoyl chain in SOS takes the bent conformation like those observed in the most stable β-type. As the final stage, the ordering of unsaturated chains occurred in the β′→β transition both for SOS and OSO. A conversion in the layered structure from bilayer to trilayer was also accompanied by the conformational ordering in the α→γ transition of SOS and in the β′→β transition of OSO.—Yano, J., K. Sato, F. Kaneko, D. M. Small, and D. R. Kodali. Structural analyses of polymorphic transitions of sn-1,3-distearoyl-2-oleoylglycerol (SOS) and sn-1,3-dioleoyl-2-stearoylglycerol (OSO): assessment on steric hindrance of unsaturated and saturated acyl chain interactions. J. Lipid Res. 1999. 40: 140–151.

Keywords

• polymorphism
• phase behavior
• infrared spectroscopy
• molecular packing
• molecular conformation
• deuteration