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

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.