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 triacyl glycerols, SOS (Sn-1,3-distearoyl-2-oleoylglcerol) and OSO (sn-1,3-dioleoyl -2-stearoyl-glycerol), have been studied by ET-IR spectroscopy using deuter ated specimens in which stearoyl chains are fully deuterated, A reversible phase transition between sub alpha and alpha and a series of irreversible t ransitions (alpha-->gamma-->beta'-->beta (beta(2), beta(1)) for SOS and alp ha-->beta'-->beta for OSO) were studied with an emphasis on the conformatio nal ordering process of stearoyl and oleoyl chains. The alpha-->sub alpha r eversible 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 fr om alpha to beta, the conformational ordering of saturated chains took plac e in the alpha-->gamma transition of SOS and in the alpha-->beta' transitio n of OSO; one stearoyl chain in SOS and OSO takes the all-trans conformatio n and the second stearoyl chain in SOS takes the bent conformation like tho se observed in the most stable P-type, As the final stage, the ordering of unsaturated chains occurred in the beta'-->beta transition both for SOS and OSO, A conversion in the layered structure from bilayer to trilayer was al so accompanied by the conformational ordering in the alpha-->gamma transiti on of SOS and in the beta'-->beta transition of OSO.