Structures and Binary Mixing Characteristics of Enantiomers of 1‐Oleoyl‐2,3‐dipalmitoyl‐sn‐glycerol (S‐OPP) and 1,2‐Dipalmitoyl‐3‐oleoyl‐sn‐glycerol (R‐PPO)

Abstract

AbstractWe performed differential scanning calorimetry (DSC) and synchrotron radiation X‐ray diffraction (SR‐XRD) experiments of polymorphic structures and binary mixing characteristics of the enantiomers of 1‐oleoyl‐2,3‐dipalmitoyl‐sn‐glycerol (S‐OPP) and 1,2‐dipalmitoyl‐3‐oleoyl‐sn‐glycerol (R‐PPO). In the two enantiomers, oleic and palmitic acid moieties are asymmetrically connected at the sn‐1 and sn‐3 positions of the glycerol groups, with palmitic acid moiety at the sn‐2 position. Pure enantiomer samples (>99 %) were synthesized and employed throughout this study. The following results were obtained. (1) A basic feature of the mixture systems of S‐OPP and R‐PPO is of a eutectic nature due to different polymorphic structures of two enantiomers and the racemic compound of PPO (rac‐PPO). (2) Polymorphic behavior of S‐OPP and R‐PPO was quite similar, both having α‐2 and β′‐3, whereas rac‐PPO contained α rac‐3, β′rac‐2, and β′rac‐3. The DSC measurements showed that the melting points of β′‐3 (S‐OPP = 35.3 °C and R‐PPO = 34.9 °C) were higher than that of β′rac‐3 (31.0 °C). β was not crystallized in the pure enantiomers, and rac‐PPO. (3) αrac‐3 was crystallized at low cooling rates (~2 °C/min), whereas α‐2 of the two enantiomers was crystallized only with very rapid cooling (~10 °C/min). (4) Triple‐chain‐length structures were formed in αrac‐3, β′S‐3 (=β′R‐3), and β′rac‐3; α‐2 with a double‐chain‐length structure was formed in both enantiomers. These results indicate the importance of the relationship between subcell packing and glycerol conformation in the polymorphism and mixing characteristics of asymmetric unsaturated–saturated‐saturated mixed‐acid triacylglycerols.