FT-IR study of polymorphic transformations in SOS, POP, and POS

Study of molecular structures with Fourier transform (FT) infrared (IR) spectroscopy has been done on polymorphic transformations of three saturated-unsaturated mixed acid triacylglycerols (TAGs): sn-1,3-stearoyl-2-oleoyl glycerol (SOS), sn-1,3-palmitoyl-2-oleoyl glycerol (POP), and sn-1,3-palmitoylstearoyl-2-oleoyl glycerol (POS), in combination with X-ray diffraction and differential scanning calorimetry (DSC). Specific attention has been paid to absorption bands corresponding to methyl end packing, olefinic conformation, subcell structure, and glycerol structure. It was confirmed that the acyl chain packing, mainly expressed in the subcell structure and the olefinic conformation, is rather disordered in the less stable forms such as α and γ. The more ordered packing is revealed in the metastable form of β′, which was called pseudo-β′ in the authors' previous article.⁵ In the most stable forms of β₂ and β₁, well-ordered acyl chain packing and the olefinic conformation of the skew-cis-skew′ type are revealed. The chain length structure is reflected in the absorption bands of the terminal methyl end packing as well as of the glycerol group. The double chain length structure is revealed in the α forms of the three TAGs and in β′ of POP, whereas the triple chain length structure appears in γ, β′ except for POP, and β forms of the three TAGs. A polarized microprobe FT-IR technique using well-defined crystals was applied to clarify subtle differences between the two β forms of SOS, which appeared in the subcell structures of the oleoyl chains and the methyl end packing.