Molecular properties of polymorphic forms of gondoic acid [cis-C 20:1 ∆11ω9 (GOA)] have been studied by X-ray diffraction (XRD), differential scanning calorimetry (DSC), optical microscopy, and Raman scattering, in comparison to those of six principal unsaturated fatty acids: oleic acid [cis-C 18:1 ∆9ω9 (OA)], erucic acid [cis-C 22:1 ∆13ω9 (ERA)], petroselinic acid [cis-C 18:1 ∆6ω12 (PSA)], asclepic acid [cis-C 18:1 ∆11ω7 (APA)], palmitoleic acid [cis-C 16:1 ∆9ω7 (POA)], and elaidic acid [trans-C 18:1 ∆9ω9 (ELA)]. In addition, phase behavior of binary mixtures of GOA and APA and OA was examined by XRD and DSC. The polymorphic structures of GOA are quite similar to those of APA, ERA, POA, and partly to OA. In particular, DSC and Raman scattering studies have shown that gondoic acid exhibits conformational disordering on heating at the ω-chain, a chain segment between the double bond and CH 3 group, as a transition from alltrans (γ form) to gauche-rich (α form) conformations. A miscible mixing phase was observed in the mixture of GOA and APA, yet eutectic phases were observed in the GOA and OA mixtures. This is a remarkable contrast because the binary mixture systems of varying combinations of cis-unsaturated fatty acids examined so far exhibited either eutectic nature or molecular compound formation. It is expected that specific molecular interactions between GOA and APA that originate from the equivalence of the length of the ∆-chain, the chain segment between the cis-double bond and COOH group, and also from the presence of the γ-α order-disorder transformation would be operating to form the miscible mixing phase. JAOCS 74, 1153JAOCS 74, -1159JAOCS 74, (1997.KEY WORDS: Asclepic acid, differential scanning calorimetry, gondoic acid, oleic acid, phase behavior of binary mixtures, polymorphism in unsaturated fatty acids, X-ray diffraction.
FIG. 1.A molecular model of γ-α transformation observed in oleic acid, erucic acid, asclepic acid, and palmitoleic acid.