To clarify the influence of cis-unsaturation on solid-state structures of triacylglycerols (TAGs), the crystal structures of three crystalline phases (alpha, beta' and beta) of triolein [C3H5(OCOC17H33)3] were investigated by powder X-ray diffractometry and IR and Raman spectroscopy. The influence on the structural change of the alpha phase in the course of cooling was also studied. With respect to the subcell structure and conformational order of hydrocarbon chains in the beta and beta' phases, triolein resembles saturated TAGs; trans-zigzag hydrocarbon chains are adopted in the T(parallel) subcell for the beta phase and in the O(perpendicular) subcell for the beta' phase. The influence of cis-unsaturation was most obvious in the structure of the alpha phase and its temperature dependence. The alpha phase of triolein does not form the ordinary hexagonal subcell but a rather loose distorted subcell, which hardly changes in cooling, forming a striking contrast to the hexagonal-->pseudohexagonal subcell transformation found in the alpha phase of saturated TAGs.

The reversible solid-state phase transition between the γ and α phases of oleic acid was followed by 13C solid-state NMR spectroscopy. All peaks in the γ and α phases were assigned based on the corresponding crystal structures determined by X-ray diffraction method (Abrahamsson and Ryderstedt-Nahringbauer, 1962, and Kaneko et al., 1996). As to the transition behavior, there was a significant difference between the two hydrocarbon chains straddling cis -olefin group. On the γ → α phase transition at 271 K, the methyl-sided methylene carbons exhibited upfield shifts due to the gauche effect. Characteristic peaks shifts due to the conformational change of the cis -olefin group were observed for cis -olefin carbons. On the other hand, the peaks due to carboxyl-sided chains showed no large shifts on this transition. Each oleic acid carbon atom showed a discontinuous decrease in T 1, except for the methyl carbon, which was in the extreme narrowing limit even in the γ phase. However, the T 1 values of the methyl-sided carbons were significantly smaller than those of the carboxyl-sided carbons in the α phase. The conformational disordering in the methyl-sided chains was accompanied by a remarkable increase of molecular motion in this moiety.

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Two polytypes for the monoclinic (M011) modification of n-hexatriacontane (n-C36H74), single-layered structure Mon and double-layered structure Orth II, were studied by means of the solid-state CP/MAS 13C NMR spectroscopy. The terminal methyl carbon showed clear differences in the chemical shift as well as spin-lattice relaxation time (T1) between Mon and Orth II, in contrast to no marked difference for the other carbons.