1960 年 80 巻 9 号 p. 1233-1237
A solution of α-lipoic acid and thiourea dissolved in butanol, with application of heat, when allowed to cool gradually, precipitates a crystalline addition compound. The analytical values of this addition compound suggest that it is composed of one mole of α-lipoic acid and 6.3 moles of thiourea. Ultraviolet spectrum, polarography, and biological activity all give values corresponding to this composition. The X-ray diffraction pattern of the adduct differs from those of α-lipoic acid, thiourea, or cyclohexane-thiourea adduct. Its lattice constants indicate that the adduct is monoclinic, differing from the rhombohedral form of the thiourea adduct. Infrared absorption spectrum of the adduct is almost the same as those of the known thiourea adducts except that the absorption of carbonyl of lipoic acid in the adduct is the same as that of lipoic acid in carbon tetrachloride solution, indicating that there is no interaction between lipoic acid molecules. Lipoic acid in this adduct is stable and does not change after 30 days at 38° in 92% relative humidity. Washing of this adduct with ethanol results in dissolution of lipoic acid and a part of thiourea remains as a rhombic thiourea. The adduct is also formed by standing a mixture of lipoic acid, thiourea, and a small quantity of methanol in a sealed vessel at 40° for four days. These facts suggest that α-lipoic acid-thiourea adduct is a new type of inclusion compound.