N-ヘテロ芳香族の電子的性状(第18報) : アスコルビン酸との相互作用によるリボフラビンの吸収スベクトルおよび溶解度の変化

抄録

The nature of the interaction between riboflavin and ascorbic acid, which was assumed from the brown coloration occurring upon mixing the components in high concentrations, was investigated by both spectrophotometric and solubility methods. The difference spectra of aqueous mixtures of riboflavin and ascorbic acid exhibited absorption maximum at 493 mμ (ε=2700), which was ascribed to semiquinone of riboflavin brought about by the formation of a complex of 1 : 1 molar ratio with ascorbic acid. The formation constant values at various pH's indicated the participation of non-ionized species in the complex formation. The values of ΔH and ΔS revealed that the complexation reaction was exothermic. These findings were further ascertained by examination of the solubility changes due to the complex formation. A new absorption band was observed at about 380 mμ in the spectra of non-aqueous mixtures of riboflavin and ascorbic acid, which were ascribable to π→π^* electronic transition. As to the correlation of the two absorption bands, it was presumed that charge transfer (appearance of the absorption band in the vicinity of 380 mμ) which takes place in non-aqueous media proceeds to the extent of formation of semiquinone (light absorption at 493 mμ) in aqueous media.