Chemistryl of Anthocyanin Pigments. 9. UV-Visible Spectrophotometric determination of the Acidity Constants of Apigeninidin and Three Related 3-Deoxyflavinium Salts

Article abstract of DOI:10.1021/ja00390a033

The equilibrium constants for the structural transformations of some 3-deoxyanthocyanidins in water at 25 deg C have been measured by using the pH-jump method.This method have been described previously.According to their particular substitution patterns, hydroxylated flavinium salts can exist in slightly acidic media in several neutral forms: the quinoidal bases A, the carbinol pseudobase B, and the chalcone pseudobase C.Two of the conpounds investigated, namely 4',5,7-trihydroxyflavinium (apigeninidin) and 4'-methoxy-4-methyl-5,7-dihydroxyflavinium chlorides, exist essentially as a mixture of the three neutral forms A, B, and C, the colored species A being the most abundant.As expected, 4',7-dihydroxyflavinium chloride is stable in the open chalcone structiure C.This result is in a good agreement with the catalytic light effect generally observed for the ring-closure reaction of this species leading to the flavinium cation AH⁺.Only the monohydroxylated pigment 4'-methoxy-4-methyl-7-hydroxyflavinium chloride is the quinoidal base A perfectly stable, whatever the pH.In contrast to natural anthocyanins, the hydration of the pyrilium ring is less efficient and occurs, therefore, at much higher pH values (pH 5-6).Proton loss from the phenolic acidic hydroxyl groups of the flavinium cation takes place in the usual acidity range (pH 4-5), indicating that these groups are strongly hydrogen bonded to the surrounding water molecules.The chalcone content is much higher than for the anthocyanins, and for 4',7-dihydroxyflavinium chloride for instance, the value for the equilibrium ratio of the chalcone to the carbinol is as high as 20.6.