Flavylium salts, anthocyanidins and anthocyanins. I. Structural transformations in acid solutions

Structural transformations in acid solutions of some flavylium salts, anthocyanidins and anthocyanins have been studied by spectrum and pH measurements. The stabilities of the coloured cationic forms were increased by substitution of hydroxyl groups in positions 7 and 5 of the A ring, but decreased by hydroxylation in the 3-position (anthocyanidins). The anthocyanin 3,5-diglycosides were stronger acids than the 3-glycosides. The effect of substituents on anhydro-base formation is also described. The results are discussed in relation to the possible use of flavylium salts as food additives.