Structural transformations of the synthetic salt 4', 7-dihydroxyflavylium chloride in acid and basic aqueous solutions. Part 1 - Ground state

A complete study of the structural pH dependent transformations of the synthetic flavylium salt 4',7-dihydroxyflavylium chloride (DHF). occurring in aqueous solutions, including the basic region, is described. The kinetic study of the transformations occuring in acidic media (quinoidal base (A) ⇄ flavylium cation (AFT) ⇄ hemiacetal (B) ⇄ cis-chalcone (C_trans) ⇄ trans-chalcone (C_trans)) allowed to conclude that the cis-trans isomerization is faster than the tautomerization and the hydration processes, which is unique in the anthocyanins family. Results obtained with the parent compound 4",7dimethoxyflavylium chloride (DMF) with relevance to this study are also presented. In equilibrated basic solutions the existence of acid-base equilibria involving the trans-Chalcone (C_trans) and its conjugated bases. (C _transand C²_trans), was detected. Freshly prepared solutions at pH >7 show also the presence of a transient species identified as the ionized quinoidal base (A^-), which is almost completely converted into C^2-_trans with a pH dependent rate constant.