Quantitative measurement of proton dissociation and tautomeric constants of apigeninidin

Depending on the pH of the medium, the anthocyanidin apigeninidin [5,7-dihydroxy-2-(4'-hydroxyphenyl) benzopyrilium chloride] may deprotonate, leading to the formation of three tautomeric neutral and anionic forms. In order to determine the relative percentages of the various forms which are present in solution at a given pH, the three possible derivatives of apigeninidin in which two hydroxys are replaced by methoxy groups and the three in which only one hydroxy is replaced by a methoxy group were synthesized. From the first three derivatives only one neutral form and from the second three only one anionic form can be generated by proton dissociation; the study of these compounds made possible the measurement of the proton dissociation constants of each tautomer of apigeninidin and allowed us to calculate the tautomeric ratio of its three neutral and anionic forms. Moreover, our results give evidence of the existence of a dianionic form present at pH close to neutrality (pK'(a) = 8.06). From the results obtained it is possible to calculate the composition of the mixture of the different species which can originate from the cationic form of apigeninidin at any pH by proton dissociation and tautomeric equilibria.