Journal of the Chemical Society - Faraday Transactions p. 1693 - 1699 (1996)
Update date:2022-08-16
Topics:
Pina, Fernando
Benedito, Lina
Melo, Maria Joao
Parola, A. Jorge
Bernardo, Alexandra
The synthetic compound 3,4′-dimethoxy-7-hydroxyflavylium chloride gives rise, in aqueous solution at moderately acidic pH, to a pH-dependent equilibrium between the flavylium cation, hemiacetal, (Z)-chalcone and a small amount of quinonoidal base. The distribution, as a function of pH, of the molar fractions of the several species present in solution have been calculated on the basis of 1H NMR and pH jump experiments monitored by stopped-flow and conventional UV-VIS spectrophotometry, and high-performance liquid chromatography (HPLC). The compound shows interesting photochemical properties: (i) at pH 4.0 it presents a photochromic effect that converts (Z)-chalcone into hemiacetal, the reaction being reversible in the dark and (ii) excited-state proton transfer is observed between the flavylium cation and quinonoidal base. An appropriate formalism to quantify the experimental results has been developed. The formalism allows determination of the pH-dependent molar fraction distribution of the several anthocyanin forms present at equilibrium, as well as predicting the distribution of the molar fractions prior to equilibrium.
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