Alejo-Armijo, A., Moro, Artur J., Parola, A. Jorge, Lima, João Carlos, Pina, Fernando, Corici, Livia, Shova, Sergiu, and Cseh, Liliana
Abstract The thermodynamic and kinetic behavior of 4-(2,4-dihydroxybenzylidene)-6-hydroxy-1,2,3,4-tetrahydroxanthylium chloride was studied in detail. Similarly to anthocyanins, this compound generates a multistate of species that are reversibly interconverted by pH stimuli. The multistate system was studied by NMR and UV–visible (conventional spectroscopy and stopped-flow techniques). A theoretical approach adapted from the one reported for anthocyanins and related compounds was used to rationalize the complex multistate. In particular, the thermodynamic diagram where all the species are positioned was extended to the ionized species. Cis and trans isomers of flavylium cation, quinoidal bases, and ionized chalcone at extremely high pH values were identified. The multistate is dominated in the range 5 < pH < 8 by the spiro form in equilibrium with a small fraction of quinoidal bases. The spiro opening rates follow a different route and pH dependence in very acidic or moderately acidic pHs. Graphical abstract Image 1 Highlights • 2,6-bis(2-hydroxybenzylidene)cyclohexanones follow the general anthocyanins multistate of species. • A thermodynamic and kinetic model adapted from the one reported for anthocyanins and related compounds, was used to rationalize the complex multistate data for 4-(2,4-dihydroxybenzylidene)-6-hydroxy-1,2,3,4-tetrahydroxanthylium chloride. • A spiro derivative is the species that differentiates the present system from anthocyanins. It was fully characterized by NMR and is the dominant species in the pH range 5