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New Procedure To Calculate All Equilibrium Constants in Flavylium Compounds: Application to the Copigmentation of Anthocyanins
journal contribution
posted on 2019-07-12, 08:13 authored by Johan Mendoza, Nuno Basílio, Victor de Freitas, Fernando PinaA new experimental
procedure to calculate all equilibrium constants
of the multistate of species of anthocyanins and related compounds,
including those in basic medium, is reported. The procedure is based
on a series of pH jumps monitored by stopped flow from an extended
pH range of solutions at pseudo-equilibrium (when there is no significant
formation of trans-chalcones) or at equilibrium to
pH = 1.0. The experimental procedure is described for the anthocyanin
model compound 4′-hydroxyflavylium, which exhibits a peculiar
behavior in moderately acidic medium, because the quinoidal base,
hemiketal, and cis-chalcone have similar mole fractions
at pseudo-equilibrium, permitting good discrimination among these
species. The experimental procedure can be extended to the copigmentation
phenomenon and allow the calculation of the 1:1 copigmentation constants
of the flavylium cation, quinoidal base, hemiketal, and cis- and trans-chalcones (this last from the equilibrium)
and their respective ionized forms. The method was applied to calculate
the copigmentation constants of the model compound 4′-hydroxyflavylium
as well as malvidin-3-glucoside with caffeine. In the last compound,
the strongest interaction takes place with the quinoidal base (K = 303 M–1) and flavylium cation (K = 134 M–1) and, to a lesser extent,
with the ionized quinoidal base (K = 43 M–1) and cis-chalcone (K = 17 M–1). The caffeine interaction with the hemiketal and
the other ionized species is negligible.