New Procedure To Calculate All Equilibrium Constants
in Flavylium Compounds: Application to the Copigmentation of Anthocyanins
Posted on 2019-07-12 - 08:13
A 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.
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Mendoza, Johan; Basílio, Nuno; Freitas, Victor de; Pina, Fernando (2019). New Procedure To Calculate All Equilibrium Constants
in Flavylium Compounds: Application to the Copigmentation of Anthocyanins. ACS Publications. Collection. https://doi.org/10.1021/acsomega.9b01066
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AUTHORS (4)
JM
Johan Mendoza
NB
Nuno Basílio
VF
Victor de Freitas
FP
Fernando Pina