Color stabilization of malvidin 3-glucoside: self-aggregation of the flavylium cation and copigmentation with the Z-chalcone form

¹H NMR spectroscopy was used to characterize the aggregation processes leading to color stabilization of the natural anthocyanin, malvidin 3-glucoside. The concentrations of the different forms in aqueous solution were determined as a function of pH for several values of the total anthocyanin concentration. The chemical shifts were measured as a function of total concentration and temperature, and the concentration dependence of the T₁ values of relevant resonances were determined for different concentrations and pH values. The data are in agreement with a model that considers the occurrence of multimeric aggregates of flavylium cations at very acidic pH and copigmentation of flavylium cations with the Z-chalcone form at moderately acidic pH. The following equilibrium constants were determined: K_h = 0.0016 M for the flavylium cation hydration, K_T = 0.26 for the hemiacetal/E-chalcone tautomerism, K_i = 0.6 for the E-chalcone/Z-chalcone isomerization, K = 3700 M^-1 for the flavylium cation self-aggregation, and K′ = 3080 M^-1 for the flavylium cation/Z-chalcone copigmentation. The relevance of these results for color enhancement is discussed.