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: Kh = 0.0016 M for the flavylium cation hydration, KT = 0.26 for the hemiacetal/E-chalcone tautomerism, Ki = 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.
- Flavylium cation