COLOR STABILIZATION OF MALVIDIN 3-GLUCOSIDE - SELF-AGGREGATION OF THEFLAVYLIUM CATION AND COPIGMENTATION WITH THE Z-CHALCONE FORM

H-1 NMR spectroscopy was used to characterize the aggregation processe s leading to color stabilization of the natural anthocyanin, malvidin 3-glucoside. The concentrations of the different Forms in aqueous solu tion were determined as a function of pH for several values of the tot al anthocyanin concentration. The chemical shifts were measured as a f unction of total concentration and temperature, and the concentration dependence of the T-1 values of relevant resonances were determined fo r different concentrations and pH values. The data are in agreement wi th a model that considers the occurrence of multimeric aggregates of f lavylium cations at very acidic pH and copigmentation of flavylium cat ions with the Z-chalcone form at moderately; acidic pH. The following equilibrium constants were determined: K-h = 0.0016 M for the flavyliu m cation hydration, K-T = 0.26 for the hemiacetal/E-chalcone tautomeri sm, 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 re sults for color enhancement is discussed.