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Effect of molecular environment on the formation kinetics of complexes of malvidin-3-O-glucoside with caffeic acid and catechin.

The formation kinetics of malvidin-polyphenol copigmentation complexes was studied in water-ethanol mixtures by time-dependent recording of the photoluminescence (PL) signal of malvidin. Two reaction channels were examined according to the order of complex formation. Both channels end with the same malvidin-polyphenol complexes formed in binary mixtures of water-ethanol. However, in the first case, the malvidin-polyphenol complexes are formed in water first, and then their solvation shells are altered according to the changed composition of the bulk solutions after the aqueous solution is diluted with ethanol. In the second case, the stock solutions are prepared as binary solvent mixtures; therefore, the bicomponent solvation shells of the interacted species are formed prior to molecular association. Our results show a faster formation of the complexes in the former case. Evaluation of the Arrhenius parameters of the reaction shows that the activation energy of the first reaction channel is lower and the frequency factor is higher, supporting a higher reaction rate. These observations are applicable to a wide range of chemistry where the molecular environment is composed of binary solutions. In particular, it has significant consequences for winemaking procedures, where one method of color improvement is based on the association of the species examined in this work.