Polyphenol interactions. The copigmentation case: thermodynamic data from temperature variation and relaxation kinetics. Medium effect

Abstract

Copigmentation of malvin (a common anthocyanic pigment) with a series of naturally occurring colourless organic molecules (copigments) has been investigated by UV–visible absorption. The temperature-dependence of the visible spectrum of the copigmented solutions yields the copigmentation enthalpy and entropy changes. The experiments are carried out at pH 3.5 (malvin flavylium cation copigmentation) and pH 5.5 (copigmentation of the malvin quinonoidal bases). Copigmentation appears to be clearly enthalpically driven (exothermic process with unfavourable entropy change) in the case of chlorogenic acid and (+)-catechin. Concerning caffeine and tryptophan, favourable entropy changes, probably arising from solvation effects, have been recorded. Similar experiments in water–ethanol binary solvents have shown that the dramatic weakening of copigmentation on cosolvent addition is essentially entropic in origin. Salt effects on copigmentation have been investigated as well. Most of the results are discussed with a view to pointing out the major contributions to the magnitude of the copigmentation. Finally, relaxation kinetic measurements on the malvin–rutin system give striking evidence of the strong slowing down that a good copigment causes to the anthocyanin fading process (hydration). From the decrease in the apparent first-order rate constant of hydration on copigment addition, a general method for a quick determination of the copigmentation stability constants is proposed.