Anthocyanin Intramolecular Interactions. A New Mathematical Approach To Account for the Remarkable Colorant Properties of the Pigments Extracted from Matthiola incana

Abstract

In the last few years, a series of investigations has brought to light a mechanism of stabilization of the colorant properties of certain anthocyanins. Intramolecular interactions take place between the chromophore moiety of the anthocyanin and one of its aromatic acid residues, which folds over the chromophore and thus confers protection against hydration and subsequent formation of colorless forms. In our continuing study of the physicochemical properties exhibited by acylated natural anthocyanins, we report here on a series of five structurally related pigments extracted from the violet flowers of Matthiola incana. These pigments all bear the same chromophore moiety, i.e., the cyanidin aglycon, but differ in the degree of glycosylation and acylation. Acidity constants for the deprotonation and hydration equilibria of the flavylium cation, together with rate constants for the hydration step alone were determined from UV-visible absorption measurements. The data support the existence of intramolecular, noncovalent interactions that strongly stabilize the colored forms of the pigments. However, none of the four more heavily substituted anthocyanins follows the above-mentioned mechanism that was previously successfully applied to the study of acylated anthocyanins. Consequently, a new mechanism with a different mathematical treatment is here developed to account for the different behavior exhibited by these distinctive pigments.