A spectroscopic method based on the anthocyanin copigmentation interaction and applied to the quantitative study of molecular complexes

Abstract

When monitored by visible absorption spectrometry, anthocyanin copigmentation appears to be of wide application in the field of quantitative investigation of molecular complexes in general. This is made possible by the strong hyperchromic effect in the visible range involving the influence of the copigment in slightly acidic aqueous solutions. For instance, when we add to a solution containing both malvin (the pigment) and chlorogenic acid (the copigment) a third species able to interact, in a non-covalent way, with the copigment, the effect produced by the presence of the new complex (chlorogenic acid–third species), on the copigmentation reaction itself, is significant. Two types of chlorogenic acid molecular associations have been investigated. The first one corresponds to the inclusion of chlorogenic acid into α- and β-cyclodextrins, both macrocycles being inert with respect to malvin. Concerning the study of inclusion complexes, our method remains valid when the guest molecule is not a copigment, provided that malvin is replaced by an anthocyanic pigment capable of entering the macrocycle cavity. Callistephin and chrysanthemin, two very abundant naturally occurring anthocyanins, fit this requirement. Inclusion of the latter pigments into β-cyclodextrin is characterized by a much better inclusion of their colourless forms than of their coloured flavylium cations, leading to what may be called an ‘anti-copigment’ effect. The second type of chlorogenic acid molecular association, investigated with our method, corresponds to the stacking of chlorogenic acid, the copigment, with the third species, which is now also a copigment. The two interacting copigments are chlorogenic acid and caffeine. The chlorogenic acid–caffeine stability constant was measured as 36 (±9) dm³ mol^–1 at 25 °C, a value in good agreement with that reported in the literature.