Specific Features of the Spectrophotometric Determination of Monomeric Anthocyanins

Abstract

It was shown that the differential spectrophotometric method for the determination of monomeric anthocyanins (in a mixture with polymeric ones) is unsuitable for measurements of anthocyanins acylated by substituted cinnamic acids. This is explained by a competition between the reaction of the nucleophilic addition of a water molecule (with the release of H+) to flavylium ions with the formation of a noncolored pseudo-base (A) and the reaction of the deprotonation of flavylium ions with the formation of quinonoid structures (B), which are also colored with a certain bathochromic shift of the absorption maximum. It was proposed to estimate the applicability of the differential spectrophotometric method by comparing differences between the spectra recorded at different pH (1, 2, 3, 4, and 4.5). The method is applicable at the prevalence of route A, when the specified differences between the spectra are undistinguishable taking into account scaling. The method is inapplicable for the main route B, when spectral differences can be distinguished. In the second case, a simplified method for the determination of anthocyanins without the account of polymeric anthocyanins is justified.