Analogs of Natural 3-Deoxyanthocyanins: O-Glucosides of the 4',7-Dihydroxyflavylium Ion and the Deep Influence of Glycosidation on Color.

3-Deoxyanthocyanidins and their -β-d-glucosides are natural pigments abundant in black sorghum. -glycosidation can perturb the acid-base properties of the chromophore and lower its electron density with a large impact on the distribution of colored and colorless forms in aqueous solution. In this work, the influence of -glycosidation on color is systematically studied from a series of 3-deoxyanthocyanin analogs. The pH- and light-dependent reversible reactions of 7-β-d-glucopyranosyloxy-4'-hydroxyflavylium (P3) and 4'-β-d-glucopyranosyloxy-7-hydroxyflavylium (P5) were completely characterized in mildly acidic solution and compared with the parent aglycone 4',7-dihydroxyflavylium ion and the -methylethers of P3 and P5. Except P5, the chalcone forms of the pigments exhibit a high - isomerization barrier that allows a pseudo-equilibrium involving all species except the -chalcone. At equilibrium, only the flavylium cation and -chalcone are observed. With all pigments, the colored flavylium ion can be generated by irradiation of the -chalcone (photochromism). Glycosidation of C7-OH accelerates hydration and strongly slows down - isomerization with the pH dependence of the apparent isomerization rate constant shifting from a bell-shaped curve to a sigmoid. The color of P5 is much more stable than that of its regioisomer P3 in near-neutral conditions.