Abstract
3-Deoxyanthocyanidins and their O-β-d-glucosides are natural pigments abundant in black sorghum. O-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 O-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 O-methylethers of P3 and P5. Except P5, the chalcone forms of the pigments exhibit a high cis-trans isomerization barrier that allows a pseudo-equilibrium involving all species except the trans-chalcone. At equilibrium, only the flavylium cation and trans-chalcone are observed. With all pigments, the colored flavylium ion can be generated by irradiation of the trans-chalcone (photochromism). Glycosidation of C7–OH accelerates hydration and strongly slows down cis-trans 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.
Highlights
3-Deoxyanthocyanidins and their O-glycosides are natural pigments that are especially abundant in black sorghum [1] and occur in other plants such as ferns [2,3,4]. 3-deoxyanthocyanidins of black sorghum are known to express a variety of health-promoting effects [5] and a better chemical stability [6] than the much more common anthocyanins, an important advantage for their potential development as natural food colorants
The flavylium ion (AH+) is stable only in highly acidic medium and those pigments constitute a multistate system of chemical species, which are reversibly interconverted by external stimuli such as pH variations and light [7,8]
Natural (3-deoxy)anthocyanins and some related pigments such as P3 possess a high cis-trans isomerization barrier. This fact has a dramatic influence on the kinetics of the multistate
Summary
3-Deoxyanthocyanidins and their O-glycosides are natural pigments that are especially abundant in black sorghum [1] and occur in other plants such as ferns [2,3,4]. 3-deoxyanthocyanidins of black sorghum are known to express a variety of health-promoting effects [5] and a better chemical stability [6] than the much more common anthocyanins, an important advantage for their potential development as natural food colorants. 3-Deoxyanthocyanidins and their O-glycosides are natural pigments that are especially abundant in black sorghum [1] and occur in other plants such as ferns [2,3,4]. 3-deoxyanthocyanidins of black sorghum are known to express a variety of health-promoting effects [5] and a better chemical stability [6] than the much more common anthocyanins, an important advantage for their potential development as natural food colorants. In the mildly acidic conditions typical of plant cell and food, the multistate system encompasses five distinct colored or colorless species (Figure 1). Corresponding rate constants for the direct and reverse reactions are noted kn and k−n, respectively The c(onrr=eas,pho,nt,dii)n. Stants for the direct and reverse reactions are noted kn and k−n, respectively (n = a, h, t, i).
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