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

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.

Binding of the five multistate species of the anthocyanin analog 7-β-D-glucopyranosyloxy-4′-hydroxyflavylium to the β-cyclodextrin derivative captisol

“…In a previous work, the equilibrium and rate constants of the GHF multistate, Scheme 1, were reported [4]. The pH-dependent absorption spectra of the pigment in the presence of 50 mM of captisol, taken immediately after a direct pH jump, at pseudoequilibrium and at equilibrium, are shown in Fig.…”

“…1b and the data fit with a single acid-base equilibrium with pK^a ¼ 3.25. The higher pK^a value (4.7) in the absence of captisol [4] shows that the macrocycle has a higher affinity for CB^(essentially, a mixture of B and Cc) than for AH þ . The same trend is observed at full equilibrium, comparing pK 0 a ¼ 0.8 (with captisol) with pK 0 a ¼ 2.3 (no captisol), in this case pointing to the preferential stabilization of the trans-chalcone, Fig.…”

“…Cc#Ct K i isomerization (4) The addition of base to equilibrated acidic solutions of the flavylium cation is defined here as the direct pH jump. After a direct pH jump, two competitive reactions take place, proton transfer, Eq.…”

“…When there is a cis-trans isomerization barrier as for anthocyanins and the present compound, the isomerization is the slowest kinetic step. Except in very acidic medium, the hydration (timescale of seconds-minutes) is slower than tautomerization (ms timescale) [4,5]. This behavior leads in general to three well separated kinetic processes: 1-equilibrium between flavylium cation and quinoidal base (only accessed by very fast techniques such as temperature jumps and in some favorable cases by flash photolysis); 2-formation of hemiketal through the hydration reaction (rate-determining step) followed by a faster tautomerization yielding the cis-chalcone; 3-isomerization, Scheme 1.…”

“…Particularly, highly water-soluble polyanionic b-cyclodextrin derivatives bearing sulfobutyl chains, captisol [11], has been used to increase the stability and water-solubility of organic dyes, such as curcuminoids [12,13], carotenoids [14,15], and flavonoids [16,17]. In this work, we describe a new procedure based on the [4] and presence of captisol. In particular, captisol has been tested as the best host (in comparison with other cyclodextrin derivatives) for anthocyanidins, improving storage stability and facilitating formulation [18].…”