title: 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 theirO-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 ofO-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 theO-methylethers of P3 and P5. Except P5, the chalcone forms of the pigments exhibit a highcis-transisomerization barrier that allows a pseudo-equilibrium involving all species except thetrans-chalcone. At equilibrium, only the flavylium cation andtrans-chalcone are observed. With all pigments, the colored flavylium ion can be generated by irradiation of thetrans-chalcone (photochromism). Glycosidation of C7-OH accelerates hydration and strongly slows downcis-transisomerization 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.
authors: LAQV-REQUIMTE, Department of Chemistry, Faculty of Science & Technology, Universidade NOVA de Lisboa, 2829-516 Monte de Caparica, Portugal. nuno.basilio@fct.unl.pt.
authors: Nuno
authors: N
authors: Basílio
authors: Faculty of Sciences, Technology & Health, University of Avignon, INRA, UMR408, 84000 Avignon, France. shirazbitar@yahoo.com.
authors: Sheiraz
authors: S
authors: Al Bittar
authors: Faculty of Sciences, Technology & Health, University of Avignon, INRA, UMR408, 84000 Avignon, France. nathalie.mora@univ-avignon.fr.
authors: Nathalie
authors: N
authors: Mora
authors: Faculty of Sciences, Technology & Health, University of Avignon, INRA, UMR408, 84000 Avignon, France. olivier.dangles@univ-avignon.fr.
authors: Olivier
authors: O
authors: Dangles
authors: LAQV-REQUIMTE, Department of Chemistry, Faculty of Science & Technology, Universidade NOVA de Lisboa, 2829-516 Monte de Caparica, Portugal. fp@fct.unl.pt.
authors: Fernando
authors: F
authors: Pina
mesh_terms: D000872:Anthocyanins / Q000737:chemistry; D047310:Apigenin / Q000737:chemistry; D005960:Glucosides / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007536:Isomerism; D010860:Pigments, Biological / Q000737:chemistry*; D011809:Quinones / Q000737:chemistry*; D045868:Sorghum / Q000737:chemistry*; D000094162:Phytoalexins
chemical_list: D000872:Anthocyanins; D047310:Apigenin; D005960:Glucosides; D010860:Pigments, Biological; D011809:Quinones; C494947:4',7-dihydroxyflavylium; C104027:apigeninidin; D000094162:Phytoalexins
keywords: 3-deoxyanthocyanidin; flavylium; glucoside; multistate; photochromism