pH-Dependent Multistate System Generated by a Synthetic Furanoflavylium Compound: An Ancestor of the Anthocyanin Multistate of Chemical Species

Abstract

The multistate of chemical species generated by 4′-hydroxy-3,2′-furanoflavylium is similar to that of anthocyanins and related compounds. This furanoflavylium multistate system was fully characterized by UV–visible and NMR spectroscopy, allowing determination of the respective equilibrium and rate constants. In contrast to the multistate generated by flavylium cations derived from anthocyanins and related compounds, the furanoflavylium multistate is characterized by much slower hydration and tautomerization (pyran ring opening–closing). In addition, the cis–trans isomerization of the chalcones of this system (2′-hydroxyaurones) is extremely slow when compared with anthocyanins. The observed similar order of magnitude for tautomerization and isomerization rate constants leads to peculiar kinetics from the flavylium cation (pH = 1) to the stable trans -chalcone (higher pH values). The hemiketal appears and disappears during the first stages of the kinetics, which gives the intermediate cis -chalcone (pseudo-equilibrium). This last species disappears in a much slower process, as fully characterized by 1 H NMR, to give the final trans -chalcone.

Natural and Synthetic Flavylium-Based Dyes: The Chemistry Behind the Color

“…The flavylium multistates of the riccionidin model compounds 4′-hydroxyfuranoflavylium and 4′,7-dihydroxyfuranoflavylium , were also described. For 4′-hydroxyfuranoflavylium, a similar order of magnitude in the rate constants for hydration and tautomerization leads to peculiar kinetics from the flavylium cation (pH = 1) to the cis -chalcone. The hemiketal appears from the flavylium cation and disappears to give cis -chalcone, the observed species at pseudoequilibrium, as in Figure a.…”

Evolution of Flavylium-Based Color Systems in Plants: What Physical Chemistry Can Tell Us

“…We verified that as 4′-hydroxyfuranoflavylium [ 11 ], 4′,7-dihydroxyfuranoflavylium [ 12 ], and riccionidin A, the aglycone of auronidin-2′-neohesperidoside [ 13 ], follow the same multistate of species of anthocyanins and 3-deoxyanthocyanins ( Scheme 10 ) [ 13 ]. Riccionidin A was identified in several liverworts, for example, in the Antarctic Cephaloziella varians , in response to an abrupt increase in UV-B radiation [ 42 ], in in vitro cultures of Ricciocarpos natans [ 43 ], and in adventitious root cultures of the Anacardiaceae Rhus javanica [ 44 ].…”

“… 4′-hydroxyfuranoflavylium [ 11 ], 4′,7-dihydroxyflavylium [ 12 ], and riccionidin A, the aglycone of auronidin-2′-neohesperidoside [ 13 ], follow the same multistate of species of anthocyanins and 3-deoxyanthocyanins. …”

“…We devoted the past two decades to the study of the kinetics and thermodynamics of the natural anthocyanins, 3-deoxyanthocyanins, and the related synthetic flavylium compounds. Following these studies, we investigated the kinetics and thermodynamics of furanoflavylium compounds [ 11 , 12 ] and, more recently, riccionidin A, the aglycone of the auronidin-2′-neohesperidoside reported by Andersen and Davies [ 10 , 13 ]. In this work we investigate the three color systems used by plants auronidins, 3-deoxyanthocyanins, and anthocyanins—providing physicochemical arguments to suggest that the three color systems in these families of plants had a chemical evolution.…”

A model compound for pyridinechalcone-based multistate systems. Ring opening-closure as the slowest kinetic step of the multistate

“…Anthocyanins in solution give rise to several species connected through pH-dependent equilibria, as exemplified in Scheme 1. This multistate of species is followed by other families of related compounds such as anthocyanidins, 1 , 2 pyranoanthocyanins, 3 deoxyanthocyanins, 4,5 naphthoflavylium, 6 styrylflavylium, 7 aurones (riccionidin type) 8 and 2,2'-spirochromenes. 9 Scheme 1.…”