3′‐(β‐D‐Glycopyranosyloxy)flavylium Ions: Synthesis and investigation of their properties in aqueous solution. Hydrogen bonding as a mean of colour variation

Abstract

This work describes a straightforward synthesis of two 3′‐(β‐D‐glycopyranosyloxy)flavylium ions thought to be good models of natural anthocyanins (pigments). For both pigments and for the non‐glycosylated flavylium ion taken as a reference, H2O addition and proton‐transfer reactions as well as formation of molecular complexes with chlorogenic acid and caffeine (copigmentation) are quantitatively investigated in mildly acidic aqueous solution. A remarkable affinity of caffeine for the trans‐chalcone form of the pigments is demonstrated. Moreover, the differences in the flavylium pKa values are interpreted in terms of possible intramolecular H‐bonding between the glycosyl residue and the chromophore. The discussion is then extended to a series of malonylated anthocyanins recently reported for their unusual pigmentation properties. A possible role for the malonyl group (frequently encountered in the structure of naturally occurring anthocyanins) in plant colour expression is outlined for the first time.