3'-(BETA-D-GLYCOPYRANOSYLOXY)FLAVYLIUM IONS - SYNTHESIS AND INVESTIGATION OF THEIR PROPERTIES IN AQUEOUS-SOLUTION - HYDROGEN-BONDING AS A MEAN OF COLOR VARIATION

This work describes a straightforward synthesis of two 3'-(beta-D-glyc opyranosyloxy)flavylium ions thought to be good models of natural anth ocyanins (pigments). For both pigments and for the non-glycosylated fl avylium ion taken as a reference, H2O addition and proton-transfer rea ctions as well as formation of molecular complexes with chlorogenic ac id and caffeine (copigmentation) are quantitatively investigated in mi ldly acidic aqueous solution. A remarkable affinity of caffeine for th e trans-chalcone form of the pigments is demonstrated. Moreover, the d ifferences in the flavylium pK(a) values are interpreted in terms of p ossible intramolecular H-bonding between the glycosyl residue and the chromophore. The discussion is then extended to a series of malonylate d anthocyanins recently reported for their unusual pigmentation proper ties. A possible role for the malonyl group (frequently encountered in the structure of naturally occurring anthocyanins) in plant colour ex pression is outlined for the first time.