Polyphenol interactions. Part 5. Anthocyanin co-pigmentation

In aqueous media, anthocyanins undergo several structural transformations and exist in a series of equilibria between carbinol-base 6a, 6b, flavylium cation 3, quinonoidal anhydro-base 4a, 4b and chalcone 7a, 7b forms. A detailed interpretation of the proton NMR (400 MHz) spectra of malvin 3 in D2O is given for the first time in the context of these equilibria. The phenomenon of co-pigmentation is reviewed and the efficacy of various phenolic flavonoids, galloyl and hexahydroxycinnamyl esters as natural co-pigments is determined quantitatively. Association constants at 22 °C in H2O/CF3CO2H (0.02% v/v) of 1686 (±58) and 987 (±37) dm3 mol–1 were measured by UV-visible spectroscopy for the complexation of 3 with quercetin-3-β-D-galactoside 11 and β-1,2,3,4,6-penta-O-galloyl-D-glucose 10, respectively. In D2O/CF3CO2D (1.1% v/v) at 45 °C an association constant of 508 (±8) dm3 mol–1 was determined by proton NMR for the interaction of 3 with 10. DNA, RNA and ATP also act as effective co-pigments for the flavylium ion 3 but both caffeine 16 and theophylline 17 preferentially stabilise the quinonoidal-base forms 4a, 4b of the anthocyanin. In media (pH 3.5 to 7.0), they give rise to stable violet through to blue and finally green colours of the anthocyanin 3. Proton NMR studies of all these intermolecular co-pigmentation reactions are reported and the phenomenon is provisionally interpreted in terms of hydrophobically reinforced 'π-π' stacking of anthocyanin and co-pigment molecules in the aqueous environment.