Origin of the exceptional colour stability of the Zebrina anthocyanin

The exceptional colour stability near neutrality of the Zebrina anthocyanin is related both to a high value of the acidity constant and to the total absence of formation of the colourless pseudobase and chalcone. The aromatic residues of the acylated groups (caffeic and ferulic) probably interact with the positively charged pyrylium nucleus. This intramolecular effect prevents further addition of nucleophiles to the pyrylium ring; in particular, addition of water is largely disfavoured compared to ordinary anthocyanins.