Anthocyanin intramolecular copigment effect

Five structurally coherent anthocyanins, extracted from red-purple cultivars of Pharbitis nil, have been investigated for their pigmentation properties in slightly acidic aqueous solution and at 25°. They all possess the same aglycone (pelargonidin) and the same glycosyl moieties (sophorosyl at C-3 and glucosyl at C-5 of the aglycone). The reference compound is not acylated, while two members are monocaffeylated, the fourth and the fifth in the series being di- and tricaffeylated, respectively. It is shown that caffeylation of the sophorosyl group has a profound effect on some properties of pelargonidin, e.g. it reduces both the rate and the extent of the pelargonidin hydration step. Our kinetic and equilibrium data strongly suggest the existence of an interaction taking place between the pelargonidin chromophore and the caffeyl group(s), the glycosyl unit playing the role of a spacer. This highlights the function of glycosyl residues in the complex anthocyanins found during the past decade.