Self-association of some anthocyanins in neutral aqueous solution

Abstract

A good contrast in optical properties caused by self-association was found between malvin and cyanin quinonoidal bases. Circular dichroism measurements of the pigments in neutral aqueous solutions show that molecules of the malvin quinonoidal base self-associate quickly and the conformational orientation of the aggregates is opposite to those formed by cyanin quinonoidal bases. Hypsochromism and hypochromism in the visible absorption also occurred on the formation of malvin aggregates. CD comparisons of malvin, hirsutin and 4′- O-methylmalvin suggest that a methoxyl group in the B-ring of the anthocyanidin strongly suppresses self-association. It is proposed that the driving forces for self-association are mainly hydrophobic interactions among the aromatic nuclei stacked parallel to each other which are surrounded by the hydrophilic glucose moieties in a suitable spatial arrangement. Furthermore, the glucose moiety at the 5-position rather than that at the 3-position plays an important role in the self-association of these anthocyanidin 3,5-diglucosides. Addition of sodium chloride promotes self-association and the greater stability of the anthocyanins in solution.