ANTHOCYANIN SELF-AGGREGATES .7. SELF-ASSOCIATION OF FLAVYLIUM CATIONS OF ANTHOCYANIDIN 3,5-DIGLUCOSIDES STUDIED BY CIRCULAR-DICHROISM AND H-1-NMR

Exciton coupling type circular dichroism was induced by increasing anthocyanin concentration in strongly acidic media. The higher concentration, the larger magnitude in the circular dichroism. All anthocyanidin 3,5-diglucosides exhibited negative signs in the first Cotton effect, which indicates that all the anthocyanins self-associate in a left-handed screw manner. Concentration dependence of proton chemical shifts were also observed with increase of anthocyanidin 3,5-diglucosides in a strongly acidic medium; upfield shifts of aromatic rings by increasing pigment concentrations demonstrated that anthocyanidin nuclei stack vertically. Analyses of proton chemical shifts based on the Dimicoli and Helene equation made possible the determination of the self-association of flavylium cations. The self-association constants for the most common natural anthocyanidin 3,5-diglucosides are given.