Effect of β-Cyclodextrin on the Multistate Species Distribution of 3-Methoxy-4′,7-dihydroxyflavylium. Discrimination of the Two Hemiketal Enantiomers

The effect of β-cyclodextrin on the mole fraction distribution of the multistate species of the anthocyanin model compound, 3-methoxy-4′,7-dihydroxyflavylium, was studied by NMR, stopped flow, circular dichroism, and UV-visible absorption spectroscopy. The formation of inclusion complexes with hemiketal and trans-chalcone, possessing transition dipole moments in a parallel orientation to the cyclodextrin n-fold axis, was unequivocally proved by means of the positive-induced circular dichroism signal. The discrimination of the two hemiketal enantiomers was achieved by the splitting of ¹H NMR peaks in the presence β-cyclodextrin. The spectroscopic data shows that the β-cyclodextrin has only moderate enantioselectivity, slightly favoring one of the optical isomers. The observed binding affinity of β-cyclodextrin for the 3-methoxy-4′,7-dihydroxyflavylium multistate species increases in the order flavylium cation < quinoidal base < hemiketal < trans-chalcone < cis-chalcone. As a result of this selectivity and the dynamic nature of the network, the equilibrium is displaced toward the formation of the chalcone species.