INTRAMOLECULAR STACKING CONFORMATION OF GENTIODELPHIN, A DIACYLATED ANTHOCYANIN FROM GENTIANA-MAKINOI

Gentiodelphin from Gentiana makinoi is unusually stable in neutral aqueous solutions and may be stabilized by intramolecular sandwich-type stacking of two caffeic acids. Although in acidic methanol solution, evidence for intramolecular stacking of gentiodelphin was obtained from electronic spectra and H-1 NMR analysis. Using computer-assisted conformational analysis with NOE constraints and bond angle constraints, we first established the intramolecular stacking conformation of aromatic acid to the anthocyanidin nucleus. Under acidic conditions, one caffeic acid attached at the glucose of the 2-position of the B-ring was stacked to the delphinidin nucleus but the other acid attached at the glucose of the 5-position was not stacked.