Further NMR Investigation and Conformational Analysis of an Acylated Flavonol Glucorhamnoside

Abstract

With the combination of various 2D NMR techniques, the structure of an acylated glycoside (1) was unambiguously determined to be kaempferol 3-O-α-(6‴-p-coumaroylglucosyl-β-1,2-rhamnoside). The complete 1H and 13C resonance assignments were obtained. Based on NMR data and molecular dynamics simulations in DMSO solvent using periodic boundary conditions, a folded conformation was elucidated for 1 in solution, implying a hydrophobic interaction between the aromatic nuclei of aglycone and the acyl group. Such hydrophobic stacking is further assisted by a weak intramolecular hydrogen bond between OH3″ on glucose and O2‴ on rhamnose, as indicated directly by the NMR behaviour (temperature coefficients dδ/dT, NOEs and coupling constants JHOCH) of the hydroxyl protons on the sugar rings.