Flavonoids from Mimosa xanthocentra (Leguminosae: Mimosoideae) and molecular modeling studies for isovitexin-2″-O-α-l-rhamnopyranoside rotamers

Abstract

Phytochemical investigation of Mimosa xanthocentra Mart. (Leguminosae: Mimosoideae), an herb comprising the diet of marsh deer and pampas deer in the Brazilian Pantanal, led to the isolation of flavones isovitexin-2″-O-α-l-rhamnopyranoside 1 and vitexin-2″-O-α-l-rhamnopyranoside 2, and a mixture of flavonols avicularin 3a and reynoutrin 3b. At room temperature, compound 1 was observed in the 1H and 13C NMR spectra as a mixture of two rotamers in equilibrium in a DMSO-d6 solution. Molecular modeling using MM2 calculations led to the first proposal of three-dimensional solution structure of both observed conformers. The energy barrier between both rotamers was evaluated theoretically by molecular modeling and experimentally by variable-temperature NMR studies and calculation of ΔG‡ of rotation using Eyring equation. This is the first report on flavonoids from M. xanthocentra.