Flavonoids from Mimosa xanthocentra (Leguminosae: Mimosoideae) and molecular modeling studies for isovitexin-2 00 -O-a-L-rhamnopyranoside rotamers Luiza Maria de Magalha ˜es Camargo a , Jean-Pierre Fe ´re ´ zou b , Luzineide Wanderley Tinoco a , Carlos Roland Kaiser c , So ˆnia Soares Costa a, * a Nu ´cleo de Pesquisas de Produtos Naturais, Universidade Federal do Rio de Janeiro, 21 941-902 Rio de Janeiro, RJ, Brazil b Laboratoire de Chimie des Proce ´de ´s et des Substances Naturelles, ICMMO, UMR CNRS 8182, Universite ´ Paris-Sud-11, F-91405 Orsay, France c Instituto de Quı´mica, Universidade Federal do Rio de Janeiro, 21 941-909 Rio de Janeiro, RJ, Brazil 1. Introduction The Mimosa L. genus (Leguminosae: Mimosoideae), native from South America, comprises about 500 species, most of them medicinally used against infectious and inflammatory symptoms (Dhillion et al., 2005; Grosvenor et al., 1995; Heard and Pettit, 2005; Hussain et al., 1979; Mors et al., 2000). Previous phytochemical studies on some Mimosa species resulted in the identification of flavonoids, saponins, alkaloids, terpenoids, non-protein aminoacid (mimosin), tannin, and fatty acids (Bum et al., 2004; Dinda et al., 2006; Lobstein et al., 2002; Yuan et al., 2006, 2007). Flavonoids, including the C-glycosylflavones orientin, isoor- ientin, 2 00 -O-rhamnosylorientin, 2 00 -O-rhamnosylisoorientin, 4 00 - hydroxymaysin and cassiaoccidentalin B, are the main secondary metabolites of the genus (Dominguez et al., 1989; Kirk et al., 2003; Lobstein et al., 2002; Ohsaki et al., 2006, Yuan et al., 2006, 2007). Mimosa xanthocentra Mart. is a sensitive herbaceous plant that has never been chemically studied. This species, along with twenty- one other plants, is part of the diet of two endangered mammals – marsh deer and pampas deer – living at a Private Natural Heritage Reserve in the Brazilian Pantanal wetland (Costa et al., 2006). In a search for flavonoids from this unexplored species, we report here the isolation and structural elucidation of four flavonoids, including one present in solution as two rotamers. 2. Results and discussion 2.1. Phytochemical investigation The aqueous extract of aerial parts from M. xanthocentra was partitioned between ethyl acetate (EF) and n-butanol (BF). The purification of EF fraction by combining chromatography on reversed-phase silanized silica gel and LH-20 Sephadex led to the isolation of a mixture of 3a and 3b flavonols, while flavones 1 and 2 were obtained by purification of BF fraction. The structures of all flavonoids were determined on the basis on 1-D and 2-D 1 H and 13 C NMR assignments and comparison with the literature. Compound 1, the major flavonoid of M. xanthocentra, was obtained as an amorphous yellow powder with [a D ] – 86.2 (c. 0.1, H 2 O). The 1 H NMR spectrum revealed the presence of a 6-C substituted apigenin as characterized by two doublets at d 7.91 (J = 8.6 Hz; H-2 0 /6 0 ) and d 6.92 (J = 8.6 Hz; H-3 0 /5 0 ), in accordance with an AA 0 XX 0 system, two singlets at d 6.75 (H-3) and d 6.50 (H-8), Phytochemistry Letters 5 (2012) 427–431 A R T I C L E I N F O Article history: Received 31 July 2011 Received in revised form 26 March 2012 Accepted 27 March 2012 Available online 11 April 2012 Keywords: Mimosa xanthocentra Flavonoid C-Glycosylflavone NMR Rotamerism MM2 calculations A B S T R A C T 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 00 -O-a-L-rhamnopyranoside 1 and vitexin-2 00 -O-a-L-rhamnopyranoside 2, and a mixture of flavonols avicularin 3a and reynoutrin 3b. At room temperature, compound 1 was observed in the 1 H and 13 C NMR spectra as a mixture of two rotamers in equilibrium in a DMSO-d 6 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 DGz of rotation using Eyring equation. This is the first report on flavonoids from M. xanthocentra. ß 2012 Phytochemical Society of Europe. Published by Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +55 21 2562 6791; fax: +55 21 2562 6512. E-mail addresses: luizammc@hotmail.com (L.M.M. Camargo), jean-pierre.ferezou@u-psud.fr (J.-P. Fe ´re ´ zou), luzitinoco@hotmail.com (L.W. Tinoco), kaiser.ufrj@gmail.com (C.R. Kaiser), sscosta@nppn.ufrj.br, sscbh@terra.com.br (S.S. Costa). Contents lists available at SciVerse ScienceDirect Phytochemistry Letters jo u rn al h om ep ag e: ww w.els evier.c o m/lo c ate/p hyt ol 1874-3900/$ – see front matter ß 2012 Phytochemical Society of Europe. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.phytol.2012.03.015