title: A new NMR approach for structure determination of thermally unstable biflavanones and application to phytochemicals from Garcinia buchananii.
abstract: Previous activity-guided phytochemical studies on Garcinia buchananii stem bark, which is traditionally used in Africa to treat various gastrointestinal and metabolic illnesses, revealed xanthones, polyisoprenylated benzophenones, flavanone-C-glycosides, biflavonoids, and/or biflavanones as bioactive key molecules. Unequivocal structure elucidation of biflavonoids and biflavanones by means of NMR spectroscopy is often complicated by the hindered rotation of the monomers around the C-C axis (atropisomerism), resulting in a high spectral complexity. In order to facilitate an unrestricted rotation, NMR spectra are usually recorded at elevated temperatures, commonly over 80 °C, which effects in a single set of resonance signals. However, under these conditions, one of the target compounds of this investigation, (2R,3S,2″R,3″R)-manniflavanone (1), undergoes degradation. Therefore, we demonstrated in the present study that the 1,1-ADEQUATE could be successfully used as a powerful alternative approach to confirm the C-C connectivities in 1, avoiding detrimental conditions. However, a moderate increase in temperature up to 50 °C was sufficient to deliver sharp signals in the proton NMR experiment of (2R,3S,2″R,3″R)-isomanniflavanone (2) and (2″R,3″R)-preussianone (3). In addition, two new compounds could be isolated, namely (2R,3S,2″R,3″R)-GB-2 7″-O-β-D-glucopyranoside (4) and (2R,3S,2″R,3″R)-manniflavanone-7″-O-β-D-glucopyranoside (5), and whose structures were elucidated by spectroscopic analysis including 1D and 2D NMR and mass spectrometry methods. The absolute configurations were determined by a combination of NMR and electronic circular dichroism (ECD) spectroscopy. The aforementioned compounds exhibited high anti-oxidative capacity in the H2O2 scavenging, hydrophilic Trolox equivalent antioxidant capacity (H-TEAC) and hydrophilic oxygen radical absorbance capacity (H-ORAC) assays.
authors.affiliation: Chair of Food Chemistry and Molecular Sensory Science, Technische Universität München, Lise-Meitner-Str. 34, Freising, 85354, Germany.
authors.forename: Timo D
authors.initials: TD
authors.lastname: Stark
authors.affiliation: Chair of Food Chemistry and Molecular Sensory Science, Technische Universität München, Lise-Meitner-Str. 34, Freising, 85354, Germany.
authors.forename: Sofie
authors.initials: S
authors.lastname: Lösch
authors.affiliation: Chair of Food Chemistry and Molecular Sensory Science, Technische Universität München, Lise-Meitner-Str. 34, Freising, 85354, Germany.
authors.forename: Mathias
authors.initials: M
authors.lastname: Salger
authors.affiliation: Department of Biological Sciences, University of Idaho, Moscow, ID, USA.
authors.forename: Onesmo B
authors.initials: OB
authors.lastname: Balemba
authors.affiliation: Chair of Food Chemistry and Molecular Sensory Science, Technische Universität München, Lise-Meitner-Str. 34, Freising, 85354, Germany.
authors.forename: Junichiro
authors.initials: J
authors.lastname: Wakamatsu
authors.affiliation: Chair of Food Chemistry and Molecular Sensory Science, Technische Universität München, Lise-Meitner-Str. 34, Freising, 85354, Germany.
authors.forename: Oliver
authors.initials: O
authors.lastname: Frank
authors.affiliation: Chair of Food Chemistry and Molecular Sensory Science, Technische Universität München, Lise-Meitner-Str. 34, Freising, 85354, Germany.
authors.forename: Thomas
authors.initials: T
authors.lastname: Hofmann
mesh_terms: D002623:Chemistry Techniques, Analytical / Q000379:methods*; D004355:Drug Stability; D044950:Flavanones / Q000737:chemistry*; D047309:Flavones / Q000737:chemistry*; D029760:Garcinia / Q000737:chemistry*; D006358:Hot Temperature; D009682:Magnetic Resonance Spectroscopy; D015394:Molecular Structure; D064209:Phytochemicals; D024301:Plant Bark / Q000737:chemistry; D010936:Plant Extracts / Q000737:chemistry*
chemical_list: D044950:Flavanones; D047309:Flavones; D064209:Phytochemicals; D010936:Plant Extracts; C000606656:manniflavanone
keywords: 1,1-ADEQUATE; 13C; 1H; GB2 7″-O-β-d-glucopyranoside; NMR; biflavanone glucosides; biflavanones; manniflavanone; manniflavanone-7″-O-β-d-glucopyranoside; variable temperature NMR