title: Covalent anthocyanin-flavone dimer from leaves of Oxalis triangularis.
abstract: The anthocyanin-flavone C-glycoside, (malvidin 3-O-(6(II)-O-alpha-rhamnopyranosyl(AIV)-beta-glucopyranoside(AII))-5-O-beta-glucopyranoside(AIII)) (apigenin 6-C-(2(II)-O-beta-glucopyranosyl(FIII)-beta-glucopyranoside(FII))) malonate(AV) (A(IV)-4-->A(V)-1, F(III)-6-->A(V)-3) (1), has been isolated from leaves of Oxalis triangularis A. St.-Hil. In the 1D (1)H NMR spectrum of 1 dissolved in CD(3)OD-CF(3)CO(2)D (95:5), MTFA, recorded 45 min after sample preparation, this covalently linked dimer occurred mainly as flavylium cation (38%) and two equilibrium forms assigned to be quinonoidal bases (54%), whereas only minor amounts of the hemiacetal forms were present. After five days storage at 300 K, the hemiacetals (39%) and flavylium cation (38%) constituted the main forms of 1. More simple anthocyanins are normally considered to be on the flavylium cation form in acidified deuterated methanol. The cross-peaks observed in NOESY NMR spectra of 1 indicated the presence of vertical 'pi-pi' stacking between the B-ring of the flavone unit and the A-ring of each of the two forms assigned to be quinonoidal bases. It was not possible to discriminate between inter- or intramolecular association mechanisms. The equilibria between the various forms of 1 were studied by two-dimensional NOESY and ROESY NMR spectroscopy. 2D HSQC-TOCSY NMR spectroscopy was among the methods used for characterization of the various forms.
authors.affiliation: Department of Chemistry, University of Bergen, Allégt. 41, N-5007 Bergen, Norway. Torgils.Fossen@kj.uib.no
authors.forename: Torgils
authors.initials: T
authors.lastname: Fossen
authors.forename: Saleh
authors.initials: S
authors.lastname: Rayyan
authors.forename: Maya H
authors.initials: MH
authors.lastname: Holmberg
authors.forename: Manfred
authors.initials: M
authors.lastname: Nimtz
authors.forename: Oyvind M
authors.initials: OM
authors.lastname: Andersen
mesh_terms: D000872:Anthocyanins / Q000737:chemistry* / Q000302:isolation & purification; D002240:Carbohydrate Sequence; D018548:Cotyledon / Q000737:chemistry; D019281:Dimerization; D047309:Flavones / Q000737:chemistry* / Q000302:isolation & purification; D005960:Glucosides / Q000737:chemistry* / Q000302:isolation & purification; D009682:Magnetic Resonance Spectroscopy; D019684:Magnoliopsida / Q000737:chemistry*; D008958:Models, Molecular; D008968:Molecular Conformation; D008969:Molecular Sequence Data; D018515:Plant Leaves / Q000737:chemistry*; D013053:Spectrophotometry
chemical_list: D000872:Anthocyanins; D047309:Flavones; D005960:Glucosides