title: Equilibrium forms of vitisin B pigments in an aqueous system studied by NMR and visible spectroscopy.
abstract: The main species in the acid-base equilibria of two pyranoanthocyanins (vitisins B), pyranomalvidin-3-glucoside I and pyranomalvidin-3-O-coumaroylglucoside II, and the respective pK(a) were determined using NMR, visible spectroscopy, and mass spectrometry techniques. The three equilibria involve protonation of the pyranoflavylium cation of vitisin B (pK(a1)) and two deprotonations (pK(a2) and pK(a3)). For pigment I, the values obtained through the titration curves of the chemical shift of some protons versus pH were (pK(a1) < 0.68; pK(a2) = 4.40 +/- 0.08; pK(a3) = 7.45 +/- 0.09) very close to the values obtained by visible spectroscopy (pK(a1) < 0.63; pK(a2) = 4.43 +/- 0.02; pK(a3) = 7.34 +/- 0.03). For pigment II, it was only possible to calculate the pK(a) by visible spectroscopy (pK(a1) < 0.75; pK(a2) = 4.66 +/- 0.10; pK(a3) = 6.76 +/- 0.10). NMR studies have shown that pigment I does not undergo hydration, and the hypothesis of the occurrence of hemiacetal forms in equilibrium was discarded.
authors.affiliation: Departamento de Química, Faculdade de Ciências, Centro de Investigação em Química, Universidade do Porto, 4169-007 Porto, Portugal.
authors.forename: Joana
authors.initials: J
authors.lastname: Oliveira
authors.forename: Nuno
authors.initials: N
authors.lastname: Mateus
authors.forename: Artur M S
authors.initials: AM
authors.lastname: Silva
authors.forename: Victor
authors.initials: V
authors.lastname: de Freitas
mesh_terms: D001572:Benzofurans / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D010636:Phenols / Q000737:chemistry*; D010860:Pigments, Biological / Q000737:chemistry*; D011522:Protons; D013057:Spectrum Analysis / Q000379:methods*; D014867:Water / Q000737:chemistry
chemical_list: D001572:Benzofurans; D010636:Phenols; D010860:Pigments, Biological; D011522:Protons; C480486:malvidin-3-O-glucoside-4 vinyl; D014867:Water