Equilibrium Forms of Vitisin B Pigments in an Aqueous System Studied by NMR and Visible Spectroscopy

• Type: Article in International Scientific Journal
• Date: 2009

Authors

Joana Oliveira (Author) Other; Nuno Mateus (Author) FCUP; Artur M S Silva (Author) Other; Victor de Freitas (Author) FCUP

Journal

Title: Journal of Physical Chemistry B

Vol. 113

Pages: 11352-11358

ISSN: 1520-6106

Publisher: American Chemical Society

Indexing

ISI Web of Knowledge - 47 Citations

Scopus - 50 Citations

Scientific classification

FOS: Natural sciences > Chemical sciences

Other information

Authenticus ID: P-003-GS6

DOI: 10.1021/jp904776k

Abstract (EN): 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.

Language: English

Type (Professor's evaluation): Scientific

No. of pages: 7