In the present work, the proton-transfer reactions of the methylpyranomalvidin-3-O-glucoside pigment in water with different pH values was studied by NMR and UV-visible spectroscopies. The results showed four equilibrium forms: the methylpyranomalvidin-3-O-glucoside cation, the neutral quinoidal base, the respective anionic quinoidal base, and a dianionic base unprotonated at the methyl group. According to the NMR data, it seems that for methylpyranomalvidin-3-O-glucoside besides the acid base equilibrium between the pyranoflavylium cation and the neutral quinoidal base, a new species is formed at pD 4.88-6.10. This is corroborated by the appearance of a new set of signals in the NMR spectrum that may be assigned to the formation of hemiketal/cis-chalcone species to a small extent. The two ionization constants (pK(a1) and pK(a2)) obtained by both methods (NMR and UV-visible) for methylpyranomalvidin-3-O-glucoside are in agreement (pK(a1) = 5.17 +/- 0.03; pK(a2) = 8.85 +/- 0.08; and pK(a1) = 4.57 +/- 0.07; pK(a2) = 8.23 +/- 0.04 obtained by NMR and UV-visible spectroscopies, respectively). Moreover, the fully dianionic unprotonated form (at the methyl group) of the methylpyranomalvidin-3-O-glucoside is converted slowly into a new structure that displays a yellow color at basic pH. On the basis of the results obtained through LC-MS and NMR, the proposed structure was found to correspond to the flavonol syringetin-3-glucoside.
ISI Document Delivery No.: 717SBTimes Cited: 7
Cited Reference Count: 47
Oliveira, Joana Petrov, Vesselin Parola, A. Jorge Pina, Fernando Azevedo, Joana Teixeira, Natercia Bras, Natercia F. Fernandes, Pedro A. Mateus, Nuno Ramos, Maria Joao de Freitas, Victor
Oliveira, Joana/G-8108-2012; Parola, A. Jorge/F-4048-2010; Pina, Fernando/C-8161-2011; Fernandes, Pedro/D-6327-2013; Bras, Natercia/D-5493-2013; Ramos, Maria/D-6183-2013; Caparica, cqfb_staff/H-2611-2013; de Freitas, Victor/H-7789-2013; REQUIMTE, AL/H-9106-2013; Chaves, Pedro/K-1288-2013
Parola, A. Jorge/0000-0002-1333-9076;
FCT (Fundacao para a Ciencia e a Tecnologia-Praxis), Portugal [BPD/65400/2009]; FCT (Fundacao para a Ciencia e a Tecnologia), Portugal [PTDC/AGR-ALI/65503/2006, PTDC/QUI/67681/2006, REDE/1517/RMN/2005]; FEDER
We thank Dr. Zelia Azevedo and Dr. Mariana Andrade for the LC/DAD-MS and NMR analysis, respectively. This research was supported by a postdoctoral grant from FCT (Fundacao para a Ciencia e a Tecnologia-Praxis BPD/65400/2009) and a grant from FCT (Fundacao para a Ciencia e a Tecnologia-PTDC/AGR-ALI/65503/2006, PTDC/QUI/67681/2006, REDE/1517/RMN/2005) all from Portugal and by FEDER funding.
Amer chemical soc
Washington