UV/vis, 1H, and 13C NMR spectroscopic studies to determine mangiferin pKa values

The acid constants of mangiferin (a natural xanthonoid) in aqueous solution were determined through an UV/vis spectroscopic study employing the SQUAD program as a computational tool. A NMR study complements the pKa values assignment and evidences a H-bridge presence on 1-C. The chemical model used was consistent with the experimental data obtained. The pKa values determined with this procedure were as follows: H4(MGF) = H3(MGF)− + H+, pKa1pKa1(6-H) = 6.52 ± 0.06; H3(MGF)− = H2(MGF)2− + H+, pKa2pKa2(3-H) = 7.97 ± 0.06; H2(MGF)2− = H(MGF)3− + H+, pKa3pKa3(7-H) = 9.44 ± 0.04; H(MGF)3− = (MGF)4− + H+, pKa4pKa4(1-H) = 12.10 ± 0.01; where it has been considered mangiferin C19H18O11 as H4(MGF). Mangiferin UV/vis spectral behavior, stability study in aqueous solution as well as NMR spectroscopy studies: one-dimensional 1H,13C, 2D correlated 1H/13C performed by (g)-HSQC and (g)-HMBC methods; are also presented. pKa values determination of H4(MGF) in aqueous solution is a necessary contribution to subsequent pharmacokinetic study, and a step towards the understanding of its biological effects.