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 pK(a) values assignment and evidences a H-bridge presence on 1-C. The chemical model used was consistent with the experimental data obtained. The pK(a) values determined with this procedure were as follows: H-4(MGF)=H-3(MGF)(-) + H+, pK(a1) (6-H) = 6.52 +/- 0.06; H-3(MGF)(-) = H-2(MGF)(2-) + H+, pK(a2) (3-H) = 7.97 +/- 0.06; H-2(MGF)(2-) = H(MGF)(3-) + H+, pK(a3) (7-H) = 9.44 +/- 0.04; H(MGF)(3-) = (MGF)(4-) + H+, pK(a4) (1-H) = 12.10 +/- 0.01; where it has been considered mangiferin C19H18O11 as H-4(MGF). Mangiferin UV/vis spectral behavior, stability study in aqueous solution as well as NMR spectroscopy studies: one-dimensional H-1, C-13, 2D correlated H-1/C-13 performed by (g)-HSQC and (g)-HMBC methods; are also presented. pK(a) values determination of H-4(MGF) in aqueous solution is a necessary contribution to subsequent pharmacokinetic study, and a step towards the understanding of its biological effects. (c) 2005 Elsevier B.V. All rights reserved.