Spectrochimica Acta Part A 64 (2006) 1002–1009
UV/vis,
1
H, and
13
C NMR spectroscopic studies
to determine mangiferin pK
a
values
Berenice G ´ omez-Zaleta
a
, Mar´ ıa Teresa Ram´ ırez-Silva
a
, Atilano Guti´ errez
a
, Enrique
Gonz´ alez-Vergara
b
, Marisol G ¨ uizado-Rodr´ ıguez
c
, Alberto Rojas-Hern´ andez
a,∗
a
Universidad Aut´ onoma Metropolitana-Iztapalapa, Depto. Qu´ ımica,
´
Area de Qu´ ımica Anal´ ıtica,
Apdo. Postal 55-534, 09340 M´ exico DF, M´ exico
b
Centro de Qu´ ımica, Instituto de Ciencias, Benem´ erita Universidad Aut´ onoma de Puebla, M´ exico
c
Depto. de Qu´ ımica Inorg´ anica, Instituto de Qu´ ımica, UNAM, Ciudad Universitaria, M´ exico DF, M´ exico
Received 14 June 2005; accepted 12 September 2005
Abstract
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
a
1
(6-H) = 6.52 ± 0.06; H
3
(MGF)
-
=H
2
(MGF)
2-
+H
+
,pK
a
2
(3-H) = 7.97 ± 0.06;
H
2
(MGF)
2-
= H(MGF)
3-
+H
+
,pK
a
3
(7-H) = 9.44 ± 0.04; H(MGF)
3-
= (MGF)
4-
+H
+
,pK
a
4
(1-H) = 12.10 ± 0.01; where it has been considered
mangiferin C
19
H
18
O
11
as H
4
(MGF). Mangiferin UV/vis spectral behavior, stability study in aqueous solution as well as NMR spectroscopy studies:
one-dimensional
1
H,
13
C, 2D correlated
1
H/
13
C 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.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Mangiferin; Acidity constants; UV/vis spectroscopic determination;
1
H NMR;
13
C NMR
1. Introduction
Mangiferin (1,3,6,7-tetrahydroxyxanthone-C
2
--D-glucosi-
de) (Scheme 1) is a glucoside xanthone widely distributed in
higher plants in families such as Anarcardiaceae [1], Gen-
tianaceae [2] and Guttiferae [3]. Some of these have been rec-
ommended as traditional medicinal plants [4,5]. Recently, in
Cuba, it has been developed an aqueous decoction of mango
(Mangifera indica L.) stem bark used as a nutritional supplement
and phytomedicine registered as Vimang
®
[7]. The Cuban stud-
ies underline its anti-tumoral effect. Something similar occurs
with Salaretin
®
, a Sri Lanka’s product that can be used in the
obesity treatment and particularly on diabetes type II [8,9]. Actu-
ally, in the present, the impact about the mangiferin is due to its
pharmacological effects as an anti-tumoral, anti-viral, and anti-
∗
Corresponding author. Tel.: +52 55 58044670; fax: +52 55 58044666.
E-mail address: suemi918@xanum.uam.mx (A. Rojas-Hern´ andez).
diabetic agent [6,10–16]. The mangiferin bioactivities observed
have been related to some antioxidative as well as free radi-
cal captodative functions [17–19] since its structure seems to
be associated with certain flavonoid systems and their activities
[20,21].
However, the studies about mangiferin generally are done
from the pharmacological perspective, not from the physico-
chemical point of view. The previous UV/vis and NMR studies
were focused on its identification [22–25]. There is no infor-
mation about the mangiferin equilibrium constants, pK
a
values
or complexation constants with inorganic cations. The present
work is focused on obtaining at least some of this informa-
tion. In this study, we considered some graphic methods and
the use of computational programs to determine the equilibrium
constants. SQUAD [26] program has been used before by our
research group, and excellent results for UV/vis spectroscopic
data [27–29] have been obtained. Thus, a similar approach was
undertaken for the equilibrium constants of mangiferin in aque-
ous solution. pK
a
values determination of H
4
(MGF) in aqueous
1386-1425/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2005.09.009