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O R I G I N A L R E S E A R C H
The tautomerism influence on the antioxidant predictionof oxederavone
Sheise A. S. Brgida Ednilson Orestes Taina G. Barros
Carlos A. L. Barros Agnaldo S. Carneiro
Alberico B. F. da Silva Rosivaldo S. Borges
Received: 14 November 2012 / Accepted: 18 February 2013
Springer Science+Business Media New York 2013
Abstract A detailed theoretical prediction of oxedaravone
(a hydro-soluble derivative of edaravone) as antioxidant wasperformed. The DFT method with B3LYP/6-31G(d) basis
sets was used for the tautomerism study in gas phase and
solvent medium (water and methanol) and the antioxidant
capacity was predicted from HOMO, ionization energy (IE),
and bond dissociation energies (BDE). All calculations
showed that the ketoenol tautomerism of oxedaravone is
thermodynamically more favored than the imineenamine
tautomerism andthe solvent effect reduced theisomerization
energy barriers for the OH or NH tautomers. HOMO and
IE values showed that the NH tautomer is a better antioxi-
dant, while the BDE values showed that the OH tautomer is
the better antioxidant. Our results showed that the oxeda-
ravone may be a good strategy for designing polar deriva-
tives of edaravone.
Keywords Edaravone derivatives DFT Antioxidant Ionization Tautomerism Scavenging
Introduction
Edaravone 1, a recently developed free-radical scavenger
for clinical use, can quench radicalar reactions by trapping
a variety of free-radical oxygen and nitrogen species
(Tabrizchi, 2000). It was developed as a medical drug forbrain ischemia process (Kawai et al., 1997; Watanabe
et al., 1986) and has been reported to be effective for
myocardial ischemia, as well (Wu et al., 2002) as an effi-
cient antioxidant, which is considered to be the basis of its
protective effect against ischemia and has been reported to
prevent against hydroperoxy polyunsaturated fatty acids,
an oxidation product generated by lipoxygenase-induced
vascular endothelial cell injury and irradiation-induced in
endothelial nitric oxide synthase expression (Yamamoto
et al., 1997).
Its pharmacological effect arises from the scavenging
activity against free-radicals. In fact, it is efficient as
scavenging hydroxyl (HO) as DPPH radicals (DPPH)
(Tabrizchi, 2000). The HO is one of the most reactive
oxygen species in nature; it is not surprising that edaravone
possesses a high HO scavenging activity (Wang and
Zhang, 2003).
The study of its antioxidant mechanism through density
functional theory (DFT) calculations revealed that the
H-atom-abstraction rather than electron-transfer reaction is
involved in the radical-scavenging process of edaravone,
and the H-atom at position 4 is ready to be abstracted
(Wang and Zhang, 2003). The bond dissociation energy of
the methylene moiety (BDECH) of edaravone is higher than
the bond dissociation energy of the hydroxyl moiety
(BDEOH) of a-tocopherol, accounting for the activity dif-
ference between the two antioxidants (Wang and Zhang,
2003). As substituents have little influence on the BDECH,
2-pyrazolin-5-one is recognized as the active center for
edaravone (Wang and Zhang, 2003). However, other
authors have considered that edaravone anion is reported to
be an active form in scavenging free-radicals by a one-
electron-transferring mechanism (Watanabe et al., 1986),
S. A. S. Brgida T. G. Barros C. A. L. Barros A. S. Carneiro R. S. Borges (&)Nucleo de Estudos e Selecao de Biomoleculas da Amazonia,
Instituto de Ciencias da Saude, Universidade Federal do Para,
CP 11101, Belem, PA 66075-110, Brazil
e-mail: [email protected]
E. Orestes A. B. F. da Silva R. S. BorgesInstituto de Qumica de Sao Carlos, Universidade de Sao Paulo,
CP 780, Sao Carlos, SP 13560-970, Brazil
123
Med Chem Res
DOI 10.1007/s00044-013-0553-0
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that is a certain contradiction between the authors, since
some authors have related the antioxidant activity of eda-
ravone with the hydrogen abstraction of the ring pirazolone
(Wang and Zhang, 2003) and other have related the anti-
oxidant activity of edaravone with the electron donation of
the corresponding anionic form, an enolate ion (Watanabe
et al., 1986).
Previous results showed that the electron abstraction isrelated to electron-donating groups (EDG) at position 3,
decreasing the ionization energy (IE) when compared to the
substitution at position 4 (Perez-Gonzalez and Galano,
2012). However, the hydrogen abstraction is related to the
electron-withdrawing groups EDG at position 4, decreasing
the BDECH when compared to other substitutions, resulting
in a better antioxidant activity (Perez-Gonzalez and Galano,
2012; Borges et al., 2012). The unpaired electron after a
hydrogen abstraction from the CH group of the pyrazole
ring is localized at 2, 4, and 6 positions. The highest scav-
enging activity prediction is related to the lowest contribu-
tion at the carbon atom. The likely mechanism is related tohydrogen transfer. It was found that the antioxidant activity
depends on the presence of EDG at the C2 and C4 positions
and there is a correlation between IE and BDE. Our and other
results have identified three different classes of new deriv-
atives more potent than edaravone (Perez-Gonzalez and
Galano, 2012; Borges et al., 2012).
Nevertheless, in previous theoretical studies few com-
pounds are water soluble or polar (Watanabe et al., 1986;
Wang and Zhang, 2003; Borges et al., 2012). Furthermore,
the ischemia process is related to the free-radical species in
plasmatic phase (Yamamoto et al., 1997). Therefore, it is
interesting to clarify the scavenging mechanism and the
influence of the hydro-soluble derivative of edaravone 1,
named oxederavone 2 (Fig. 1), which will be helpful to
elucidate the structureactivity relationship in the oxidation
process.
Methodology
All calculations were performed with the Gaussian 03
molecular package (Frisch et al., 2003). Prior to any DFT
(Parr et al., 1999) calculation, all isomers were submitted
to a PM3 (Stewart, 1986) geometry conformational search.
All the lowest geometries obtained by semi-empirical PM3
were reoptimized by means of DFT/B3LYP (Becke, 1993;
Lee et al., 1988) with the 6-31G* basis sets (Hehre et al.,
1986).
The IE were calculated as the energy difference between
a neutral molecule (EMXH) and the respective cation free-radical (EMXH?) (Eq. 1), where M is the molecule and X
is the methylene, hydroxyl, or imine groups.
IE EMXH EMXH 1
The bond dissociation energies of methylene, hydroxyl,
and amine moieties (BDEXH) were calculated as the energy
difference between a neutral molecule (EMXH) and the
respective semiquinone (EMX) plus hydrogen radical or
hydrogen atom (EH) (Eq. 2).
BDEXH EMX EH EMXH 2
All calculations were performed in gas phase with thepurpose of obtaining the intrinsic properties of the tautomers
studied, free of any interaction. The solvent effect was
calculated in the presence of water or methanol simulated
using the polarizable continuum model (PCM) (Hehre et al.,
1986) implemented in the Gaussian 03W package. Then, the
molecular electrostatic potential surfaces were drawn using
the Gaussian 03 (Frisch et al., 2003). The molecular
visualization was performed with the program Molekel 4.2
(Portmann and Luthi, 2000).
In the present work, we aimed at exploring the hydro-
soluble derivative of edaravone 1, oxedaravone 2, for the
antioxidant properties and tautomerism taking into accountsolvent effects. We are interested in knowing the role
played for the different five tautomer of this molecule 2
(Fig. 2), to a better understanding of the oxedaravone
oxidation and its possible application as an antioxidant.
Results and discussion
Oxedaravone tautomerism
The dicarbonylic form (2a) is the most stable tautomer of
oxederavone and the values of the relative barriers of tau-
tomerization energies related to other tautomers are 15.62
(2b),66.73(2c), 62.95 (2d), and 71.56 (2e). These values are
shown in Table 1. Consequently, the imine and hydroxylated
forms are less favored thermodynamically. Therefore, as
show in the Fig. 3, there is no difference between the ox-
ederavone form 2a and 2b in water or methanol by PCM
methods. A low relative barrier was found between 2a and 2b
with values of 15.62, 16.67, and 16.34 kJ mol-1 in gas
phase, water, and methanol, respectively. Nevertheless, the
(1)
NN
H3C
O
(2)
NN
HO
O1
2
34
5
67
8
9
10
11
12
13
Fig. 1 Structure and numbering of edaravone (1) and oxederavone
(2)
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imine and hydroxylated forms have the lowest values in
water or methanol than in gas phase.
These results are in accordance with the study of the
edaravone tautomerism (Queiroz et al., 2010). In this
study, the more stable carbonyl tautomer is responsible for
radical-scavenging reaction of edaravone and was mea-
sured in gas phase and solvents such as water and methanol
(Ohara et al., 2006).
Our result showed that keto tautomerism of oxedaravone
is more favored than the enol form. However, it is possible
that the keto form has the larger radical-scavenging activity
than the enol form. In fact, previous works have demon-
strated that the BDE of C7H is higher than C4H for
edaravone. Nevertheless, the BDENH and BDEOH corre-
sponding to positions 3 and 6, respectively, are much lower
than the BDECH corresponding to position 4. Moreover, the
relative barrier between CH and NH or OH tautomers
of edaravone is 32.76 and 43.22 kJ mol-1, respectively
(Wang and Zhang, 2003; Queiroz et al., 2010).
In accordance with Wang and Zhang, CH tautomer of
edaravone is unstable. Nevertheless, the solvent effect by
PCM methods decreased the energy barriers in NH or OH
tautomerization of edaravone for 11.59 or 25.14 kJ mol-1 in
water and 12.71 or 26.15 kJ mol-1 in methanol, respec-
tively. Furthermore, the solvent effects decreased the energy
barriers for the NH or OH of edaravone tautomerization(Wang and Zhang, 2003; Queiroz et al., 2010).
The edavarone tautomerization can be associated with
the scavenging activity of edavarone in ischemia and the
hydrogen at the 2, 4, 6, and 13 positions are acid. The
ionized form of edaravone (anion) is reported to be an
active form in scavenging free-radicals by a one-electron-
transfer mechanism (Watanabe et al., 1986) and its effi-
ciency as radical scavenger was assumed to be due to the
increasing of its anion form as active form (Nakagawa
(2a)
(2b)(2e)
(2d) (2c)
HNN
O
O
NN
HO
O
NN
HO
OHHN
N
HO
O
HNN
O
OH
Fig. 2 Five tautomeric forms
of oxedaravone
Table 1 Relative energy barriers of oxedaravone tautomers
Position DEgas phase DEin water DEin methanol
2a 0 0 0
2b 15.62 16.67 16.34
2c 66.73 51.99 53.01
2d 62.95 37.14 39.09
2e 71.56 46.16 47.05
Fig. 3 Relative energy barriers of the five tautomeric forms of the
oxedaravone
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et al., 2006). Nevertheless, the ischemia is a result of the
marked pH reduction of the tissue by generation of lactic
acid as well as protons (Ohara et al., 2006).
This way, the changes of EDG in 3-position of pyraz-
olone ring through substitution of methyl to carbonyl or its
tautomer hydroxyl group may facilitate the increase of
polarity and acidity of the pyrazolone ring.
The difference between the electronic behaviors of oxe-daravone tautomers can be observed by molecular electro-
static potentials (MEPs). These MEPs for the five
oxedaravone tautomers are presented in Fig. 4. All car-
bonyl, hydroxyl, imine, and enamine moieties present
negative charges (red and yellow), but the intermediate
charges are located on the carbon atoms (green) of the
aromatic or aliphatic moieties. Otherwise, the positive
charges (blue) are located on the hydrogen atoms. Actually,
the heteroatoms directly attached to the pyrazolone ring are
the ones responsible for the electronic characteristics pre-
sented by the molecules studied here. In fact, the electro-
static profiles for keto groups are significantly differentfrom the hydroxyl compounds.
Electron abstraction
The antioxidant capacity of keto, hydroxyl, imine, or enam-
ine tautomers of oxedaravone were theoretically predicted
using HOMO and IE values. The HOMO energy is an
important parameter for molecular electronic structure. The
molecule which has the lower HOMO energy has a weak
electron donating ability. On the contrary, the higher
HOMO energy implies that the molecule is a good electron
donor (Queiroz et al., 2009). The HOMO disposition
around the hydroxylpyrazolone rings can indicate the site
of reaction with free-radicals qualitatively because the
H-abstraction reaction undergoes electron transfer. These
tautomers have every position rich in electrons, as shown in
the HOMO maps (see Fig. 5). In fact, all phenyl, methyl,and hydroxyl or carbonyl groups at the 1 and 2-positions
have great contribution for the electronic formation of
HOMO, increasing the chemistry reactivity of these
molecules.
The HOMO values are showed in Tables 1, 2. The
tautomers showed the following HOMO values in gas
phase: -6.06 to -5.61 eV. Therefore, all hydroxyl and
imine tautomers are more nucleophilic than the carbonyl
tautomers.
Also, the IE values in gas phase for the tautomers are
760.75700.14 kJ mol-1. These results are in accordance
with the HOMO values. In fact, previous work hasdemonstrated a good tendency between HOMO and IE
(Queiroz et al., 2009). Therefore, the result indicates that
the OH tautomer increased the HOMO and decreased the
IE values.
All calculations showed that the 2b tautomer is more
nucleophilic than all other tautomers, decreasing the IE
values. Therefore, the electron donation is more favored. In
this case, the electron transfer is stabilized by the electron
conjugation between hydroxyl and imine groups. In accor-
dance with the electron abstraction, the hydroxyl group is
Fig. 4 MEPs of the five tautomeric forms of oxedaravone Fig. 5 HOMO of the five tautomeric forms of oxedaravone
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responsible for the more antioxidant property of oxede-
ravone than edaravone.
In fact, the spin density distribution showed a character-
istic contribution of the tautomer 2b on the 3C, 4C, 5C, and
6C positions (see Table 3). Consequently, the tautomer that
has the lowest global contribution for the phenyl and pyr-
azolone rings is more nucleophilic. These results showed
that the hydroxyl-imine tautomer can be responsible for the
antioxidant capacity by electron abstraction due to its higherHOMO and lower IE values.
Hydrogen abstraction
The bond dissociation energies (BDEXH) for oxedaravone
tautomers for different CH, OH, or NH substitutions
were calculated. These values represent the easiness of the
hydrogen donation of oxedaravone tautomers to produce
semiquinone derivatives. The hydrogen abstraction was
another antioxidant mechanism studied (Queiroz et al.,
2009; Cao et al., 2003). Therefore, molecules with the
lowest BDEXH are more active.As can be seen from Fig. 6, the BDEXH values to the
tautomer of lower energy (2a) in gas phase are
398.47 kJ mol-1 for CH and 335.56 kJ mol-1 for NH.
The next tautomer with lower energy (2b) has BDEXH values
of 348.58 kJ mol-1 for CH and 319.93 kJ mol-1 for OH.
The BDEXH values for other tautomers are of higher energies
(2c, 2d, and 2e).
The hydrogen donation at CH position is more difficult.
Nonetheless, OH and NH bond dissociation energies are
favored due to electronic interactions mainly for the tau-
tomer 2d. This result suggests that hydroxyl or carbonyl
tautomers of oxedaravone have more electronic stabiliza-tion than the methyl moiety.
In accordance with the electron abstraction, the hydro-
gen abstraction showed that the NH and OH tautomers
can be responsible for the antioxidant capacity by hydrogen
abstraction due to its higher HOMO and lower IE and
BDEXH values. Nevertheless, edaravone has the lowest
BDEXH values, i.e., 343.67, 310.91, and 300.45 kJ mol-1
for CH, NH, or OH, respectively.
Contrarily to the electron abstraction, the spin density
distribution after hydrogen abstraction with semiquinone
compounds showed a lower spincontribution on the oxygen or
nitrogen atoms, 1,3-positions of pyrazolone ring and globalspin contribution for the phenyl and pyrazolone rings (see
Table 4). Consequently, these results showed that the anti-
oxidantcapacity of oxedaravone can be determined mainly by
the stability of the semiquinone radical, generated after the
hydrogen donation. Therefore, a symmetric contribution
between the phenyl and pyrazolone rings was needed for the
more stable tautomer. The result of this work canbe important
for the design of new edaravone derivatives.
Conclusion
The theoretical prediction of the oxedaravone tautomerism
was carried out using DFT-B3LYP/6-31G(d). Gas phase
and solvent effects showed that the ketoenol tautomers of
oxedaravone are thermodynamically more favored than the
Table 2 Calculated properties of oxedaravone tautomers
Position HOMO (eV) IE (kJ mol-1)
1 -5.73 723.99
2a -6.06 760.75
2b -5.66 700.14
2c -5.61 717.27
2d-
5.69 726.03
2e -5.67 724.27
Table 3 Spin density contribution in cation radical of the oxedaravone tautomers calculated by electron abstraction
Position 2a 2b 2c 2d 2e
1N 0.305918 0.294384 0.264365 0.324966 0.299915
2N 0.143442 0.042923 0.019815 0.087726 0.070777
3C -0.027008 0.080433 0.133693 -0.022708 -0.093880
4C 0.004539 -0.005398 0.062706 0.013520 0.302434
5C -0.058345 -0.028949 -0.010669 -0.058069 -0.106994
6O 0.138687 0.095594 0.016676 0.185821 -0.016531
7C 0.082953 0.054494 0.077003 0.085143 0.049817
8C 0.101649 0.148802 0.109278 0.113520 0.079937
9C -0.042654 -0.077893 -0.055846 -0.049831 -0.036699
10C 0.258679 0.305536 0.241651 0.269930 0.173009
11C -0.074866 -0.091523 -0.060561 -0.079264 -0.048886
12C 0.129823 0.160784 0.112177 0.137585 0.085885
13O 0.056023 0.048396 0.113920 0.002962 0.261715
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imineenamine tautomers. The HOMO and ionization
energy values showed that carbonyl or hydroxyl tautomers
are better antioxidants than the imine or enamine tautom-
ers. The calculated bond dissociation energies showed that
the OH tautomer is a better antioxidant than the NH or
CH ones. Our results revealed that the OH tautomer is
more important for the antioxidant property of oxedarav-one due to its lower bond dissociation energies.
Acknowledgments The authors are grateful to the Brazilian
Agencies CNPq and UFPA by financial support.
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Position 2a 2b 2c 2d 2e
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2N 0.489441 0.489277 0.399726 0.402683 -0.017143
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13O 0.211571 0.211594 0.010378 0.014831 0.285859
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