Synthesis, characterization and studies on antioxidant and molecular docking of metal complexes of 1-(benzo[d]thiazol-2-yl)thiourea

Article abstract of DOI:10.1007/s12039-015-0999-3

In the present study, a new thiourea derivative bearing benzothiazole ligand, 1-(benzo[d]thiazol-2-yl)thiourea (btt) and its ternary metal (Cu(II), Co(II) and Ni(II)) complexes were synthesized. The structural characterization was carried out by micro analysis, IR, ¹H-NMR, EPR, UV-Visible spectral analyses, molar conductance and thermal analysis studies. Spectral studies of complexes revealed that the metal complexes have distorted octahedral geometry. Molecular modelling study was performed to evaluate the recognition of target compounds at the 3MNG binding pocket. The docking results revealed that copper complex selectively binds to the crucial amino acid residues in the active site of 3MNG. The in vitro antioxidant activity of the ligand and its metal complexes was assayed by radical scavenging activity (DPPH, H₂O₂ and NO) and ferric reducing antioxidant power (FRAP) methods. The ligand showed moderate antioxidant activity whereas the metal complexes exhibited better antioxidant activity than that of the ligand. The results of the four methods proved that the copper complex is the most potent antioxidant among all the tested compounds.

Full text of DOI:10.1007/s12039-015-0999-3

J. Chem. Sci. Vol. 128, No. 1, January 2016, pp. 43–51. ꢀc Indian Academy of Sciences.
DOI 10.1007/s12039-015-0999-3
Synthesis, characterization and studies on antioxidant and molecular
docking of metal complexes of 1-(benzo[d]thiazol-2-yl)thiourea
a
b
a
HARINATH YAPATI , SUBBA RAO DEVINENI , SURESH CHIRUMAMILLA and
a,∗
SESHAIAH KALLURU
Inorganic and Analytical Chemistry Division, Sri Venkateswara University, Tirupati 517 502,
a
Andhra Pradesh, India
b
Research and Development Centre, Micro Labs Ltd., API Division, Bommasandra-Jigini Link Road,
KIADB INDL Area, Bangalore 560 100, Karnataka, India
e-mail: seshaiahsvu@gmail.com
MS received 24 July 2015; revised 20 October 2015; accepted 21 October 2015
Abstract. In the present study, a new thiourea derivative bearing benzothiazole ligand, 1-(benzo[d]thiazol-2-
yl)thiourea (btt) and its ternary metal (Cu(II), Co(II) and Ni(II)) complexes were synthesized. The structural
1
characterization was carried out by micro analysis, IR, H-NMR, EPR, UV-Visible spectral analyses, molar
conductance and thermal analysis studies. Spectral studies of complexes revealed that the metal complexes
have distorted octahedral geometry. Molecular modelling study was performed to evaluate the recognition of
target compounds at the 3MNG binding pocket. The docking results revealed that copper complex selectively
binds to the crucial amino acid residues in the active site of 3MNG. The in vitro antioxidant activity of the
ligand and its metal complexes was assayed by radical scavenging activity (DPPH, H O and NO) and ferric
2
2
reducing antioxidant power (FRAP) methods. The ligand showed moderate antioxidant activity whereas the
metal complexes exhibited better antioxidant activity than that of the ligand. The results of the four methods
proved that the copper complex is the most potent antioxidant among all the tested compounds.
Keywords. Benzo[d]thiazol; thiourea; metal complexes; antioxidant activity; molecular docking.
1
. Introduction
The thiazoles, containing N and S heterocycles, are
8
–11
found in many naturally occurring compounds.
Thia-
and antioxi-
They also form biologically potent metal
12,13
Thioureas are important functional derivatives posses-
sing good biological activities like antiviral, antibacte-
rial, antifungal, antitubercular, anti-thyroidal, herbicidal
zoles are used as neuro protectors
14,15
dants.
complexes.
16–19
Metal complexes of thiazoles recently
1
–4
and insecticidal.
These are also found as precur-
attracted attention of inorganic, metallo-organic as well
as bio-inorganic chemists because of their extensive
applications in wide ranging areas from material to
sors or intermediates in the synthesis of a variety of
heterocyclic systems such as imidazole-2-thiones, 2-
imino-1,3-thiazolines, N-4-substituted-benzyl-N-tert-
butylbenzyl thioureas as vanilloid receptors and antag-
20
biological sciences.
Thioazoles attached with thiourea functional group
5
21
onists in rat DRG neurons. Further, nitrogen and sulfur
are used as pharmaceutical agents. The biological and
synthetic significance of thiazole, and thiourea deriva-
tives prompted us to synthesize new molecules:
donor atoms in thiourea provide a multitude of bonding
possibilities, hence, its derivatives are versatile ligands
and are able to coordinate to metal centers either as
neutral ligands, mono anions or di-anions and form
stable complexes. They can behave as thiolate-type
ligands, since a substituted thiourea bearing at least one
1
-(benzo[d]thiazol-2-yl)thiourea and its (Cu(II), Co(II)
and Ni(II)) complexes. Transition metal ions such as
Cu(II), Co(II) and Ni(II) were selected since they
6
22,23
enhance biological activity.
The synthesized ligand
1
2
3
hydrogen, R R NC(=S)NHR can exist in a tautomeric
and metal complexes were characterized by elemen-
1
2
3
1
thiolate form, i.e., R R C(SH)=NR . Metal complexes
of thiourea derivatives are bio-active compounds and
show wide range of biological activities.7
tal analyses, IR, H-NMR, ESR, UV-Visible spectra,
molar conductance and thermal analysis studies. Fur-
ther, the in vitro antioxidant activity and molecular
modelling study against 3MNG binding pocket were
performed.
∗
For correspondence
43

4
4
Harinath Yapati et al.
Found: C, 45.86; H, 3.41; N, 20.05; S, 30.61%. IR
2
. Experimental
−1
1
(
cm ); 3318 (w), 1565 (m), 842 (m), 710 (s). H-NMR
2
.1 Materials and physical measurements
(
7
DMSO-d ): δ 2.54 (s, 1H, NH), 9.30 (2H, s, NH ),
.20-7.85 (4H, m, Aromatic). C-NMR (DMSO-d6) in
6
2
13
Analytical grade (AR) chemicals with high purity were
purchased from Sigma Aldrich, Spectrochem and S D
Fine Chem. and used without further purification. The
analysis of CHNS of ligand and metal complexes were
δ (ppm): 144.3, 169.01, 116.31, 123.3, 124.0, 127.2,
−1
1
3
27.7, 128.3. Electronic spectra (DMF, cm ); 47619,
7735, 43516, 26667.
carried out on Euro elemental analyzer. IR spectra
1
were recorded on Bruker spectrometer. H NMR was 2.3 Synthesis of Metal Complexes
recorded on a Bruker Advanced Drx-200 MHz spec-
All the three metal complexes (5-7) were prepared by
a general procedure. To the hot solution (60 C) of
trometer with TMS as an internal standard. The melting
points of the compounds were determined on Beijing
XT-4-100x microscopic melting point apparatus and are
uncorrected. TG/DTA curves for the complexes were
recorded on Nietzsche thermo balance (model-STA
◦
the appropriate metal chloride (1 mmol) in ethanol and
water mixture (1:1, 25 mL), 1-(benzo[d]thiazol-2-yl)
thiourea (4) (0.41 g, 2 mmol) was added to the same sol-
vent mixture (20 mL). The resulting mixture was stirred
under reflux for 1–2 h, the precipitate was filtered off,
washed twice with a 1:1 ethanol: water mixture.
4
0 g) with PtVsP 10% Rh thermocouple in dynamic air
t
◦
conditions between the room temperature (∼20 C) and
◦
1
020 C (rate 10 K/min).
2
.2 Preparation of the ligand: 1-(benzo[d]thiazol-2-yl)
2.3a [Cu(btt)
M.p.: 246 C, Anal(%). Calcd(%). for C16
2
Cl
2
] (5): Green solid, Yield: 83%,
CuN
◦
thiourea
H
12Cl
2
6
S4: C, 34.90, H, 2.23, N, 15.22, S, 23.26, Cu, 11.54, Cl,
Aniline (0.1 mole, 9.10 mL) (1) and concentrated 12.89. Found: C, 34.88, H, 2.23, N, 15.22, S, 23.26, Cu,
−1
hydrochloric acid (15 mL) were mixed and the solu- 11.54, Cl, 12.89%. IR data (KBr, cm ); 3329 (w), 1584
1
tion was warmed for 30 min. A saturated solution of (m), 762 (m), 718 (s). H-NMR (DMSO-d6; δ 10.23
ammonium thiocyanate in water (5 g in 10 mL) was (2H, s, NH ), 7.25-7.91(4H, m, Aromatic). Electronic
2
−1
added slowly to the above solution. The mixture was spectra (DMF, cm ); 15267, 16525, 17211, 21739.
boiled until the solution attained the turbidity. This
solution was poured in cold water. The precipitate
was filtered and re-crystallized from aqueous ethanol
to afford pure phenylthiourea (2). The phenylthiourea
2
.3b [Co(btt) Cl ] (6): Reddish-brown solid, Yield:
◦
16 12 2
2
2
7
9%, M.p.: 320 C, Anal(%). Calcd(%). For C H Cl
CoN S : C, 35.20, H, 2.25, N, 15.36, S, 23.45, Co,
1
2
(20 mmol, 3.04 g) in chloroform (20 mL) was bromi-
6 4
0.81, Cl 13.02. Found: C, 35.17, H, 2.21, N, 15.38, S,
nated by using bromine solution in chloroform (5%)
till the orange-yellow colour appeared. The slurry was
kept overnight at ambient temperature. The obtained
precipitate was filtered and washed with chloroform
until the color disappeared. The precipitate as hydro-
bromide was dissolved in rectified spirit (30 mL) and
basified with ammonia solution (20 mL). The precipi-
tate was filtered, washed with water, dried and recrys-
tallized from ethanol:dichloromethane mixture (1:2) to
get pure benzo[d]thiazole-2-amine (3). The compound
−1
3.47, Co, 10.79, Cl, 12.98. IR data (KBr, cm ): 3332
1
(
w), 1585 (m), 786 (m), 724 (s). H-NMR (DMSO-d ;
6
δ 9.07 (2H, s, NH ), δ 7.40–7.82 (4H, m, Aromatic).
Electronic spectra (DMF, cm ); 17856, 21734.
2
−1
2
.3c [Ni(btt) Cl ] (7): Pale green solid, Yield: 82%,
◦
16 12 2 6
2
2
M.p.: 284 C, Anal(%). Calcd(%). For C H Cl NiN
S4: C, 35.22, H, 2.25, N, 15.36, S, 23.45, Ni, 10.81,
Cl 13.02. Found: C, 35.19, H, 2.21, N, 15.39, S, 23.48,
Ni, 10.75, Cl, 12.98. IR data (KBr, cm ); 3326 (w),
(3) was treated with concentrated hydrochloric acid
−1
(20 mL) and warmed for 30 min. A saturated solution
1
1
585 (m), 774 (m), 719 (s). H-NMR (DMSO-d ; δ 9.71
6
of ammonium thiocyanate in water (5 g in 10 mL) was
added slowly to the above solution. The mixture was
boiled until the solution got turbidity. The turbid solu-
tion was poured in cold water. The resulting precipitate
was filtered and recrystallized from aqueous ethanol
(
2H, s, NH ), δ 7.25-7.91 (4H, m, Aromatic). Electronic
2
−1
spectra (DMF, cm ); 24271, 15360, 9576.
2.4 Molecular docking
(
(
80%) to obtain pure 1-(benzo[d]thiazol-2-yl)thiourea
4). Yellow solid, Yield: 80%, M.p.: 174 C, Anal. Calc. Molecular modeling studies were performed using
◦
for C H N S : C, 45.91; H, 3.37; N, 20.08; S, 30.64%. Molecular Operating Environment (MOE) version
8
7
3 2

Synthesis of biologically active metal complexes
45
2
008.10 for the synthesized compounds, which exhib- was carried out in triplicate and the average values
ited promising and lower antioxidant activity in vitro calculated.
ꢀ
ꢁ
ꢂ
to find the preferred binding conformations in the
receptor. Three dimensional structures of the ligand
and metal complexes were outlined by Hyper Chem
%
of scavenging = 1 − A
− A
sample
sample−blank
24
ꢃ
/
A
× 100 (1)
control
software which is a refined molecular modeling envi- Here, Acontrol is the absorbance of the control (DPPH
ronment that uniting with quantum chemical calcula- solution without the test compound) and Asample is the
25
tions, molecular mechanics and dynamics. The start- absorbance of the test sample (DPPH solution with the
ing coordinates of the human antioxidant enzyme in test compound) and A
is the absorbance of
sample−blank
26,27
complex
with the competitive inhibitor DTT (PDB: the sample solution (the test compound solution without
3MNG) were taken from the Protein Data Bank (http:// DPPH).
www.rcsb.org/pdb). The docking study was carried out
by the protein loaded in MOE and the errors of the
protein were corrected by the structure preparation
processes in MOE. After the corrections, the ligand
molecules were removed and the polar hydrogen atoms
were added to the isolated target with their standard
geometry. Energy minimization (MMFF94x, gradient:
2
.5b Hydrogen peroxide (H O ) scavenging activity:
2 2
•
The hydroxyl radical (OH ) in aqueous media was gen-
erated through the Fenton reaction. The solutions of
the test compounds were prepared in DMF. The reac-
tion mixture containing 2.5 mL of 0.15 M phosphate
buffer (pH = 7.4), 0.5 mL of 114 μM safranin, 1 mL
of 945 μM EDTA-Fe(II), 1 mL of 3% H O and 30 μL
0.05) was performed and the lowest energy conform-
2
2
ers were selected as the global minimal for model-
ing study. The active site was identified from PDB
Sum as well as correlated with ‘Site Finder’ module
of MOE to define the docking site for the ligands. The
standard protocol implemented in MOE 2008.10 was
followed for docking procedure. The selected ligands
were docked against the lead competitive inhibitor lig-
and DTT at the crystal enzyme structure of the target
protein and the best energy conformations of receptor-
ligand were studied, and the energy of binding was
calculated as the difference between the energy of the
complex and the individual energies of enzyme and
ligand.
of the test compound solution were prepared. The reac-
tion mixture without the test compound was used as the
control. The reaction mixtures were incubated at 37 C
◦
for 60 min on a water bath. Absorbances (A , A , A ) at
i
0
c
•
5
20 nm were measured. The scavenging ratio for OH
was calculated from equation 2.
A − A
i
0
Scavenging ratio (%) =
× 100 (2)
A − A
c
0
Where, A is the absorbance in the presence of the
i
tested compound, A is the absorbance in the absence
0
of the tested compound and A is the absorbance in the
c
absence of the tested compound, EDTA-Fe(II) and
H O .
2
2
2.5 Antioxidant activity
2.5c Nitric oxide (NO) scavenging activity: Nitric
oxide scavenging activity of the tested samples was
measured using a slightly modified protocol of Green
2
.5a 1,1-Diphenyl-2-picryl-hydrazyl (DPPH) assay:
DPPH has been widely used to evaluate the free
radical scavenging capacity of different antioxidants.
The radical in the DPPH has a strong absorption
maximum at 517 nm and the absorbance of DPPH
decreases, when the radical reacts with the antioxi-
dant. All the tested samples in various concentrations
28,29
et al. and Marcocci et al.
All the tested samples
in various concentrations (50, 75 and 100 μg/mL) were
prepared in DMF and homogeneous solutions were
achieved by stirring. Nitric oxide radicals (NO) were
generated from 1 mL of sodium nitroprusside (10 mM)
and 1.5 mL of phosphate buffer saline (0.2 M, pH 7.4)
(50, 75 and 100 μg/mL) were prepared in MeOH and
were added to the test compounds and incubated for
the homogeneous solutions were achieved by stirring.
Aliquot of test sample (1 mL) was added to 4 mL of
◦
1
50 min at 25 C. The reaction mixture of the above
samples (1 mL each) was treated with 1 mL of Griess
reagent (1% sulfanilamide, 2% H PO and 0.1% naph-
thylene diamine dihydrochloride). The absorbance was
measured at 546 nm. Butylated hydroxy toluene (BHT)
was used as a positive control. The ability to scavenge
the NO radicals was calculated by equation 3.
0
.004% (w/v) methanol solution of DPPH and the
reaction mixture was vortexed for 1 min and kept at
room temperature for 30 min in the dark to complete
the reaction. The absorbance was read against blank
at 517 nm. The synthetic antioxidant BHT was used
as positive control. The ability of the tested sam-
ples at tested concentration to scavenge DPPH radi-
cals was calculated using equation 1. The experiment
3
4
A
control − Asample
%
of S ca vengi ng =
× 100 (3)
A
control

46
Harinath Yapati et al.
2
.5d Ferric reducing antioxidant power (FRAP):
N
The reducing power of synthesized compounds was
determined according to the method of Oyaizu. Vari-
ous concentrations of the tested compounds, 50, 75 and
N
30
NH2
S
1
00 mM were mixed with 2.5 mL of phosphate buffer
Cl
S
S
(0.2 M, pH 6.6) and 2.5 mL of 1% potassium ferric
◦
M
cyanide and incubated at 50 C for 20 min. 2.5 mL of
Cl
1
0% trichloro acetic acid was added to this mixture and
centrifuged at 3000 rpm for 20 min. The upper layer
2.5 mL) was mixed with 2.5 mL of deionized water
S
H N
2
(
N
and 0.5 mL of 0.1% ferric chloride and the absorbance
was measured at 700 nm using a spectrophotome-
ter (Shimadzu 160A). Increase in the absorbance of
the reaction mixture indicates higher reducing power.
The experiment was repeated in triplicate and mean
reducing power values are summarized in Figure S11
N
M = Cu, Co, Ni
(in Supplementary Information, SI). BHT was used as
Scheme 2. Proposed structure for metal complexes.
standard and compared with the reducing power of the
synthesized complexes.
metal complexes is low in water, ethanol, chloroform,
acetone and most organic solvents, but have solubility in
DMSO, DMF and decompose at higher temperature.
3
. Results and Discussion
Keeping in mind the biological importance of the
thiourea derivatives bearing heterocyclic scaffold and
3.1 IR Spectral Analysis
metal complexes, the ligand, 1-(benzo[d]thiazol-2- The FT-IR spectra of ligand and its metal complexes
yl)thiourea (4) was synthesized from the aniline (1) as are shown in figure S1. The bands at 1517 and
−1
depicted in scheme 1. All the synthesized intermediates 751 cm due to thioamide stretching and bending
and final product were recrystallized from methanol and vibrations, respectively, of the free ligand are shifted
used for further study. Later, the desired metal com- to lower values indicating coordination of thiolate sul-
plexes, copper(II), cobalt(II) and nickel(II) were pre- phur to the metal ions (table S1). This negative shift of
pared (scheme 2). Analytical data indicated the forma- ν (C=S) in the complexes has already been indicated by
31
tion of 1:2 metal complexes of ligands with Co(II), Campbell. In the spectra of all the complexes, a new
−1
Cu(II) and Ni(II) metal ions. The molar conductance strong band is found in the range of 1612–1649 cm ,
−3
values of the complexes (measured in 10 M DMF) which may be due to the newly formed ν(C=N) bond
−
1
2
−1
are in the range of 15–18 ꢀ cm mol indicating the as a result of enolization. A strong band observed at
−1
non-electrolytic nature of the complexs. Solubility of 751 cm is attributed to ν(C–S–C) of thiazole ring.
H
N
NH2
NH2
N
HCl
Br2, CHCl3
+
4
NH SCN
NH2
S
S
(
1)
(2)
(3)
HCl
N
NH
S
S
NH2
(
4)
1-(benzo[d]thiazol-2-yl)thiourea
Scheme 1. Synthetic route for ligand.

Synthesis of biologically active metal complexes
47
Complexation of the ligand with metals leads to shift 3.4 Electronic spectrum of the 1-(benzo[d]thiazol-2-
in the band to lower frequency. This is probably due to yl)thiourea and its complexes
the coordination properties of the ligand and suggesting
Electronic spectral data of the ligand and its metal com-
plexes are shown in table S2. The electronic spectrum
the involvement of sulfur atoms in the bonding with the
32
−1
metal ions. The bands near 435–450 cm are ascribed
−1
33
of the ligand exhibited absorption bands at 47,619 cm
to ν (M–S). This presents a good evidence for the
participation of thiazole and thiourea sulfur atoms in
complex formation.
−1
and 37,735 cm corresponding to π-π* transitions of
36
the benzenoid system of benzothiazole moiety. The
−1
absorption band at 43,516 cm is attributed to the π-
37
π* transitions of the thiazole moiety. The broad band
3.2 Nuclear magnetic resonance spectroscopy
−1
at 26,667 cm is due to the transition within the mole-
cule, essentially an intra molecular charge transfer in-
teraction. The electronic absorption spectrum of Cu(II)
complex exhibited three broad, low intensity bands
1
The formation of ligand was supported by the H
NMR spectrum, by the appearance of a sharp singlet at
2
.54 ppm, corresponding to the N(2)H proton figure
−
1
−1
−1
1
centered at 15,267 cm , 16525 cm and 17,211 cm .
These bands might reasonably be assigned to Cu(II)
S2). The H-NMR spectra of the complexes when com-
pared with the parent ligand did not show the signal for
N(2)H at 2.54 ppm indicating that the ligand is coor-
dinated to the metal ions in all the complexes in the
thiolate form. The multisignals within the range of
2
2
2
2
2
2
2
d–d transitions, B → A (d
–d ), B → B
1
g
1g
x −y
z
1g
2g
2
2
2
2
2
2
(
d
–d ) and B → Eg (d
–d , d ) respec-
xz yz
x −y
xy
1g
x −y
tively. These d–d transitions are normally close in
2
energy. These transitions are due to the Eg and
T2g states of the octahedral Cu(II) ion (d9) split
7.20–7.85 ppm are assigned to the aromatic protons of
2
benzothiazole ring. These signals are shifted towards
low field in the metal complexes. A singlet for two
under the influence of the asymmetric filling causing
the tetragonal distortion. A shoulder band observed
protons at δ 9.30 is assigned to NH protons and
2
−1
at 21,739 cm is due to ligand–metal charge trans-
fer (LMCT). The electronic spectrum of Ni(II) com-
shifted towards up field in the complexes. This informa-
tion suggests the adjustment of electronic current upon
coordination of >C=S group to the metal ion.
−
1
plex displayed three broad bands at 24,271 cm
(
−1
−1
ν1), 15,360 cm (ν2) and 9576 cm (ν3) which
are assigned to the spin-allowed transitions from the
3
3
3
3
ground state A (F)→ T (P), A (F)→ T (F) and
2g
1g
2g
1g
3.3 Electron paramagnetic resonance spectrum of the
3
3
A (F)→ T (F) states, respectively. The positions of
2g
2g
Cu(II) complex
bands indicated that the Ni(II) complex has six coor-
dinated tetragonal geometry. The electronic spectrum
of Co(II) complex showed two spin-allowed transi-
The X-band EPR spectrum of Cu(II) complex was re-
corded at room temperature (figure S3). The giso values
and the geometric parameter G, i.e. the measurement of
exchange interaction between the copper centers were
evaluated by using equations 4 and 5:
−1
tions at 17856 and 21734 cm which are assignable
4
4
4
4
to T (F)→ A (F) and T (F)→ T (P) transitions
1g
2g
1g
1g
respectively. The positions of bands indicated that
the Co(II) complex has six coordinated tetragonal
geometry.
g + g
|
|
⊥
giso
=
(4)
(5)
3
3.5 Thermal analysis of the metal complexes
g − 2.0023
|
|
In the present investigation, the thermo gravimetry anal-
ysis (TG-DTA) of the complexes was carried out in
G =
g − 2.0023
⊥
◦
the temperature range of 20–1020 C with a sample
◦
The evaluated value of g tensor parameters shows heating rate 10 C/min in a static nitrogen atmosphere.
the order as g > g > 2.0023 which reveals that During the heating, the complexes have undergone a
|
|
⊥
2
2
34
d
is the ground state and also indicates that the series of thermal changes associated with a weight
x −y
2
2
unpaired electron is localized in d
Cu(II) ion. The tetragonal structure is thus confirmed plexes (figures S4–S6) show three stages of decom-
for the complexes. The complexes in the present study position within the temperature range of 160–770 C.
orbital of the loss in the samples. The TGA curves of the com-
x −y
◦
◦
showed the value of G<4 (G = 2.1383) which indicated The first stage of decomposition is observed at 170 C
◦
the effective interaction between the copper centers and to 320 C which corresponds to the loss of HCl. This
35
the ligand.
process is accomplished by exothermic peaks observed

48
Harinath Yapati et al.
◦
at 270–280 C. Second stage occurred within the Besides, one more H-bond was formed with the
◦
38
temperature range of 280–620 C. This stage involves Thr 44. The docked conformer of ligand (figure S7b)
the loss of thiourea moiety. This degradation is accom- showed no hydrogen bonds among active site residues
◦
plished by exo-effects with maxima 370–390 C. Third of 3MNG protein, but it has hydrophobic and electro-
stage of decomposition is observed in the temperature static interactions among ligand and active site residues
◦
range of 400–770 C. For cobalt complex, fourth stage of 3MNG. The molecular docking of the copper com-
◦
is observed in between 760 to 870 C. This process plex (figure S7c) indicated that it forms one stable
◦
is accompanied by an endo-effect observed at 870 C. hydrogen bonding with a bond length 2.76 Å from CN
Above this temperature, no weight change is observed, of copper complex with the CN group of the key pocket
leaving the metal oxide as the final residue.
residue of Gly 46. Nickel complex was found to be
forming two hydrogen bondings (figure S7d) with bond
of lengths 2.89 and 2.57 Å from –CN of the complex
with the –CO and –CN group of the key pocket residue
of Asn24. Cobalt complex showed two hydrogen bonds
with bond lengths of 2.73 and 2.41 Å from –NC of
the complex with –CO groups of Glu16 and Val94
3.6 Molecular docking studies
The antioxidant assay of the synthesized ligand and
its metal complexes has revealed that the copper com-
plex exhibited more antioxidant activity, almost equal
to the standard, BHT. In order to interpret the bind-
ing mode of the compounds in comparison to the
competitive antioxidant inhibitor, 1,2-dithiane-4,5-diol
(
figure 1). Finally, the molecular docking studies for
the selected compounds revealed that the synthesized
compounds are antioxidant competitive inhibitors in
comparison to antioxidant inhibitor, DTT at 3MNG
binding pocket.
2
6
(
DTT), the molecular modeling study was conducted
for these compounds at the human antioxidant enzyme
3MNG) pocket. The starting coordinate of the human
(
27
antioxidant enzyme in complex with the competi-
tive inhibitor DTT (PDB: 3MNG) was obtained from
the Protein Data Bank (http://www.rcsb.org/pdb). The
3.7 Antioxidant activity of ligand and complexes
In vitro antioxidant activity of 1-(benzo[d]thiazol-2-
yl)thiourea and its metal (Cu(II), Ni(II) and Co(II))
complexes was evaluated by using radical scavenging
methods, DPPH, H O , NO and reducing power method
3
MNG pocket was loaded into Molecular Operating
24
Environment MOE version 2008.10 with high resolu-
tion of 2.70 Å and a library was constructed for these
selected molecules with receptor. The active site was
identified from PDB Sum and recognized the residues,
Gly 46, Gln 133, Asn 24, Val 94, Glu 16, Arg 86 to
be interacting with DNA as well as correlated with
2
2
(FRAP). The antioxidant property of the tested sam-
ples was evaluated at different concentrations (50, 75
and 100 μg/mL) and butylated hydroxyl toluene (BHT)
was used as a standard for the comparison of the
activity.
‘Site Finder’ module of MOE to define the docking site
for the ligands. Hydrophobic and hydrophilic spheres
are used to identify the interactive positions which will
be the potential ligand binding sites in each possible 3.7a DPPH radical scavenging activity: DPPH rad-
position. ical scavenging activity (RSA) evaluation is a stan-
The binding recognition profile of these synthesized dard assay in antioxidant activity studies. A freshly pre-
molecules was attained by performing the docking pared DPPH solution exhibits a deep purple colour with
simulation at 3MNG pocket. The most common an absorption maximum at 517 nm due to DPPH free
H-interactions in the docked structures, bond lengths, radical. This purple colour generally fades/disappears
bond angles and binding energies are shown in table 1. when an antioxidant is present in the medium. More
In the docked conformers, all the compounds were potent antioxidants are able to reduce the absorbance
showing best bonding energies in the order of 5 rapidly.
(
>
−97.4802 kcal/mol) > 4 (−30.3237) > 7 (−25.7574)
DPPH radical scavenging activity data (figure S8) of
6 (−20.5211) > DTT (−12.3869). Interestingly, it the synthesized ligand and metal complexes states that
was observed that the active molecules, even the least all the compounds showed potency, high to moderate
active molecule 6, showed better score than antioxidant activity. The metal complexes exhibited more radical
competitive inhibitor, DTT.
scavenging activity than that of the ligand. Cu(II) metal
The competitive antioxidant inhibitor, DTT (Figure complex exhibited potent antioxidant activity almost
S7a) formed two bifurcated H-bonds with the key close to the standard, BHT and Co(II) complex showed
amino acid residues, Gly 47 and Cys 47; the latter is lower antioxidant activity, and Ni(II) complex and the
an essential residue for binding and biological activity. ligand showed moderate activity.

Synthesis of biologically active metal complexes
49
Table 1. Molecular docking parameter of ligand and its metal complexes.
Interactions
Protein—Ligand
Binding energies Bond
Bond
◦
S.No.
Product
(kcal/mol)
angle( ) length (Å)
N
NH
S
S
NH2
Lys 32, Lys 33,
Gly 133, Asp 134
−30.3237
–
–
1
N
N
NH
2
S
Cl
S
S
Cu
Cl
S
2
H N
N
2
3
N
Gly 46CN—NC
−97.4802
−20.5211
107.6
2.76
N
N
NH
2
S
Cl
S
S
Ni
Cl
S
2
H N
N
Asn 24CO—NC
Asn 24CN—NC
119.0
136.5
2.89
2.57
N
N
N
NH
2
S
S
Cl
S
Co
Cl
S
H N
2
N
Glu 16CO—NC
Val 94CO—NC
−25.7574
120.6
136.4
2.73
2.41
4
N
Figure 1. Interaction mode of the cobalt complex docked and minimized in the 3MNG binding pocket.

50
Harinath Yapati et al.
3.7b Hydrogen peroxide scavenging activity: Hy- 4. Conclusions
drogen peroxide is formed in vivo by many oxidizing
enzymes such as superoxide dismutase in biological
systems. It can cross membranes and may slowly oxi-
The ligand, 1-(benzo[d]thiazol-2-yl)thiourea (btt) and
its metal(II) complexes (Cu, Co and Ni) have been syn-
thesized and characterized by spectral and analytical
data. The spectral data showed that the thiourea deriva-
tive exists as bidendate ligand by bonding to the metal
ions through the deprotonated thiol group and thiazole
sulphur. The analytical data showed the presence of
one metal ion per two ligand molecules and suggest a
mononuclear structure for the complexes. The correla-
tion of the experimental data allows assigning a tetrag-
onal geometry to all the three synthesized complexes.
Biological activity studies revealed that the antioxi-
dant activity of the ligand was found to be enhanced
on complexation with metal ions. Among the studied
complexes, copper complex exhibited high antioxidant
activity. Finally, the molecular docking studies of the
synthesized compounds were carried out. Molecular
docking studies revealed that all the synthesized com-
pounds have relatively less binding energy as compared
to the standard drug and may be considered as a good
antioxidant agents. Hence this study has widened the
scope of developing this type of metal based drugs as
promising antioxidant agents.
39
dize a number of compounds. It is used in the respi-
ratory burst of activated phagocytes. It is known that
H O is toxic and induces cell death. Hydrogen perox-
2
2
ide can attack many cellular energy producing systems.
The ability of 1-(benzo[d]thiazol-2-yl)thiourea (btt) and
its metal complexes to scavenge hydrogen peroxide
at different concentrations was studied and the results
(
figure S9) were compared with the standard. The
results showed that the free ligand and its metal com-
plexes exhibited an effective hydrogen peroxide scav-
enging activity.
3
.7c Nitric Oxide (NO) Scavenging Activity: NO
·−
may divert O2 mediated toxic reactions to other
oxidative and less damaging, pathways, thus protect-
ing O2 sensitive target molecules. Nitric oxide under-
goes a facile radical-radical reaction with O , to yield
ONOO , a reaction that is three times faster than the
SOD-catalyzed dismutation of O . This diversion of
O2 through ONOO oxidation and decomposition
pathways would also limit H O accumulation and sub-
·−
·
−
2
−
·
−
2
·−
−
2
2
sequent reactions of H O , by decreasing the amount of
O2 available for spontaneous or SOD-catalyzed dis-
2
2
Supplementary Information
·−
mutation. We find that the inhibitory effect of the com- Figures S1 to S11, and tables S1 and S2 are provided in
•
pounds tested on NO is concentration-dependent and the supplementary information. Supplementary Infor-
suppression ratio increases with increasing sample con- mation is available at www.ias.ac.in/chemsci.
centrations (figure S10). The copper(II) complex is the
most effective among all the complexes.
Acknowledgements
The authors wish to thank the CSIR, Government of
India (Project 02(0136)/ 13/ EMR-II) for their generous
support.
3
.7d Ferric reducing antioxidant power (FRAP):
Figure S11 shows the reducing power of 1-(benzo[d]
thiazol-2-yl)thiourea (btt) and its metal complexes. In
this assay, the yellow colour of the test solution changes
to various shades of green and blue depending upon
the reducing power of each compound. The presence
of reducers (i.e., antioxidants) causes the reaction of
Fe /ferricyanide complex to the ferrous form giving,
after addition of trichloro acetic acid and ferric chloride,
the Perl’s Prussion blue that can be monitored at 700 nm.
The reducing power of the standard (BHT) at various
concentrations showed higher absorbance value compa-
red to that of the new compounds. The reducing power
of the new complexes solutions increased with increase
in concentration. Metal complexes showed much better
activity than the ligand. All the three complexes showed
moderate to high activity which are slightly less than
that of the standard: BHT>Cu>Ni>Co> Ligand.
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