Synthesis and characterization of copper(II) complexes of sulfadiazine with amino acids: Catalytic activity toward oxidation of phenol and catechol

Article abstract of DOI:10.1080/00958972.2012.671936

New copper complexes of DL-methioninoylsulfadiazine (MTS) and L-cystinoylsulfadiazine (CYS) were prepared and characterized using elemental analysis, IR, electronic spectroscopy, EPR spectroscopy, and thermal analysis. The mode of binding indicates that copper binds to MTS through carbonyl oxygen with the amino group nitrogen while for Cu^II-CYS the copper binds through carbonyl oxygen and SH with removal of its proton. The proposed structures were supported by conformational analysis which showed predominance of the trans form of copper(II)-L-cystinoylsulfadiazine. The two complexes enhanced oxidation of phenol and catechol in the presence of H₂O ₂ under mild conditions. The catalyst shows proficiency toward oxidation of phenol and catechol compared to the auto-catalytic oxidation. Cu^II-MTS exhibited higher catalytic activity than Cu ^II-CYS. The phenol and catechol oxidation is inhibited by Kojic acid.

Full text of DOI:10.1080/00958972.2012.671936

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Synthesis and characterization of
copper(II) complexes of sulfadiazine
with amino acids: catalytic activity
toward oxidation of phenol and
catechol
Ahmed I. Hanafy a b , Zeinhom M. El-Bahy a b & Ibraheem O. Ali b
a Chemistry Department, Faculty of Science, Taif University, Taif,
Saudi Arabia
b Chemistry Department, Faculty of Science, Al-Azhar University,
Nasr City, 11884 Cairo, Egypt
Published online: 30 Mar 2012.
To cite this article: Ahmed I. Hanafy , Zeinhom M. El-Bahy & Ibraheem O. Ali (2012): Synthesis and
characterization of copper(II) complexes of sulfadiazine with amino acids: catalytic activity toward
oxidation of phenol and catechol, Journal of Coordination Chemistry, 65:8, 1459-1474
To link to this article: http://dx.doi.org/10.1080/00958972.2012.671936
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Journal of Coordination Chemistry
Vol. 65, No. 8, 20 April 2012, 1459–1474
Synthesis and characterization of copper(II) complexes
of sulfadiazine with amino acids: catalytic activity toward
oxidation of phenol and catechol
AHMED I. HANAFY*yz, ZEINHOM M. EL-BAHYyz
and IBRAHEEM O. ALIz
yChemistry Department, Faculty of Science, Taif University, Taif, Saudi Arabia
zChemistry Department, Faculty of Science, Al-Azhar University,
Nasr City, 11884 Cairo, Egypt
(Received 10 September 2011; in final form 27 January 2012)
New copper complexes of DL-methioninoylsulfadiazine (MTS) and L-cystinoylsulfadiazine
(CYS) were prepared and characterized using elemental analysis, IR, electronic spectroscopy,
EPR spectroscopy, and thermal analysis. The mode of binding indicates that copper binds to
MTS through carbonyl oxygen with the amino group nitrogen while for CuII–CYS the copper
binds through carbonyl oxygen and SH with removal of its proton. The proposed structures
were supported by conformational analysis which showed predominance of the trans form of
copper(II)-L-cystinoylsulfadiazine. The two complexes enhanced oxidation of phenol and
catechol in the presence of H2O2 under mild conditions. The catalyst shows proficiency toward
oxidation of phenol and catechol compared to the auto-catalytic oxidation. CuII–MTS
exhibited higher catalytic activity than CuII–CYS. The phenol and catechol oxidation is
inhibited by Kojic acid.
Keywords: Sulfadiazine; Amino acids; Oxidation; Phenol; Catechol; Inhibition
1. Introduction
Since environmental and economic factors make the use of harmful oxidants
increasingly unacceptable, hydrogen peroxide is used in oxidation of phenolic
compounds. Oxidation can be carried out by a large number of natural enzymes that
catalyze the oxidation in the presence or absence of oxidant. Catechol oxidases are
enzymes that belong to type 3 copper proteins and catalyze only the oxidation of
catechols to quinones in the presence of dioxygen [1, 2]. The two other known type 3
copper proteins are hemocyanins and tyrosinases. Hemocyanins, the dioxygen carrier
protein in arthropods and mollusks, reversibly bind molecular oxygen. Tyrosinases
catalyze both the hydroxylation of phenols to o-diphenols (monophenolase activity)
and the two-electron oxidation of catechol to quinones (catecholase activity) [3, 4].
There is interest in the catechol oxidase activity of copper complexes using catechol and
*Corresponding author. Email: ahmedih@yahoo.com
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.671936

1460
A.I. Hanafy et al.
3,5-di-tert-butylcatechol as convenient model substrates. Copper complexes serve as
functional mimics for catechol oxidases or as new biomimetic catalysts for oxidation
reactions [5–10]. In oxidative catalysis, transition metal complexes with polydentate
ligands are often used to model structural or functional properties of non-heme
enzymes [11]. For catechol oxidation by copper complexes, several mononuclear and
dinuclear copper complexes have been studied serving as models for the active sites of
copper proteins and the observed biomimetic activity has been correlated with their
structural features [12, 13]. The binding and activation of small molecules like dioxygen
under mild conditions by enzymatic systems and biomimetic complexes is a very active
research field connecting chemical synthesis and catalysis with technology [14, 15].
Herein, we present the synthesis and characterization of simple and safe complexes
derived from sulfadiazine and two amino acids. Sulfadiazine and amino acids have been
chosen due to the antibacterial activity of sulfadiazine and the ability of the prepared
ligands to bind transition metals, mimicking the active site center in natural enzymes.
The copper complexes have been used as oxidase synthetic models in phenol and
catechol oxidation in the presence of the green oxidizing agent, H2O2.
2. Experimental
2.1. Materials
Sulfadiazine, DL-methionine, L-cystine, N,N0-dicyclohexylurea, phenol, catechol, and
Fmoc (9H-fluoren-9-ylmethoxycarbonyl) were purchased from Sigma-Aldrich. Kojic
acid, 3-methyl-2-benzothiazolinone hydrazone (MBTH) and copper chloride were
purchased from Merck company.
2.2. Synthesis of ligands
A mixture of 0.01 mol of sulfadiazine and 0.01 mol of Fmoc-DL-methionine or Fmoc-
L-cystine was dissolved in ꢀ40 mL tetrahydrofuran. The mixture was cooled to 0ꢁC,
then 2.06 g (0.01 mol) N,N0-dicyclohexylcarbodiimide (DCCD) dissolved in ꢀ10 mL
tetrahydrofuran was added. The reaction mixture was stirred for 3–5 h at 0ꢁC and
allowed to stand for 24 h at room temperature. A few drops of acetic acid and water
were added, then the precipitate of N,N0-dicyclohexylurea was filtered off. The filtrate
was concentrated in vacuo to dryness. The residual material was recrystallized
from ethanol to water and obtained in 80% yield (schemes 1 and 2). The
DL-methioninoylsulfadiazine and L-cystinoylsulfadiazine were chromatographically
homogeneous when developed with iodine–benzidine solution and gave negative
ninhydrin test.
2.2.1. DꢂL1-methioninoylsulfadiazine (MTS). IR (KBr); 3348, 3260 cmꢂ1 ꢀ(NH2),
3100 cm
ꢀ(NH), 3036 cmꢂ1 ꢀ(CH-arom.), 2936 cmꢂ1 ꢀ(CH-ali.), 1650 cmꢂ1 ꢀ(CO),
1586 cmꢂ1 ꢀ(CN), 1494 cmꢂ1 ꢁ(NH2), 1440 cmꢂ1 ꢀ(SO2); H NMR (300 MHz, DMSO)
ꢂ ¼ 11.11 (b, s, 2 H-NH2), 8.47 (s, 2 H, 2CH-pyrimidin), 7.96–6.59 (m, 5 H- (4 H,
ArH þ 1 H), CH-pyrimidin), 5.99 (s, 1 H-NHSO2), 3.386–2.09 (m, 8 H-aliphatic protons);
1

Copper(II) sulfadiazine with amino acids
1461
H2
H
C
S
N
C
CH3
CH
C
O
H2
N
O
NH2
S
N
N
O
H
H2
C
S
N
C
CH3
CH
C
H2
N
O
NH2
S
N
N
OH
O
H
Scheme 1. Tautomeric form of DL-methioninoylsulfadiazine (MTS).
SH
H
CH2
CH
N
C
O
N
O
NH2
S
N
N
O
H
SH
CH2
CH
N
C
N
NH2
O
S
N
N
OH
O
H
Scheme 2. Tautomeric form of L-cystinoylsulfadiazine (CYS).
Anal./Calcd for C15H19N5O3S2 (%): C (47.2); H (4.9); N (18.3). Found: C (47.2); H
(5.0); N (18.4).
2.2.2. L-cystinoylsulfadiazine (CYS). IR (KBr); 3380, 3225 cmꢂ1 ꢀ(NH2), 3036 cmꢂ1
ꢀ(NH), 2850 cmꢂ1 ꢀ(CH-arom.), 1650 cmꢂ1 ꢀ(CO), 1596 cmꢂ1 ꢀ(CN), 1494 cmꢂ1 ꢁ(NH2),
1
1440 cmꢂ1 ꢀ(SO2), 2650 cmꢂ1 ꢀ(SH); H NMR (300 MHz, DMSO) ꢂ ¼ 11.26(bs, 2 H-
NH2), 8.47(s, 2 H, 2CH-pyrimidin), 7.96–6.55 (m, 5 H- (4 H, ArH þ 1 H), CH-pyrimidin),
6.00 (s, 1 H-NHSO2), 2.88 (s, H, CH-Cys), 2.73 (s, 2 H, CH2-Cys), 2.08 (s, H, SH-Cys);
Anal./Calcd for C13H15N5O3S2 (%): C (44.2); H (4.2); N (19.8). Found: C (44.1); H
(4.3); N (19.8).
2.3. Synthesis of copper complexes. Copper(II) chloride (0.01 mol) was dissolved in
ꢀ40 mL absolute ethanol, then it was added to 0.01 mol of the prepared ligand
dissolved in ꢀ40 mL absolute ethanol. The mixture was heated under reflux for ꢀ2 h. A
precipitate formed was filtered off and finally washed with hot ethanol several times.
2.3.1. CuII-MTS; [CuL(OH)2]. IR (KBr); 3365, 3238 cmꢂ1 ꢀ(NH2), 3100 cmꢂ1 ꢀ(NH),
3037 cmꢂ1 ꢀ(CH-arom.), 2935 ꢀ(CH-ali.), 1632 cmꢂ1 ꢀ(CO), 1586 cmꢂ1 ꢀ(CN), 1504 cmꢂ1
ꢁ(NH2), 1440 cmꢂ1 ꢀ(SO2); electronic spectrum: ꢃmax ¼ 280 nm, ꢃmax ¼ 414 nm, and
ꢃmax ¼ 650 nm. Anal./Calcd for CuC15H20N5O5S2 (%): C (37.7); H (4.2); N (15.6); Cu
(13.3). Found: C (37.9); H (3.8); N (15.8); Cu (12.8).

1462
A.I. Hanafy et al.
2.3.2. Cꢂu1II–CYS; [Cu(L–H)2 E (H2O)2]. IR (KBr); 3382, 3226 cmꢂ1 ꢀ(NH2),
3037 cm
ꢀ(NH), 2850 cmꢂ1 ꢀ(CH-arom.), 1634 cmꢂ1 ꢀ(C¼O), 1596 cmꢂ1 ꢀ(CN),
1494 cmꢂ1 ꢁ(NH2), 1440 cmꢂ1 ꢀ(SO2); electronic spectrum: ꢃmax ¼ 270 nm, ꢃmax
¼
320 nm, and ꢃmax ¼ 700 nm. Anal./Calcd for CuC26H32N10O8S4 (%): C (38.8); H (3.9);
N (17.4); Cu (7.9). Found: C (39.4); H (3.9); N (17.5), Cu (8.3).
2.4. Physical methods
Carbon, hydrogen, and nitrogen were determined at the Microanalytical Unit, Cairo
University, Egypt. Infrared (IR) spectra of the ligands and their solid complexes were
measured in KBr on a Mattson 5000 FTIR spectrometer. All electronic spectra and
kinetic measurements were performed using a Varian Cary 4 Bio UV-Vis spectro-
photometer. 1H NMR spectra of the ligands were recorded on Joel-90Q Fourier
Transform (300 MHz) spectrometers in [D6] DMSO. Thermal analyses (TGA, DTA)
were recorded on a Shimadzu thermo-gravimetric analyzer model TGA-50 H using
20 mg samples. The flow rate of nitrogen and heating rate were 20 cm3 minꢂ1 and
10ꢁC minꢂ1, respectively. Electron paramagnetic resonance (EPR) spectra were
obtained on a Bruker EMX spectrometer working in the X-band (9.78 GHz) with
100 kHz modulation frequency. The microwave power and modulation amplitudes were
set at 1 mW. Magnetic susceptibility measurements for the complexes were determined
with a Gouy balance using Hg[Co(NCS)4] as a calibrant at room temperature. The
molar conductivities of CuII–MTS and CuII–CYS in DMSO were carried out using a
TDS model 72 conductivity bridge.
2.5. Molecular modeling methods
Conformational analysis: Initial molecular structures of the ligands and complexes were
built using the HyperChem program 7.5. Conformational analysis has been performed
by use of MMþ [16] force field (calculations in vacuo, bond dipole option for
electrostatics, Polake Ribiere algorithm, and RMS gradient of 0.01 kcal molꢂ1).
Furthermore, the geometrical optimization with semi-empirical (PM3) molecular
orbital method was preformed [17]. The resulting conformations were confirmed as
minima by vibrational analysis.
2.6. Kinetic reactions for phenol and catechol oxidation
The catalytic activity of CuII–MTS and CuII–CYS toward the homogeneous oxidation
of phenol and catechol in ethanolic solution at 25ꢁC was determined by measuring the
initial rate of oxidation. MBTH was added to copper(II) complex together with very
dilute solution of phenol or catechol dissolved in ethanol, then H2O2 was added to the
solution in a UV-cell to perform the oxidation. The increase of absorption at 500 nm
(" ¼ 3.25 ꢃ 104 (mol L
)
ꢂ1 ꢂ1 cmꢂ1) due to the formation of adduct between MBTH with
the oxidized form of phenol or catechol [18] with time was obtained on a Varian Cary
3 E spectrophotometer. The initial rate was determined from linear increase in
concentration with time. To study the effect of the catalyst concentration on the rate of
the reaction, various amounts of the complex (40–500 mmol Lꢂ1) were used with

Copper(II) sulfadiazine with amino acids
1463
200 mmol Lꢂ1 H2O2 for oxidation of 1.0 mmol Lꢂ1 phenol or catechol at 25ꢁC;
100 mmol Lꢂ1 of copper complex (CuII–MTS or CuII–CYS) has been used in the
oxidation of different concentrations of the substrate (200–4500 mmol Lꢂ1) in the
presence of 200 mmol Lꢂ1 H2O2 to study the effect of phenol or catechol concentration
on the reaction. The rate laws were determined and rate constants were obtained. The
dependence of H2O2 on substrate oxidation by 100 mmol Lꢂ1 copper complex was
determined by measuring the oxidation rate at different concentrations of hydrogen
peroxide (20–400 mmol Lꢂ1) in the presence of 1.0 mol Lꢂ1 phenol or catechol in ethanol
at 25ꢁC. The auto-oxidation rate of the substrate was determined under the same
conditions in the absence of copper complex. Inhibition studies were carried out in a
similar fashion as the kinetic measurements using 100 mmol Lꢂ1 copper complex in the
presence of 200 mmol Lꢂ1 H2O2 and different amounts of Kojic acid.
3. Results and discussion
1
3.1. IR and H NMR spectroscopy of the ligands
The ligands have been prepared using Fmoc as a protective group. Fmoc is generally
removed from nitrogen of a peptide chain by acidolysis using trifluoroacetic acid (TFA)
[19]. IR spectral data of MTS and CYS show two bands for ꢀ(NH2) (3348, 3260 cmꢂ1
)
for MTS and (3380, 3225 cmꢂ1) for CYS. The bands assigned to ꢀ(NH) for MTS and
CYS appear at 3100 and 3036 cmꢂ1, respectively. The two ligands also show bands
attributed to ꢀ(C¼O), ꢁ(NH2), and ꢀ(O¼S¼O) at 1650, 1494, and 1440 cmꢂ1
,
respectively [20, 21]. L-cystinoylsulfadiazine shows a weak band at 2600 cmꢂ1 attributed
to ꢀ(S–H). 1H NMR spectra of MTS and CYS in dimethyl-sulfoxide-d6 exhibited
signals at ꢂ ¼ 11.11 and 11.26, respectively, assigned to 2 H, NH2. L-cystinoylsulfadia-
1
zine shows a signal at ꢂ ¼ 2.08 assigned to 1 H, SH. IR and H NMR spectroscopies
together with elemental analyses suggest the structure of the ligands as shown in
schemes 1 and 2.
3.2. IR spectroscopy of the copper(II) complexes
To investigate the binding of copper to ligands, IR spectral data of the ligands were
compared to that of the copper complexes. The bands attributed to ꢀ(NH2) in the IR
spectrum of MTS ligand shift (3365, 3238 cmꢂ1) in CuII–MTS. The band attributed to
ꢁ(NH2) shifts to 1504 cmꢂ1 in the complex. The carbonyl ꢀ(C¼O) (at 1650 cmꢂ1 in
MTS) shifts to lower wavenumber (1632 cmꢂ1) in CuII–MTS with less intensity than
that of the free ligand. Bands assigned to ꢀ(O¼S¼O) and ꢀ(NH) are unchanged,
indicating no coordination. Spectral data show that copper(II) binds to the carbonyl
oxygen and NH2 nitrogen. Comparison of IR spectral data of CYS with its copper
complex shows that the copper binds to the carbonyl oxygen and sulfur of deprotonated
SH. This mode of binding was supported by disappearance of ꢀ(SH) band in CuII–CYS
spectrum as well as shifting of carbonyl to lower wavenumber (1634 cmꢂ1) with reduced
intensity. There is no change in ꢀ(NH), ꢀ(O¼S¼O), and ꢀ(NH2), indicating no
participation of these groups in binding. IR spectra show a broad band at ꢀ3450 cmꢂ1
assigned to ꢀ(OH) in both copper(II) complexes.

1464
A.I. Hanafy et al.
Figure 1. Electronic spectra of CuII–CYS and CuII–MTS complexes in DMSO.
3.3. Electronic spectral data
The magnetic moment (1.8 BM) of CuII–MTS at room temperature corresponds to one
unpaired electron. Electronic spectrum of CuII–MTS in DMSO (figure 1) reveals
absorption bands centered at 650 and 414 nm, attributed to d–d transition
2
(2B1g ! A1g) and charge transfer, respectively. These bands suggest square-planar
geometry around copper [22]. The electronic spectrum of CuII–CYS in DMSO
(figure 1) shows two bands at 270 and 320 nm, which can also be assigned to a CT band
from filled orbitals of Cu(II) to the anti-bonding ꢄ* orbitals of the CYS ligand [23]. In
addition, a copper(II) complex with d9 configuration is expected to experience Jahn–
2
2
Teller distortion which leads to further splitting of the Eg and T2g levels. Moreover,
2
2
2
2
they give rise to the B1g ! A1g (ꢀ1), B2g (ꢀ2), and Eg (ꢀ3) transitions which are
expected to be close in energy and generally appears as a broad band. Therefore the
2
broad band centered at 700 nm is assigned to the envelope of 2B1g ! A1g, 2B2g, and 2Eg
transitions [24] which support (with the magnetic moment ¼ 1.9 BM) a distorted
octahedral geometry around copper(II). The electronic spectral pattern exhibited by
CuII–CYS in Nujol mull is similar to that obtained for the complex in DMSO,
indicating that the geometry around the copper metal ion is likely to be the diaqua
complex.
The EPR spectra of CuII–MTS and CuII–CYS were recorded as polycrystalline
samples at room temperature and show hyperfine splitting, which indicates that there is
an interaction between the metal center and the nitrogen adjacent to the copper(II).
Both complexes exhibit an anisotropic signal. Analysis of the EPR spectral data
(Supplementary material) gives g ¼ (2.26499, 2.204911) and g ¼ (2.14629, 2.11621) for
k
?
CuII–MTS and CuII–CYS, respectively. The g 4 g 4 2.00 indicate that the ground
k
?
state of Cu(II) is predominately dx2ꢂy2 [25, 26]. The observed g values for CuII–MTS
k
and CuII–CYS are less than 2.3, indicating that the bonds between ligand and copper
have more covalent character than ionic. According to Hathaway and Billing [27], the
magnitude of the ratio G ¼ (g ꢂ 2)/(g ꢂ 2), which measures the exchange interaction
k
?
between the copper centers in a polycrystalline solid, is 1.811 and 1.763 for CuII–MTS
and CuII–CYS, respectively (G less than 4.0). This indicates considerable exchange
interaction in the solid complex.

Copper(II) sulfadiazine with amino acids
1465
H
H2
C
N
C
S
C
C
CH3
H2
N
O
O
NH2
S
N
N
O
Cu
H
O
O
H
H
Scheme 3. Copper(II) complex of MTS, CuII–MTS.
N
O
S
NH2
CH
H
N
N
N
H
C
O
S
O
CH2
S
H
H
O
Cu
O
O
C
H
H
H2C
H
O
N
N
N
S
CH
H
O
N
NH2
Scheme 4. Copper(II) complex of CYS, CuII–CYS.
3.4. Thermal analysis
The TGA confirms solvent inside and/or outside the coordination sphere and gives
information about the stability of the compound. The thermogram of CuII–MTS
(Supplementary material) shows four stages of mass loss from 25ꢁC to 800ꢁC. The first
stage at 88ꢁC corresponds to removal of one water molecule (Calcd ¼ 3.7%,
found ¼ 3.0%). The second peak at 310ꢁC corresponds to removal of NH3, H2O,
CO2, and C2H5–S–CH3 (Calcd ¼ 32.4%, found ¼ 32.3%). The third stage at 470ꢁC is
assigned to H2NSO2C6H4NH2 (Calcd ¼ 36.0%, found ¼ 36.3%). The TGA of CuII–
CYS (Supplementary material) shows five inflections from 25ꢁC to 800ꢁC. The first
stage at 135ꢁC corresponds to removal of two water molecules (Calcd ¼ 4.5%,
found ¼ 4.0%). The second stage at 210ꢁC is assignable to two CH3SH and one CO2
(Calcd ¼ 17.4%, found ¼ 17.6%). The third and fourth stages together in the temper-
ature range of 242.9–369.2ꢁC correspond to two NH3 and two CO2 (Calcd ¼ 15.2%,
found ¼ 15.2%). The weight loss (Calcd ¼ 23.2%, found ¼ 24.0%) in the fifth stage at
460ꢁC is assigned to two C6H5NH2.
The molar conductivities of CuII–MTS and CuII–CYS in DMSO at 25ꢁC are 5 and
0 ꢀꢂ1 cm2 molꢂ1, respectively, indicating non-electrolytes [28]. The IR spectral data,
electronic spectroscopy, EPR spectroscopy, thermal analysis, and elemental analysis
support the structures for CuII–MTS and CuII–CYS (schemes 3 and 4).
3.5. Molecular modeling
For insight on the molecular structure of the preferred tautomers of ligand and
complexes, the conformational analysis was performed. The computed molecular

1466
A.I. Hanafy et al.
Table 1. Calculated energies of keto and enol forms of MTS and CuII–MTS complex.
Method PM3
Keto
Enol
Complex
Heat of formation (kcal molꢂ1
)
ꢂ46.238
ꢂ95,332.67
ꢂ4476.00
ꢂ9.20
ꢂ38.422
ꢂ95,324.86
ꢂ4468.18
ꢂ9.11
ꢂ243.18
ꢂ136,636.01
ꢂ4924.86
ꢂ9.00
Total energy (kcal molꢂ1
)
Binding energy (kcal molꢂ1
HOMO (eV)
LUMO (eV)
)
ꢂ9.21
ꢂ9.44
ꢂ7.00
Dipole (Debye)
3.58
7.908
8.76
Table 2. Calculated energies of keto and enol forms of CYS ligand.
Method PM3
Keto
Enol
Heat of formation (kcal molꢂ1
)
ꢂ35.41
ꢂ88,435.61
ꢂ3914.99
ꢂ9.421
ꢂ27.101
ꢂ88,427.3
ꢂ3906.67
ꢂ9.34
Total energy (kcal molꢂ1
)
Binding energy (kcal molꢂ1
HOMO (eV)
LUMO (eV)
)
ꢂ9.44
ꢂ9.65
Dipole (Debye)
6.74
7.83
Table 3. Calculated energies of cis and trans isomeric forms of CuII–CYS.
Method PM3
Cis
Trans
426.42
Heat of formation (kcal molꢂ1
)
710.06
ꢂ115,382.47
ꢂ6235.78
ꢂ8.460
Total energy (kcal molꢂ1
)
ꢂ206,638.33
ꢂ6902.13
ꢂ8.55
Binding energy (kcal molꢂ1
HOMO (eV)
LUMO (eV)
)
ꢂ7.25
ꢂ7.311
4.38
Dipole (Debye)
2.818
parameters, total energy, binding energy, heat of formation, the lowest unoccupied
molecular orbital (LUMO), the highest occupied molecular orbital (HOMO) energies,
and the dipole moment for studied compounds were calculated and presented in
tables 1–3. Experimental determination of the tautomeric ligand structure is compli-
cated and often unpredictable. The calculated molecular parameters have been used to
investigate the most stable keto-enol isomer and showed that the most stable tautomer
is keto for MTS and CYS. The optimized keto-enol tautomerization for MTS is
illustrated in figure 2. Conjugation of the ꢄ-electrons of the carbonyl groups with the
ꢄ-system of the molecular skeleton reduces the energy of the keto form, which leads to
its predominance over the enol.
The possibility of existence of CuII–CYS in both cis and trans isomers was examined
by calculating total energy, binding energy, heat of formation, LUMO, and HOMO
energies (table 3). The most stable isomer is the trans (figure 3). The bond length
between carbon and oxygen in the keto form of CuII–CYS was shorter than that in case
of COH in the enol form. The CO and SH opposite to each other in the molecular
skeleton lowered the energy of the trans isomer.

Copper(II) sulfadiazine with amino acids
1467
Figure 2. Ball and stick rendering for the keto and enol forms of MTS (from above to below, respectively),
as calculated by PM3 semi-empirical molecular orbital calculations.
The bond length and bond angle for the keto and enol forms of the ligands together
with the most stable complex isomer are calculated and listed in tables 1s–4s in
‘‘Supplementary material.’’
3.6. Oxidation of phenol and catechol
Although hydrogen peroxide has been used in oxidation of phenolic compounds, it
requires additional activation by a catalyst for effective oxidation. CuII–MTS and
CuII–CYS have been used in this study to activate H2O2 in oxidation of phenol and
catechol. Phenol and catechol oxidations, which are very slow, are spectacularly
enhanced by 100 mmol Lꢂ1 copper complex, reaching saturation at high concentration
of the substrate. Such kinetic pattern indicates an enzyme-like pre-equilibrium kinetic
pathway.
In order to study the catalytic activity of CuII–MTS and CuII–CYS and their
interaction with H2O2 toward oxidation of phenolic compounds, phenol and catechol
have been used as substrates to provide detailed kinetic information. The oxidation
rates of phenol by 100 mmol Lꢂ1 CuII–MTS and 100 mmol Lꢂ1 CuII–CYS at different
concentrations of phenol (figure 4) have been determined in the presence of

1468
A.I. Hanafy et al.
Figure 3. Ball and stick rendering for the most stable tautomer form of CYS (keto form) and trans isomer of
its complex (from top to bottom, respectively; hydrogen atoms were removed for clarity), as calculated by
PM3 semi-empirical molecular orbital calculations.
200 mmol Lꢂ1 H2O2 at room temperature. The rate of phenol oxidation using either
CuII–MTS or CuII–CYS is nonlinear, reaching saturation at high substrate concentra-
tions which suggests enzyme-like pre-equilibrium kinetics. This kinetics can be
described as the binding of the substrate (S ¼ phenol) with the catalyst to form an
intermediate S–CuII-complex, followed by conversion of the bound substrate into
products (equation (1)).
k1
kcat
CuII-complex þ S
CuII-complex-S ꢂꢂꢂ! CuII-complex þ Prod:
ð1Þ
ꢂꢂꢂꢂꢂꢂ*
)ꢂꢂꢂꢂꢂꢂ
kꢂ1
where CuII-complex ¼ CuII-MTS or CuII-CYS, S ¼ phenol or catechol.

Copper(II) sulfadiazine with amino acids
1469
Figure 4. Oxidation of phenol using 100 mmol Lꢂ1 of CuII–MTS (ꢅ) and 100 mmol Lꢂ1 CuII–CYS (ꢁ) in the
presence of 200 mmol Lꢂ1 H2O2 at 25ꢁC.
Figure 5. Oxidation of catechol using 100 mmol Lꢂ1 CuII–MTS (ꢅ) and 100 mmol Lꢂ1 CuII–CYS (ꢁ) in the
presence of 200 mmol Lꢂ1 H2O2 at 25ꢁC.
The rate law for this reaction can be obtained with steady-state approximation
similar to the Michaelis–Menten kinetics in enzyme catalysis and can be expressed as in
equation (2),
kcat½CuII-complexꢄ½Sꢄ
Rate ¼
ð2Þ
K0 þ ½Sꢄ
where K0 ¼ (kꢂ1 þ kcat)/k1 is the dissociation constant of the S–CuII-complex.
This reaction produces a first-order rate constant kcat ¼ 1.58 ꢃ 10ꢂ2 sꢂ1 (t1/2 ¼ 43.9 s)
for CuII–MTS and kcat ¼ 1.27 ꢃ 10ꢂ2 sꢂ1 (t1/2 ¼ 54.6 s) for CuII–CYS. Oxidation of
1.0 mmol Lꢂ1 of phenol or catechol using 100 mmol Lꢂ1 CuII–MTS in the absence of
H2O2 affords kcat of 1.40 ꢃ 10ꢂ5 sꢂ1 and 1.42 ꢃ 10ꢂ5 sꢂ1, respectively. Oxidation
of catechol using 100 mmol Lꢂ1 CuII–MTS or 100 mmol Lꢂ1 CuII–CYS in the presence
of 200 mmol Lꢂ1 H2O2 (figure 5) is similar to phenol oxidation with the saturation

1470
A.I. Hanafy et al.
Table 4. Oxidation of phenol and catechol using CuII–MTS and CuII–CYS in the presence of H2O2 at room
temperature.
Catalyst
Substrate
kcat (sꢂ1
)
K0 (mmol Lꢂ1
)
t1/2 (s)
C.P.
kcat/K0 (mol Lꢂ1 ꢂ1 sꢂ1
)
CuII–MTS
CuII–CYS
CuII–MTS
CuII–CYS
CuII–MTS
CuII–CYS
Phenol
Phenol
Catechol
Catechol
H2O2
1.58 ꢃ 10ꢂ2
1.27 ꢃ 10ꢂ2
1.58 ꢃ 10ꢂ2
1.43 ꢃ 10ꢂ2
1.83 ꢃ 10ꢂ2
1.60 ꢃ 10ꢂ2
0.51
0.48
0.21
0.53
0.42
0.43
43.87
54.57
43.87
48.47
37.87
43.32
3.43 ꢃ 105
2.77 ꢃ 105
3.34 ꢃ 104
3.02 ꢃ 104
3.98 ꢃ 105
3.49 ꢃ 105
30.98
26.45
75.23
26.98
43.57
37.20
H2O2
C.P. ¼ catalytic proficiency.
pattern to produce first-order rate constant kcat ¼ 1.58 ꢃ 10ꢂ2 sꢂ1 (t1/2 ¼ 43.9 s) for CuII–
MTS and kcat ¼ 1.43 ꢃ 10ꢂ2 sꢂ1 (t1/2 ¼ 48.5 s) for CuII–CYS. The dissociation constants
K0 were also obtained and are listed in table 4. Free-radical initiated oxidation would
not show such pre-equilibrium kinetics and the substrate bound intermediate [29]. To
investigate the existence of free radical in the reaction mechanism, various concentra-
tions of (CH3)2SO were used in oxidation of 1.0 mmol Lꢂ1 phenol or catechol in the
presence of H2O2 using CuII–MTS and CuII–CYS. The free radical scavenger (CH3)2SO
[30] did not inhibit the reaction, suggesting the absence of free radical to induce the
oxidation reaction.
The complexes show significant catalytic efficiency kcat/K0 (table 4). The catalytic
activity of CuII–MTS toward oxidation of phenol shows kcat/K0 ¼ 30.98 (mol L
)
ꢂ1 ꢂ1sꢂ1
as the second-order rate constant. Moreover, the catalysis shows 3.43 ꢃ 105 times rate
enhancement in terms of the first-order rate constant (kcat/ko, wherein ko ¼ 4.6 ꢃ
10ꢂ8 sꢂ1 is the rate constant for the uncatalyzed phenol oxidation). Catalytic activity of
catechol oxidation using CuII–MTS is 75.23 (mol Lꢂ1 ꢂ1 sꢂ1 as the second-order rate
)
constant, showing 3.34 ꢃ 104 rate enhancement in terms of first-order rate constant
compared to uncatalyzed catechol oxidation reaction ko ¼ 4.74 ꢃ 10ꢂ7 sꢂ1. Table 4
shows that the copper complex of methionionylsulfadiazine (CuII–MTS) is more active
in oxidation of either phenol or catechol in the presence of H2O2 at room temperature
than CuII–CYS. This activity may be due to different geometries around Cu(II) in the
two complexes. The square-planar CuII–MTS leaves two positions free around copper,
which may facilitate the binding of the substrate to the metal ion or forming OH bridge.
The octahedral geometry in CuII–CYS occupies all six coordination sites around
copper, slowing the binding of the substrate to copper. Oxidation of phenol and
catechol using the copper complexes as a function of H2O2 also shows a saturation
pattern at high concentrations of H2O2 (figure 6), indicating direct binding of this
oxidant as a substrate to the active metal center. Both substrates bind to the enzyme
model to produce a ternary complex [18, 31, 32]. The order of binding can either be
random (in a random mechanism) or substrates have to bind in a particular sequence
(in an ordered mechanism). The kinetics indicates that H2O2 can bind to (Cu-complex)
to form a ternary complex H2O2–(Cu-complex)–(cat. or ph) [31]. The first-order rate
constants kcat, t1/2, and catalytic activity are listed in table 4. The rate constants in the
studies herein are comparable or higher than those of a number of structurally defined
CuII-complexes toward oxidation of catechol. For example, kcat for the oxidation of
catechol is in the range (1.6–2.2) ꢃ 10ꢂ4 sꢂ1 by CuII-macrocycle complexes [33]
and (2.75–5.13) ꢃ 10ꢂ4 sꢂ1 and (3.98–13.2) ꢃ 10ꢂ3 sꢂ1 by some dinuclear CuII-complexes

Copper(II) sulfadiazine with amino acids
1471
Figure 6. Oxidation of phenol using 100 mmol Lꢂ1 CuII–MTS (ꢅ) and 100 mmol Lꢂ1 CuII–CYS (ꢁ) in the
presence of different concentrations of H2O2 at 25ꢁC.
[34, 35]. The CuII–MTS and CuII–CYS complexes showed kcat ¼ ꢀ6.2 ꢃ 103 and
1.1 ꢃ 103 times more toward oxidation of catechol than copper(II)-(4-vinylbenzyl)
cytosine (kcat ¼ 2.53 ꢃ 10ꢂ6 sꢂ1) [36] and copper-metallated nucleobase polymeric
matrix (9-allyladenine-1,4-divinylbenzene cross polymer) (1.32 ꢃ 10ꢂ5
s
ꢂ1) [37]. The
rate constants herein are only about 8 times lower than that of the CuII-complex of a
vinylpyridine-acrylamide copolymer (kcat ¼ 0.12 sꢂ1) [32]. Oxidation of catechol using
CuII–MTS and CuII–CYS showed kcat ꢀ 3.1 ꢃ 103 and 1.3 ꢃ 104 times more than that of
oxidation of 4-t-butylcatechol (kcat ¼ 4.65 ꢃ 10ꢂ6 sꢂ1 and 1.15 ꢃ 10ꢂ6 sꢂ1) [36, 37]. The
first-order rate constants of phenol oxidation using CuII–MTS and CuII–CYS are only
about 13–16 times lower than that of CuII-ꢁ amyloid1-20 (kcat ¼ 0.213 sꢂ1) [38].
To investigate the effect of the catalyst concentration on oxidation of phenol,
different concentrations of CuII–MTS were used in oxidation of 1.0 mmol Lꢂ1 phenol in
the presence of 200 mmol Lꢂ1 H2O2 at room temperature. The observed rate was linear
till ꢀ100 mmol Lꢂ1 of copper complex and then reached saturation (figure 7), indicating
that the optimum concentration for the copper complex is 100 mmol Lꢂ1. The same
concentration has been used in CuII–CYS for data comparison.
3.7. Inhibition of phenol and catechol oxidation by Kojic acid
Kojic acid, which has a quinoid structure, can act as a tyrosinase inhibitor (to inhibit
melanin formation) and as antioxidant for fats and oils in cosmetics (skin whitening or
depigmenting agent). Applications also include the prevention of discoloration of
crustacea, meat, and fresh vegetables. In this study, Kojic acid was used to inhibit the
CuII–MTS complex toward oxidation of both phenol and catechol. Figure 8 shows that
both phenol and catechol oxidation is inhibited significantly by Kojic acid with IC50
ꢀ100 and ꢀ95 mmol Lꢂ1 for phenol and catechol, respectively. Phenol oxidation has
been inhibited by Kojic acid using CuII–CYS, showing no big change for CuII–CYS
rather than using CuII–MTS.
These copper(II) complexes can be used as an oxidative model to discover new
inhibitors that can be used in the cosmetic and medicinal products used to prevent

1472
A.I. Hanafy et al.
Figure 7. Oxidation of phenol using different concentrations of CuII–MTS in the presence of 200 mmol Lꢂ1
H2O2 at 25ꢁC.
Figure 8. Inhibition of phenol oxidation (ꢅ) and catechol oxidation (ꢁ) using different concentrations of
Kojic acid by 100 mmol Lꢂ1 CuII–MTS in the presence of 200 mmol Lꢂ1 H2O2 at 25ꢁC.
hyperpigmentation [39]. Moreover, these inhibitors can be used as antioxidants and
anticancer agents.
4. Conclusion
We have prepared new copper(II) complexes of sulfadiazine with DL-methionine and
L-cystine amino acids. These complexes can be used as environmentally acceptable
catalysts due to the presence of antimicrobial sulfadiazine and amino acids. The keto
form was predominant over enol. CuII–MTS and CuII–CYS can promote the oxidation

Copper(II) sulfadiazine with amino acids
1473
of phenol and catechol in the presence of the green oxidizing agent H2O2. The rate
constants are comparable or higher than those published. CuII–MTS is more active in
the oxidation of phenol or catechol than CuII–CYS, due to the octahedral geometry
around copper in CuII–CYS. The copper(II) complexes provide model systems for
further investigation of Cu-centered oxidation and oxygenation chemistry.
Acknowledgments
The authors acknowledge Taif University for supporting this work by Taif University
research program (2-432-711).
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