Synthesis, characterization and DNA-binding studies of 1-cyclohexyl-3-tosylurea and its Ni(II), and Cd(II) complexes

Article abstract of DOI:10.1016/j.saa.2008.01.005

1-Cyclohexyl-3-tosylurea (HL) and its two complexes, ML₂·2H₂O [M{double bond, long}Ni(1), and Cd(2)], have been synthesized and characterized on the basis of elemental analyses, molar conductivities, IR spectra and thermal analyses. In addition, the DNA-binding properties of the ligand and the two complexes have been investigated by electronic absorption, fluorescence, CD spectroscopy and viscosity measurements. The experiment results suggest that the ligand and its two complexes bind to DNA via a groove binding mode, and the binding affinity of the complex 2 is higher than that of the complex 1 and the ligand.

Full text of DOI:10.1016/j.saa.2008.01.005

Available online at www.sciencedirect.com
Spectrochimica Acta Part A 71 (2008) 523–528
Synthesis, characterization and DNA-binding studies of
1-cyclohexyl-3-tosylurea and its Ni(II), and Cd(II) complexes
∗
Pin-xian Xi, Zhi-hong Xu, Xiao-hui Liu, Feng-juan Cheng, Zheng-zhi Zeng
College of Chemistry and Chemical Engineering and State Key Laboratory Applied Organic Chemistry,
Lanzhou University, Lanzhou 730000, PR China
Received 17 July 2007; received in revised form 3 January 2008; accepted 3 January 2008
Abstract
-Cyclohexyl-3-tosylurea (HL) and its two complexes, ML
of elemental analyses, molar conductivities, IR spectra and thermal analyses. In addition, the DNA-binding properties of the ligand and the two
complexes have been investigated by electronic absorption, fluorescence, CD spectroscopy and viscosity measurements. The experiment results
suggest that the ligand and its two complexes bind to DNA via a groove binding mode, and the binding affinity of the complex 2 is higher than that
of the complex 1 and the ligand.
1
2
·2H
2
O [M Ni(1), and Cd(2)], have been synthesized and characterized on the basis
©
2008 Elsevier B.V. All rights reserved.
Keywords: 1-Cyclohexyl-3-tosylurea ligand; Sulfonylurea metal complexes; DNA-binding; Fluorescence spectroscopy; CD spectroscopy; Viscosity measurements
1
. Introduction
pounds of urea have been reported to act as enzyme inhibitors
9], nickel(II) complexes of relevance to the active site of the
[
One of the important drugs regulating biological functions of
enzyme [10] some of which have a urea molecule bound to
the nickel centre and are useful due to their pharmacological
applications [11,12].
pancreatic islets is the sulfonylurea, which has been used for the
treatment of type 2 diabetes mellitus because of its insulinotropic
activity on pancreatic islets [1]. The design of small complexes
that bind and react at specific sequences of DNA becomes impor-
tant. Researching the co-operation between the metal ions and
the more complete understanding of how to target DNA sites
with specificity will lead to novel chemotherapeutics to probe
DNA and to develop highly sensitive diagnostic agent [2]. The
metal complexes also have many noticeable bioactivities [3].
The synthesis of the complexes of sulfonylurea plays an impor-
tant role in exploring the mechanism of the molecular biology
We have now extended our efforts and prepared the tran-
sition metal complexes with a sulfonylurea ligand such as
1-cyclohexyl-3-tosylurea. We have prepared and characterized
1-cyclohexyl-3-tosylurea and its complexes, we described a
comparative study of the interaction of the ligand and its com-
plexes with calf thymus DNA (CT-DNA), using electronic
absorption, fluorescence, CD spectroscopy and viscosity mea-
surements. Information obtained from this study will be helpful
to understand the mechanism of the interaction between the
sulfonylurea complexes and nucleic acids.
[
4].
Transition metal complexes have attracted considerable
attention as catalytic systems for use in the oxidation of organic
compounds [5], probes in electron-transfer reactions involving
metallo-proteins [6], and intercalators with DNA [7]. The inter-
actions between Cd2 ions and DNA have recently been reported
by Hossain and Huq [8]. They believed that Cd ions covalently
bind into adenine and guanine in DNA. The coordination com-
2. Experimental
+
2.1. Instrumentation
2+
The melting points of the compounds were determined on
a Beijing XT4-100× microscopic melting point apparatus (the
thermometer was not corrected). Elemental analyses were car-
ried out on an Elemental Vario EL analyzer. The metal contents
of the complexes were determined by titration with EDTA
∗
Corresponding author.
E-mail address: zengzhzh@yahoo.com.cn (Z.-z. Zeng).
1
386-1425/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2008.01.005

5
24
P.-x. Xi et al. / Spectrochimica Acta Part A 71 (2008) 523–528
Fig. 1. The preparation of the ligand.
(
xylenol orange tetrasodium salt used as an indicator and hex-
(0.5–3 ␮M), and each compound was introduced into CT-DNA
solution (5 ␮M) present in the viscometer. Data were presented
as (η/η0)1 versus the ratio of the concentration of the com-
pound to CT-DNA, where η is the viscosity of CT-DNA in the
presence of the compound and η0 is the viscosity of CT-DNA
alone. Viscosity values were calculated from the observed flow
time of CT-DNA containing solution corrected from the flow
time of buffer alone (t0), η = (t − t0)/t0 [14].
amethylidynetetraimine as buffer). The IR spectra were obtained
in KBr disks on a Therrno Mattson FTIR spectrometer in the
4
Varian VR 300-MHz spectrometer in CDCl3 or (CD3)2SO with
TMS as internal standard. All conductivity measurements were
performed in DMF with a DDS-11A conductor at 25 C. The UV
spectrawererecordedonaVarianCarry100UV–visspectropho-
tometer. The thermal behaviour was monitored on a PCT-2
differential thermal analyzer. Mass spectra were performed on
a VG ZAB-HS (FAB) instrument and electrospray mass spectra
/3
−
1
1
000–400 cm region. H NMR spectra were recorded on a
◦
Absorption titration experiment was performed with fixed
concentrations of the drugs, while gradually increasing concen-
tration of DNA. While measuring the absorption spectra, an
equal amount of DNA was added to both compound solution
and the reference solution to eliminate the absorbance of DNA
itself. From the absorption titration data, the binding constant
was determined using [15].
(
ESI-MS) were recorded on a LQC system (Finngan MAT, USA)
using CH3OH as mobile phase. Fluorescence measurements
were made on a Hitachi RF-4500 spectrofluorophotometer. The
CD spectra were recorded on an Olos RSM 1000 at increasing
complex/DNA ratio (r = 0.0 and 0.5). Each sample solution was
scanned in the range of 220–320 nm. A CD spectrum was gen-
erated which represented the average of three scans from which
the buffer background had been subtracted. The concentration
of DNA was 1.0 × 10− M.
[
DNA]
[DNA]
1
=
+
(
εa − εf)
(ε − ε ) K (ε − ε )
b
f
b
b
f
where [DNA] is the concentration of DNA in base pairs, εa
corresponds to the extinction coefficient observed (Aobsd/[M]),
εf corresponds to the extinctioncoefficientofthe free compound,
εb istheextinctioncoefficientofthecompoundwhenfullybound
4
2
.2. Materials and methods
to DNA, and K is the intrinsic binding constant. The ratio of
b
Calf thymus DNA (CT-DNA) was purchased from Sigma
slope to intercept in the plot of [DNA]/(εa − εf) versus [DNA]
gives the values of Kb. Over a range of DNA concentrations from
50 to 350 ␮M.
without further purification. EDTA and transition metal nitrates
were produced in China. All chemicals used were of analytical
grade. Alltheexperimentsinvolvinginteractionoftheligandand
the complexes with CT-DNA were carried out in doubly distilled
waterbuffercontaining5 mMTrisand50 mMNaClandadjusted
to pH 7.2 with hydrochloric acid. A solution of CT-DNA gave a
ratio of UV absorbance at 260 and 280 nm of about 1.8–1.9,
indicating that the CT-DNA was sufficiently free of protein
To affirm quantitatively the affinity of the complexes bound
to DNA, the intrinsic binding ability of the complexes to DNA
were obtained by fluorescence titration method. Fixed amounts
(10 ␮M) of the complexes were titrated with increasing amounts
of CT-DNA. Excitation and emission wavelengths of the sam-
ples were 279 and 330 nm, slit width 5/5 nm. All experiments
◦
[
13]. The CT-DNA concentration per nucleotide was determined
were conducted at 20 C in a buffer containing 5 mM Tris–HCl
spectrophotometrically by employing an extinction coefficient
(pH 7.2) and 50 mM NaCl concentrations. All experiments were
−1
−1
◦
of 6600 M cm at 260 nm. The ligand and the complexes
were dissolved in a mixture solvent of 1% CH3OH and 99%
Tris–HCl buffer (5 mM Tris–HCl, 50 mM NaCl, pH 7.2) at con-
conducted at 20 C in a buffer containing 5 mM Tris–HCl (pH
7.2) and 50 mM NaCl concentrations.
−
5
centration1.0 × 10 M. Anabsorptiontitrationexperimentwas
performed by maintaining 10 ␮M compounds and varying the
concentration of nucleic acid. While measuring the absorption
spectra, an equal amount of CT-DNA was added to both the
compound solution and the reference solution to eliminate the
absorbance of CT-DNA itself.
2.3. Preparation of the ligand (HL)
The preparation of the ligand is shown in Fig. 1. A mixture of
p-toluene sulfonamide (5.14 g, 0.03 mol) and finely pulverized
K2CO3 (11.00 g, 0.078 mol) in 35 mL of acetone was stirred and
heated to reflux for 30 min. Acetone solution (10 mL) of ethyl
chloroformate (3.80 mL, 0.040 mol, 1.33 equiv.) was added to
the mixture and heating continued for another 4 h. The mixture
of the results were poured into 100 mL of H2O, then the aqueous
Viscosity experiments were conducted on an Ubbelodhe vis-
cometer, immersed in a thermostated water-bath maintained
◦
at 25 ± 0.1 C. Titrations were performed for the complexes

P.-x. Xi et al. / Spectrochimica Acta Part A 71 (2008) 523–528
525
νSO(SO2NH): 1092 cm 1, νCO(–CO–): 1520 cm , ν(C N):
1
−
−1
−
1
678 cm . Umax (nm): (CH3OH) 208, 228, 274.
3. Results and discussion
All of the complexes are air stable for extended periods and
remarkably soluble in DMSO and DMF; soluble in methanol
and slightly soluble in ethanol, insoluble in benzene, water and
diethyl ether. The molar conductivities in DMF solution indicate
2
−1
that the complex 1 and complex 2 (10.8 and 13.2 S cm mol
)
are in the range expected for no electrolytes [16]. The elemen-
tal analyses, ESI-MS and molar conductivities show that the
formulas of the complexes conform to ML2·2H2O (M Ni, Cd).
3.1. Infrared spectra
Fig. 2. The suggested structure of the complexes (M Ni, Cd).
The spectra of the ligand exhibit νNH(–CONH–) vibration
phases was acidified with 10 mL of 1.00 N aqueous HCl to get
the solid, washed with 80 mL H2O for several times. Recrys-
tallization from 20 mL ethanol provided 6.35 g (87%) of ethyl
−1
bands at the 3340 cm
the 3047 cm , in the complex the νNH(–CONH–) and the
and νNH(–SOONH–) vibrations at
−1
−1
νNH(–SOONH–) vibration were blue shifted to 3353 cm and
N-(3-tossulfonyl) carbamate as a white crystalline solid: m.p.
−1
3
1
255 cm , respectively. The band of νCO(–CO–) appeared at
◦
7
6–78 C.
−1
624 cm in the ligand while the band of the complexes was
A solution of ethyl N-(3-tossulfonyl) carbamate (0.73 g,
.00 mmol) and cyclohexanamine (0.45 mL, 3.90 mmol,
.30 equiv.) in 40 mL PhMe was heated to reflux for 6 h. After
−1
−1
exhibited at 1608 cm ; ꢀν(
is to 16 cm . This
ligand−complexes)
3
1
shift confirms that the group loses its original characteristics and
forms coordinative bonds with the metal [17]. The band at the
the solution stood overnight at room temperature, the resulting
precipitate was collected, washed twice time with PhMe, and
dried in vacuum to afford 0.67 g (75%) of a white solid: m.p.
−1
1
130 cm was νSO(–SO2NH–) vibration in the ligand. In the
−1
complexes these bands are presented at 1088 cm . The band
at about 1678 cm in the complexes can be assigned to (C N)
−1
◦
+
1
56–158 C; FAB-MS: m/z = 297 [M + H] . Anal. calcd for
which suggested that the H atom of the group (–SOONH–) had
C14H20N2O3S: C, 56.73; H, 6.80; N, 9.45. Found: C, 56.80;
−1
been substituted by the metal atom. Weak bands at 552 cm are
−
1
H, 6.75; N, 9.56. IR νmax (cm ): νNH(–CONH–): 3340,
νNH(–SOONH–): 3047, νCO(–CO–): 1604, νSO(–SO2NH–):
assigned to ν(M O). These shifts demonstrate that the ligand
2+
2+
coordinated Ni and Cd ions through the oxygen of carbonyl
and sulphanilamide.
1
130. Umax (nm): (CH3OH) 203, 227.
2
.4. Preparation of the complexes
3.2. UV spectra
The ligand (600 mg, 2.03 mmol) was added to an ethanol
The study of the electronic spectra in the ultraviolet and vis-
solution of NaOC2H (142.80 mg, 2.10 mmol), then the Ni
5
ible ranges for the complexes and the ligand were carried out
in the buffer solution. The electronic spectra of ligand had a
strong band at λmax = 203 nm, a medium band at λmax = 227 nm.
There are three bands at 208, 229 and 273 nm for complex 1 and
complex 2. These changes indicate that complexes are formed.
(
NO3)2·3H2O (290.80 mg, 1 mmol) (10 ml ethanol) was added
to the system. Immediately there was a green precipitate
in the solution. After stirring for 4 h at room temperature,
the precipitate was separated by the centrifugal and washed
seven times with ethanol and one time with ether, and finally
dried in vacuo. The Cd (II) complex was synthesized by the
same way. Anal. calcd for complex 1 C28H42N4O8S2Ni:
C, 49.72; H, 6.47; N, 8.00; Ni, 8.38. Found: C, 50.03; H,
3.3. Thermal analyses
2
−1
◦
6
[
3
.36; N, 7.91; Ni, 8.91. Λm(s cm mol ): 10.8. ESI-MS
The complexes begin to decompose at 212 C or so and there
+
◦
CH3OH, m/z]: 649.2 ({[Ni(L)2·2H2O] − 2H2O + H} ),
are two exothermic peaks appearing around 212–353 C. The
+
−1
53.1({[Ni(L)2·2H2O] − 2H2O − L} ), IR νmax (cm ):
corresponding TG curves show a series of weight loss. Under
−
1
−1
◦
νSO(SO2NH): 1088 cm , νCO(–CO–): 1520 cm , ν(C N):
200 C, there are no endothermic peak and no weight loss on
−1
1
678 cm . Umax (nm): (CH3OH) 208, 229, 273. Anal.
calcd for complex 2 C28H42N4O8S2Cd: C, 46.18; H,
.01; N, 7.43; Cd, 14.90. Found: C, 46.36; H, 5.86;
corresponding TG curves. It indicates that there are no crystal
◦
or coordinate solvent molecules. While being heat to 800 C, the
6
complexes become their corresponding oxides, the residues are
in accordance with calculation.
On the basis of above evidence and analyses, the possible
structure of the complexes is shown in Fig. 2.
2
−1
N, 7.49; Cd, 14.77. Λm(s cm mol ): 13.2. ESI-MS
+
[
4
CH3OH, m/z]: 705.1 ({[Cd(L)2·2H2O] − 2H2O + H} ),
+
−1
09.0 ({[Cd(L)2·2H2O] − 2H2O − L} ), IR νmax (cm ):

5
26
P.-x. Xi et al. / Spectrochimica Acta Part A 71 (2008) 523–528
Fig. 3. (a) Electronic spectra of the ligand (10 ␮M) in the presence of 0, 10,
−
3
2
0, 30, 40, 60 and 80 ␮l 1.0 × 10 M CT-DNA. (b) Electronic spectra of the
Fig. 4. (a) Electronic spectra of the ligand (10 ␮M) in the presence of 0, 5, 10,
complex 1 (10 ␮M) in the presence of 0, 10, 20, 30, 40, 50, 60, 70 and 80 ␮l
−3
1
5, 15, 20, 25, 30, 35, 40, 45 and 50 ␮l 1.0 × 10 M CT-DNA. (b) Electronic
−
3
1
.0 × 10 M CT-DNA. (c) Electronic spectra of the complex 2 (10 ␮M) in the
spectra of the complex 1 (10 ␮M) in the presence of 0, 5, 10, 15, 15, 20, 25,
3
complex 2 (10 ␮M) in the presence of 0, 5, 10, 15, 15, 20, 25, 30, 35, 40, 45
and 50 ␮l 1.0 × 10 M CT-DNA. Arrow shows the absorbance changes upon
−3
presence of 0, 10, 20, 30, 40, 50, 60, 70 and 80 ␮l 1.0 × 10 M CT-DNA. Arrow
shows the absorbance changes upon increasing CT-DNA concentration. Inset:
plots of [DNA]/(εa − εf) vs. [DNA] for the titration of complexes with DNA;
−3
0, 35, 40, 45 and 50 ␮l 1.0 × 10 M CT-DNA. (c) Electronic spectra of the
−3
(
ꢀ) experimental data points; (solid line) linear fitting of the data.
increasing CT-DNA concentration.

P.-x. Xi et al. / Spectrochimica Acta Part A 71 (2008) 523–528
527
Fig. 6. Effect of increasing amounts of the complexes on the relative viscosity
of CT-DNA at 25.0 ± 0.1 C.
Fig. 5. CD spectrum of CT-DNA adduct with compound. Solid line: free CT-
DNA; dash line: complex 1 with CT-DNA, ri = 0.5; dot line: complex 2 with
CT-DNA, ri = 0.5 (ri = molar ratio compound: CT-DNA).
◦
alone, the fluorescence intensity decrease with the increase of
CT-DNA concentration. As shown in Fig. 4, the fluorescence
intensity of the complex 1 and 2 are quenched steadily with the
increasing concentration of the CT-DNA. This phenomenon of
the quenching of luminescence of the complex by DNA may be
attributed to the photoelectron transfer from the guanine base of
DNA to the excited MLCT state of the complex [25,27–33].
3
.4. Electronic absorption titration
Electronic absorption spectroscopy is universally employed
to determine the binding characteristics of metal complex with
DNA [18–20]. The absorption spectra of the ligand, complex
1
and complex 2 in the absence and presence of CT-DNA are
given in Fig. 3a, b and c, respectively. There exist in Fig. 3a two
well-resolved bands at 203 and 227 nm for the ligand, and in
Fig. 3b and c also have two well-resolved bands at about 208
and 229 nm for the complexes. With increasing DNA concen-
trations, the hypochromisms are 7.31% at 203 nm for ligand;
3.6. CD spectroscopy
CD spectral variations of CT-DNA were recorded by the
respective addition of the ligand and complex 1 and 2 to CT-
DNA. Fig. 5 shows the CD spectra of CT-DNA which was added
with the ligand, complex 1 and 2. The observed CD spectrum
of calf thymus DNA consists of a positive band at 277 nm due
to base stacking and a negative band at 245 nm due to helic-
ity, which is characteristic of DNA in the right-handed B form.
While groove binding interaction of small molecules with DNA
show little or no perturbations on the base stacking and helicity
bands, intercalation enhances the intensities of both the bands,
stabilizing the right-handed B conformation of CT-DNA. In all
two cases, the intensities of both the negative and positive bands
decrease significantly. This suggests that the DNA-binding of
the complexes induces certain conformational changes, such as
the conversion from a more B-like to a more C-like structure
within the DNA molecule [26,34]. These changes are indicative
of a non-intercalative mode of binding of these complexes and
offer support to their groove binding nature [29,35–36].
3
0.21% at 208 nm for complex 1; 55.43% at 208 nm for com-
plex 2. The λmax for the ligand increased from 203 to 204, for
complex 1 increased from 208 to 210 nm and that for complex
2
increased from 208 to 211 nm. Such a small change in λmax is
more in keeping with groove binding, leading to small perturba-
tions. Such small increases in the λmax and the hypochromicity
have been observed in the case of some porphyrin and copper
complexes on their interaction with DNA [21,22].
The binding constants, K for the complexes 1 and 2 have
b
been determined from the plot of [DNA]/(εa − ε ) versus [DNA]
f
4
−1
4
and found to be 4.03 × 10 M and 8.72 × 10 , respectively,
4
−1
and the K for the ligand (0.23 × 10 M ) is very small. The
b
K value obtained here is lower than that reported for clas-
b
sical intercalator (for ethidium bromide and [Ru(phen)DPPZ]
whose binding constants have been found to be in the order
6
7
of 10 –10 M) [23–26]. The results indicate that the binding
strength of complex 2 is stronger than that of 1 and the ligand.
3.7. Viscosity studies
3
.5. Fluorescence spectra
Hydrodynamic measurements that are sensitive to the length
Fixed amounts (10 ␮M) of the complexes were titrated with
increasing amounts of CT-DNA. The ligand and complexes
emit luminescence in Tris buffer with a maximum appearing at
33 nm. The fluorescence titrations spectra of the ligand, com-
plex 1 and complex 2 inthe absence and presenceof CT-DNAare
change (i.e., viscosity and sedimentation) are regarded as the
least ambiguous and the most critical tests of a binding model
in solution in the absence of crystallographic structural data
[14,37,38]. As a means for further clarifying the binding of
these compounds with DNA, viscosity studies were carried out.
Intercalating agents are expected to elongate the double helix
3
given in Fig. 4a, b and c. Compared to the ligand and complexes

5
28
P.-x. Xi et al. / Spectrochimica Acta Part A 71 (2008) 523–528
to accommodate the ligands in between the base leading to
an increase in the viscosity of DNA. In contrast, complexes
that binds exclusively in the DNA grooves by partial and/or
non-classical intercalation, under the same conditions, typically
cause less pronounced (positive or negative) or no change in
DNA solution viscosity [39]. Fig. 6 shows the relative viscos-
ity of DNA (50 ␮M) in the presence of varying amounts of the
ligand, complex 1 and 2. The results reveal that the complex
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5
[
[
2
1
and 2 show relatively small changes in DNA viscosity, indi-
cating that they bind weakly to DNA which is consistent with
DNA groove binding suggested above [40,41]. The increased
degree of viscosity which follows the order of 2 > 1 > ligand,
may depend on its affinity to DNA. This is consistent with our
foregoing hypothesis.
2
[
[
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In this paper, we have investigated and characterized HL and
2406–2414.
its two transition metal complexes ML2·2H2O [M = Ni(1), and
Cd(2)]. In addition, the DNA-binding properties were investi-
gated by electronic absorption, fluorescence, CD spectroscopy
and viscosity measurement. The results support the fact that
the complexes 1, 2 and the ligand can bond to CT-DNA by the
mode of groove binding, and the complex 2 have stronger bind-
ing affinity than complex 1 and the ligand. Information obtained
from the present work is helpful to the development of nucleic
acids molecular probes and new therapeutic reagents for some
diseases.
[
[
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