Synthesis, crystal structure and DNA interaction studies of a 2D cadmium(II) coordination polymer constructed from 2-(2-pyridyl)benzimidazole

Article abstract of DOI:10.1080/10610278.2014.959015

A new complex associated with Cd²⁺, [Cd(OH-H2Bdc)(2-Pbim)]n (1), (OH-H2Bdc = 5-hydroxyisophthalic acid, 2-Pbim = 2-(2-pyridyl)benzimidazole), has been synthesised under hydrothermal conditions and characterised by elemental analysis, FT-IR spec

Full text of DOI:10.1080/10610278.2014.959015

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Synthesis, crystal structure and DNA interaction
studies of a 2D cadmium(II) coordination polymer
constructed from 2-(2-pyridyl)benzimidazole
Yan Yanga, Chang-Gui Lib, Xu-Jian Luoa, Zhi-Hui Luoa, Rong-Jun Liua, Yue-Xiu Jiangb & Wei-
Jiang Lianga
a School of Chemistry and Material, Yulin Normal University, Yulin537000, P.R. China
b School of Chemistry and Chemical Engineering, Guangxi University, Nanning530004, P.R.
China
Published online: 24 Sep 2014.
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To cite this article: Yan Yang, Chang-Gui Li, Xu-Jian Luo, Zhi-Hui Luo, Rong-Jun Liu, Yue-Xiu Jiang & Wei-Jiang Liang (2015)
Synthesis, crystal structure and DNA interaction studies of a 2D cadmium(II) coordination polymer constructed from 2-(2-
pyridyl)benzimidazole, Supramolecular Chemistry, 27:4, 281-286, DOI: 10.1080/10610278.2014.959015
To link to this article: http://dx.doi.org/10.1080/10610278.2014.959015
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Supramolecular Chemistry, 2015
Vol. 27, No. 4, 281–286, http://dx.doi.org/10.1080/10610278.2014.959015
Synthesis, crystal structure and DNA interaction studies of a 2D cadmium(II) coordination
polymer constructed from 2-(2-pyridyl)benzimidazole
Yan Yanga*, Chang-Gui Lib, Xu-Jian Luoa, Zhi-Hui Luoa, Rong-Jun Liua, Yue-Xiu Jiangb* and Wei-Jiang Lianga*
b
aSchool of Chemistry and Material, Yulin Normal University, Yulin 537000, P.R. China; School of Chemistry and Chemical
Engineering, Guangxi University, Nanning 530004, P.R. China
(Received 20 June 2014; accepted 25 August 2014)
A new complex associated with Cd2þ, [Cd(OH-H2Bdc)(2-Pbim)]n (1), (OH-H2Bdc ¼ 5-hydroxyisophthalic acid,
2-Pbim ¼ 2-(2-pyridyl)benzimidazole), has been synthesised under hydrothermal conditions and characterised by elemental
analysis, FT-IR spectroscopy, TG/DTG and fluorescence spectrum. Its in vitro cytotoxicity towards four selected tumour cell
lines has been evaluated by an microculture tetrozolium assay, the results suggest that complex 1 displays greater inhibition
than the free benzimidazole ligand. On the basis of the combination of absorption titration and fluorescence emission
titration, the binding mode of complex 1 to calf thymus DNA has been investigated. Complex 1 can interact with the base
pairs of double-helical DNA via the combined mode of intercalation and groove binding with larger binding constants.
Keywords: cadmium(II) complex; crystal structure; fluorescent property; DNA interaction
Introduction
2-Pbim ¼ 2-(2-pyridyl)benzimidazole). Its in vitro cyto-
toxicity towards four selected tumour cell lines has been
evaluated by the microculture tetrozolium (MTT) (3-[4,5-
dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide)
method. The combination of absorption titration and
fluorescence emission titration has been used to study the
interaction of this compound with DNA.
The interaction of transition metal complexes with DNA
has been extensively studied for their use as probes for
DNA structure and their potential applications in molecular
light switches, DNA footprinting agents, nucleic acid
probes, chemotherapy and photodynamic therapy (1–3).
One of the most important DNA related activities of the
transition metal complexes is that some of the complexes
show the ability to insert DNA (4). Structures containing
benzimidazole, well-known to have a wide range of
biological properties, have commercial applications in
various realms of therapy, including antiulcerative, anti-
hypertensive, antiviral, antifungal, anti-tumour and anti-
histaminic agents, and antihelminthic agents in veterinary
medicine (5). So we begin to study the 2-(2-pyridyl)
benzimidazole (Pbim). In fact, as a chelate ligand Pbim has
multiple coordination modes in forming metal coordination
polymers. It can chelate not only as a capping ligand but
also as a bridging ligand when it is deprotonated (6–8),
which is different from the benzimidazole ligand (9). As a
rigid and versatile bridging, 5-hydroxyisophthalic acid has
been extensively studied for designing a new coordination
polymer because its two carboxylic groups can bond with
metal centres and the hydroxyl group, an electron-
withdrawing group coexisting in isophthalic acid, can not
only act as a hydrogen bond acceptor, but also exhibit steric
effects (10, 11). We have reported some complexes(12–14)
in recent years. Herein, we report the synthesis,
characterisation and DNA interaction of [Cd(OH-H2Bdc)
(2-Pbim)]n (1) (OH-H2Bdc ¼ 5-hydroxyisophthalic acid,
Experimental
Materials and methods
All reagents were bought from commercial sources and used
without further purification. IR spectra were recorded in the
range 4000–400 cm–1 on Perkin–Elmer Spectrum One FT/
IR spectrometer using a KBr pellet. Elemental analysis
(C, H, N) was performed on a Perkin–Elemer 2400II CHN
elemental analyser. TGA (thermogravimetric analysis) was
recorded on Perkin–Elmer Pyris Diamond TG/DTA
analyser. Emission spectra were measured on Ls55
spectrofluorophotometer (Horiba Jobin Jvon, France).
UV–vis absorption titration was performed on a Cary 100
Conc. UV–vis spectrophotometer (Agilent Technologies,
Australia). Fluorescence emission titration was performed
on a Shimadzu RF-5301/PC spectrofluorometer (Tianmei
Technologies, Japan). The crystal structure was determined
by a Bruker APEX area-detector diffractometer (Bruker,
Germany) and employing the SHELXTL crystallographic
software. Calf thymus DNA (CT-DNA) was purchased from
Sigma. Buffer (5 mM tris(hydroxymethyl)aminomethane
(Tris) hydrochloride, 50mM NaCl, pH 7.35) was used for
*Corresponding authors. Email: yy135175@163.com; gxu2110@163.com; liang2179@163.com
q 2014 Taylor & Francis

282
Y. Yang et al.
Table 1. Crystal data and structure refinement for 1.
emission titration experiments. The concentration of CT-
DNA was determined spectrophotometrically assuming a
molar absorption of 6600M—1 cm–1 (260nm) (15–16). The
emission titration of the complex was performed by using a
fixed complex concentration to which increments of the
DNA stock solution were added. The concentration of
complex solution was 10–6 M. Complex–DNA solutions
were allowed to incubate for 10min before the emission
spectra were recorded.
Empirical formula
Formula weight
Temp (K)
C20H13N3O5Cd
487.75
296(2)
Monoclinic
P21/c
10.577(4)
10.037(4)
17.369(6)
90
97.717(4)
90
1827.2(12)
0.20 £ 0.25 £ 0.30
4
1.773
12.3
968
Crystal system
Space group
˚
a (A)
˚
b (A)
˚
c (A)
a (8)
b (8)
g (8)
3
V (A )
˚
Synthesis of [Cd(OH-H2Bdc)(2-Pbim)]n (1)
Crystal size (mm)
Z
A solution of 2-Pbim (0.5 mmol) and 5-hydroxyisophthalic
acid (0.5 mmol) in acetonitrile (10 mL) was added to water
solution (5 mL) of Cd(NO3)2·4H2O (1 mmol). The mixture
was stirred for 30 min, and then 0.6 mL triethylamine was
added. The mixture was placed in a 23 mL Teflon-lined
autoclave and heated at 1408C for 5 days and then cooled to
room temperature. Colourless crystals were obtained (yield
19% based on OH-H2Bdc), filtered off, washed with
distilled water and dried in air. Elemental analysis (%):
Anal. calcd C, 49.25; H, 2.69; N, 8.62; found C, 49.09; H,
2.92; N, 8.57. IR (KBr, cm–1): v ¼ 3422, 3252, 3087, 1596,
1547, 1525, 1478, 1451, 1437, 1391, 1322, 1297, 1207,
1138, 1119, 1053, 976, 795, 743, 620.
Dc (g cm–3
m (cm)
F(0 0 0)
)
Limiting indices
–10 # h # 13
–12 # k # 12
–21 # l # 19
10151
3566
3201
262
0.02
1.17
0.0471
Reflections
Independent
Observed data
Npar
Rint
GOF
Ra1 (I . 2s(I))
wRa2(all data)
Max/min electron density (e A
0.0908
1.14/–1.12
2.3, 26.0
–3
˚
)
u range (8)
X-ray diffraction experiment
˚
Table 2. Selected bond lengths (A) and bond angles (8) for 1.
Diffraction data were collected on a Bruker Smart Apex
CZN diffractometer with graphite-monochromated MoKa
Zn(1)–O(1)
Zn(1)–O(2)
Zn(1)–O(3)
2.264(3) Zn(1)–N(1)
2.068(2) Zn(1)–N(3)
1.984(3)
2.146(3)
2.046(3)
˚
radiation (l ¼ 0.71073 A) at 296 K. Absorption correction
O(1)–Zn(1)–O(2) 60.59(9) O(2)–Zn(1)–C(15) 30.76(11)
O(1)–Zn(1)–O(3) 89.64(9) O(3)–Zn(1)–N(1) 96.33(10)
O(1)–Zn(1)–N(1) 171.05(10) O(3)–Zn(1)–N(3) 144.45(12)
O(1)–Zn(1)–N(3) 98.36(9) O(3)–Zn(1)–C(15) 93.38(10)
O(1)–Zn(1)–C
(15)
O(2)–Zn(1)–O(3) 101.40(11) N(1)–Zn(1)–C(15) 142.23(12)
O(2)–Zn(1)–N(1) 111.48(11) N(3)–Zn(1)–C(15) 110.73(10)
O(2)–Zn(1)–N(3) 112.74(11)
was applied by SADABS (17). The structure was solved by
direct methods and refined with full-matrix least-squares
technique using SHELXTL (18). All non-hydrogen atoms
were refined with anisotropic displacement parameters.
The crystal data, details on the data collection and
refinement are summarised in Table 1, and selected bond
lengths and angles are presented in Table 2. The hydrogen
bond lengths (nm) and bond angles (8) are listed in Table 3.
Figures 1–4 illustrate the structure of 1.
30.09(10) N(1)–Zn(1)–N(3) 80.62(10)
aSymmetry code: (a) 2x, 21/2 þ y, 1/2 2 z; (d) x, 3/2 2 y, 21/2 þ z.
o
Table 3. Distances (A) and angles ( ) of hydrogen bonds for 1.
˚
Cytotoxicity assay
D–H···A
d(D–H) d(H···A) d(D···A) ,(D–H···A)
Cell lines: MDA-MB-231, A549, HeLa and MG-63 were
obtained from the Shanghai Cell Bank in the Chinese
Academy Sciences. Tumour cell lines were grown in PPMI-
1640 medium supplemented with 10% (v/v) foetal bovine
serum, 2 mM glutamine, 100 U/mL penicillin and 100 U/mL
streptomycin at 378C, in a highly humidified atmosphere of
95% air/5% CO2. The cytotoxicity of MT and complex 1
against MDA-MB-231, A549, HeLa and MG-63 cell lines
was examined by the MTTassay (19). The experiments were
carried out using reported procedure (20). The growth
inhibitory rate of treated cells was calculated using the data
N3–H3···O4a 0.8601
O5–H5···O1b 0.8198
1.9565 2.735(5)
1.9033 2.690(5)
2.5787 3.163(7)
2.3670 3.160(7)
149.91
160.56
121.26
142.96
C1–H1···O3c
C8–H8···O1
0.9309
0.9304
Notes: Symmetry code: (a) 1–x,–1/2 þ y,1/2–z; (b) –x,–1/2 þ y,1/2–z;
(c) –x,–1/2 þ y,1/2–z.
from three replicate tests by (ODcontrol –ODtest)/
ODcontrol £ 100%. The complex was incubated with cell
lines, respectively, for 24, 48 and 72h with concentration
gradient of 3.125, 6.25, 12.5, 25 and 50mg/mL.

Supramolecular Chemistry
283
Figure 1. (Colour online) Coordination environment of Cd(II)
metal ion in 1.
Figure 4. (Colouronline)Simplified(4,4)topologystructureof1.
groups from Bdc ligand and two N atoms of 2-Pbim
ligands to give a distorted octahedral coordination
environment (Figure 1). In the coordination network of
1, the Cd(II) atoms occur in pairs and are bridged by a m2–
COO–. Each pair of Cd(II) atoms is connected by a Bdc
ligand with 1D ladder chains (Figure 2). In the structure,
the Bdc ligand adopts m3-bridging coordination fashion:
one carboxylate group adopts chelating/bridging mono-
bidentate coordination mode to connect the Cd(II) centres,
and another carboxylate group acts as a bridging mono-
dentate ligand, while 2-Pbim ligand acts as a chelating
bidentate mode to link Cd atoms. The 1D chains are
connected by a Bdc ligand with a 2D layer along the bc
plane, as illustrated in Figure 3. This layer can be reduced
to a (4, 4) topology (Figure 4).
Figure 2. (Colour online) 1D ladder chain in 1 along b axis.
Thermal stability analysis
TGA was carried out to examine the thermal stability of
complex 1 (Figure 5). The crushed single-crystal sample
Figure 3. (Colour online) 2D structure of 1 along bc plane.
Crystal structure of 1
Single-crystal X-ray diffraction studies performed on a
block of colourless crystals of 1 reveal the presence of a
2D layer constructed from 1D ladder chains running along
the crystallographic b direction. In the asymmetric unit
there is one unique Cd atom that is bridged by carboxylate
Figure 5. (Colour online) TG-DTG curve of 1.

284
Y. Yang et al.
was heated up to 10008C in N2 at a heating rate of
108C min–1. The TG/DTG curves for 1 show that it
collapsed with a total loss of 75.3% (calcd 76.9%),
consistent with the pyrolysis of mixed ligands. The
pyrolysis of mixed ligands occurred in the temperature
range 100–8008C. The above-mentioned different thermal
behaviours may attribute to their structural features, the
presence of abundant coordination bonds involving
carboxylate ligands and the chelating effect of five-
membered rings, which change the bond angles for
chelating, resulting in the formation of a rich variety of
polymeric structures.
identical experimental conditions, complex 1 and its
ligands exhibit different antitumour activity.
As shown in Figure 6, complex 1 displays greater
inhibition than the free benzimidazole ligand towards four
tumour cell lines. The inhibition rate is 50% or so, which is
higher than that of free ligand, 2-Pbim, but lower than that
of free ligand, OH-H2Bdc.
DNA interaction studies
Absorption titrations
The UV–vis absorption spectrum of complex 1 in the
absence and presence of increasing amounts of DNA is
shown in Figure 7. Complex 1 has two absorption peaks at
258 and 312 nm, which can be assigned to the p ! p* (at
258 nm) and n ! p* (at 312 nm) transitions of the
benzimidazole rings. The titration of DNA induces the
hyperchromism of 104.5% and bathochromism of 3 nm
(Figure 7) around 258 nm for complex 1. Such an
observation is likely to reflect levels of intercalation of the
complex with the DNA base stacks. DNA is a long
polynucleotide chain assembled by 50-deoxynucleotides
through phosphodiester bonds, in which its base stacks are
inside the duplex DNA. Under physiological conditions,
negative phosphates along the DNA backbone are
hydrophilic, and base pairs are hydrophobic. The base-
stacking forces, hydrogen bonding and electrostatic
interactions between negative phosphates and positive
sodium or potassium ion can stabilise the double-helical
structure of DNA. When the ratio of [DNA]/[complex]
increases from 0 to 10, the complex can penetrate into the
adenine base stacks of the double-helical DNA, which can
cause hydrophobic interaction made up of base-stacking
forces and van der Waals force to vary correspondingly,
affecting the stability of DNA conformation and unwind-
ing the double-helical structure of DNA and increasing the
Photoluminescence properties
The emission spectrum of complex 1 in the solid state was
investigated at room temperature. Excitation at 365 nm
leads to a strong blue fluorescent emission band at 449 and
562 nm for 1. These emissions are neither metal-to-ligand
charge transfer nor ligand-to-metal charge transfer in
nature since Cd(II) ions are difficult to be oxidised or to be
reduced due to their d10 configuration (21). For the free
H2Bdc ligand, the emission band at 387 nm
(lex ¼ 351 nm) can be assigned to p–p* transition.
In addition, for the free 2-Pbim, the main emission bands
at 503 nm (lex ¼ 490 nm) are assigned to the intraligand
p–p* transition (22). Therefore, we assign the emissions
described above for 1 to ligand-to-ligand charge transfer
excited states.
In vitro antitumour activity assay
The cell growth inhibition rates of complex 1 against four
selected tumour cell lines were evaluated by MTT
methods, as shown in Figure 6. After incubation of
tumour cells and the complex at 10 mM for 48 h under
Figure 7. Absorption spectrum of complex 1 in 5 mM Tris
buffer (pH 7.35) in the absence and presence of ct-DNA.
Figure 6. (Colour online) Cytotoxic activity of complex 1
against four human tumour cell lines.

Supramolecular Chemistry
285
As shown in Figure 8, emission peaks are observed to
increase for complex 1 upon addition of DNA, which also
suggests that complex 1 intercalates with the base pair of
duplex DNA rather strongly. The fluorescence enhance-
ment has resulted from the fact that fluorescence
molecules in the hydrophobic environment between base
pairs of double-helical DNA are effectively protected
against collision of solvent molecules and energy
dissipation with the environmental water and oxygen
(27, 28). In general, complex 1 can interact with the base
pairs of DNA helix via the combined mode of intercalation
and groove binding.
Summary
Figure 8. Emission spectrum of complex 1 in 5 mM Tris buffer
(pH 7.35) in the absence and presence of ct-DNA.
In summary, we have successfully synthesised the novel
coordination polymers endowed with 5-hydroxyisophtha-
lic acid and benzimidazole analogues by the hydrothermal
method. In this study, we have attempted to unravel the
DNA interactions of the novel mixed–ligand benzimida-
zole complexes. Based on UV titration and fluorescence
titration, the combined mode of intercalation and groove
binding for complex 1 is suggested. Complex 1 can
interact with the base pairs of the double-helical DNA via
an intercalative mode with intrinsic binding constant, Kb,
of 2.48 £ 106, presenting high DNA-binding affinity.
UV-vis absorption of purine and pyrimidine bases (23).
On the other hand, the behaviour may result from the
association of the compound bound to DNA, which causes
damage to hydrogen bonds between aggregates and
complexes in solution brought about by the penetration
of complexes into the DNA base stacks (24, 25).
To compare quantitatively the binding strength of the
complex, the intrinsic binding constants Kb with DNA are
determined from the increase of the absorbance with
increasing concentrations of DNA using the Equation (26):
Supplementary materials
½DNAꢀ
½DNAꢀ
1
¼
þ
;
CCDC No. 827563, contains the supplementary crystal-
lographicdata. ThesedatacanbeobtainedviatheCambridge
Crystallographic Data Centre (deposit@ccdc.cam.ac.uk;
http://www.ccdc.cam.ac.uk/deposit).
1a 2 1f 1b 2 1f Kbð1b 2 1f Þ
where 1a, 1f and 1b are the extinction coefficients of the
given, free in solution and fully bound complex,
respectively, Kb is the equilibrium binding constant,
[DNA] is the DNA concentration in 0.1 M Tris buffer
(pH 7.35) containing 5% DMSO. In the plot of [DNA]/
(1a 2 1f) versus [DNA], a slope of 1/(1b 2 1f) and an
intercept of 1/[Kb (1b 2 1f)] are given; the intrinsic binding
constants, Kb, can be obtained by the ratio of the slope of
the intercept. Complex 1 can interact with the base pairs of
the double-helical DNA via an intercalative mode with
intrinsic binding constant, Kb, of 2.48 £ 106, presenting
high DNA-binding affinity.
Funding
We acknowledge financial support by the National Natural
Science Foundation of China [grant number 21341005].
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