Synthesis, characterization, DNA interaction and antibacterial activities of two tetranuclear cobalt(II) and nickel(II) complexes with salicylaldehyde 2-phenylquinoline-4-carboylhydrazone

Article abstract of DOI:10.1016/j.inoche.2011.06.005

Salicylaldehyde 2-phenylquinoline-4-carboylhydrazone (H₂L), and its novel tetranuclear cobalt(II), nickel(II) complexes have been synthesized and characterized. The crystal structure of (NiL)₂?[NiL(H ₂O)(DMF)]₂?H₂O?DMF obtained from DMF solutions of compound 2 was determined by X-ray diffraction analysis. The DNA-binding investigation suggests that the two complexes bind to DNA via groove binding mode. The in vitro antibacterial activity of complexes against Escherichia coli, Staphylococcus aureus, Bacillus subtilis was screened and compared to the activity of the free ligand, the antibacterial activity of complex 1 is active than complex 2 which is in consistent with their DNA-binding behaviors.

Full text of DOI:10.1016/j.inoche.2011.06.005

Inorganic Chemistry Communications 14 (2011) 1569–1573
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization, DNA interaction and antibacterial activities of two
tetranuclear cobalt(II) and nickel(II) complexes with salicylaldehyde
2-phenylquinoline-4-carboylhydrazone
a
a
a
b,1
Zhi-hong Xu a, , Xiao-wei Zhang , Wan-qiang Zhang , Yuan-hao Gao , Zheng-zhi Zeng
⁎
a
College of Chemistry and Chemical Engineering, Xuchang University, Xuchang, 461000, PR China
College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou, 730000, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Salicylaldehyde 2-phenylquinoline-4-carboylhydrazone (H2L), and its novel tetranuclear cobalt(II), nickel(II)
complexes have been synthesized and characterized. The crystal structure of (NiL)2·[NiL(H2O)(DMF)]2·
H2O·DMF obtained from DMF solutions of compound 2 was determined by X-ray diffraction analysis. The
DNA-binding investigation suggests that the two complexes bind to DNA via groove binding mode. The in
vitro antibacterial activity of complexes against Escherichia coli, Staphylococcus aureus, Bacillus subtilis was
screened and compared to the activity of the free ligand, the antibacterial activity of complex 1 is active than
complex 2 which is in consistent with their DNA-binding behaviors.
Received 28 March 2011
Accepted 7 June 2011
Available online 14 June 2011
Keywords:
Salicylaldehyde 2-phenylquinoline-4-
carboylhydrazone
Tetranuclear complex
Groove binding
© 2011 Elsevier B.V. All rights reserved.
Antibacterial activity
Interaction of DNA with transition metal complexes has attracted
considerable interests due to its various applications in cancer therapy
and molecular biology [1–7]. Among them, Schiff base metal com-
plex is a kind of attractive reagent due to their special activities in
pharmacology and physiology [8–12]. Previous works had demon-
strated that 4-quinolinecarboxylic acid amides and hydrazides,
substituted at position 2, exhibit pronounced antiinflammatory and
analgesic activity at a quite low toxicity [13]. However, to the best of
our knowledge, less attention was paid on the interaction of DNA with
Schiff base metal complexes derived from 2-phenylquinoline-4-
carboxylic acid.
In our previous works [14–18], mononuclear complexes were
obtained and exhibited different DNA-binding mode while the H2L
interacted with different salts (nitrate, chloride), where intercalation
mode was obtained when the salts were chloride and groove mode
was obtained when the salts were nitrate. However, it was more
interesting in the present work, tetranuclear cobalt(II) and nickel(II)
complexes of the salicylaldehyde 2-phenylquioline-4-carboylhydra-
zone (H2L, Scheme 1) were obtained and exhibited groove binding
mode when the salts were substituted by acetate. In addition, the
antibacterial properties have been studied with a view to evaluate
their pharmaceutical activities.
The ligand H2L was prepared according to a method in the
literature [14,19], and complexes were synthesized by reacting the
ligand H2L and Co(AcO)2·4H2O and Ni(AcO)2·4H2O in ethanol [20].
The complex 2 crystallized in the triclinic lattice with a space
group Pī (X-ray diffraction data shown in Table S1 and Table S2). The
structure of the compound 2 is shown in Fig. 1. The structure of the
compound is similar to the literature [21]. The compound consists of a
tetranuclear planar neutral molecular unit. The molecule lies in the
conversion center with the Ni…Ni separation of 3.098 and 7.626 Å
for the phenoxide oxygen bridged pair, Ni1…Ni1(#1), and the N―N
bridged pair, Ni1…Ni2 (symmetry code (#1) −x+2,−y, −z+2),
respectively. The Ni1―N3―N2―Ni2 torsion angle is ca. 178.3(3) Å.
The Ni1 is coordinated in a distorted octahedral environment to the
deprotonated phenolic oxygen (O2), azomethine nitrogen (N3), and
deprotonated enolimide oxygen (O1) of a ligand forming respectively
five- and six-membered chelate rings, and a water O (O6) donor and
an O (O5) of DMF in the opposite axial sites, the sixth coordination site
is occupied by the symmetry related phenoxy oxygen of another
ligand. The Ni2 bonds in the square plane to an ONO tridentate
donors of a ligand and a N (N2) from another ligand. The five- and six-
membered chelate ring Ni1O1C1N3C18O2 is planar with the mean
deviation of 0.018 Å. The bond distances of Ni2―O3, Ni2―N6 and
Ni2―O4 agree with the bond lengths of the Ni complex of the
analogous dinegative tridentate aroylhydrazone [22]. The other Ni―O
and Ni―N bond distancesconsist with the Ni complex of the
aroylhydrazone [23–27].
⁎
Corresponding author. Tel./fax: +86 3744369257.
E-mail addresses: xuzhihong1980@yahoo.com (Z. Xu), zengzhzh@yahoo.com.cn
Even though the DMF is coordinated to nickel(II) in crystalline form
of complex 2, the DMF will be replaced by water in aqueous solution
(Z. Zeng).
1
Tel./fax: +86 9318912582.
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.06.005

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Z. Xu et al. / Inorganic Chemistry Communications 14 (2011) 1569–1573
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 helicity, which is characteristic of DNA in the right-
handed B form. While groove binding and electrostatic 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. The CD spectra of DNA taken after incubation of the complexes
with CT-DNA are shown in Fig. 3. In all two cases, the intensities of
both the negative and positive bands decrease significantly (shifting
to zero levels). 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 [31].
These changes are indicative of a non-intercalative mode of binding of
these complexes and offer support to their groove binding nature [32].
As shown in Fig. 4, the fluorescence intensity of complex 1 and 2
are quenched steadily with the increasing concentration of the CT-
DNA. The Stern–Volmer quenching plot from the fluorescence
titration data is shown in Fig. 5. The fluorescence quenching constant
(Ksv) evaluated using the Stern–Volmer equation is 2.46×104 M−1
and 3.57×104 M−1. The Stern–Volmer plot is linear, indicating that
only one type of quenching process occurs. This phenomenon of the
quenching of fluorescence 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 [30,33–36].
Scheme 1. 2-phenylquinoline-4-carboylhydrazone, H2L.
(See Fig. S1). The complexes will therefore exist as (NiL)2·[NiL
(H2O)2]2·4H2O in our experimental section.
The absorption spectra of complex 1 and 2 in the absence and
presence of CT-DNA (at a constant concentration of complexes) are
given in Fig. 2. In the presence of DNA, the absorption bands of 1 and 2
at about 267 nm exhibited hypochromism of about 21 and 43.2% and
bathochromism of about 5 and 8 nm, respectively. The spectroscopic
changes suggest that the complexes have stronger interaction with
DNA.
The intrinsic binding constants Kb of the two complexes with DNA
were obtained by monitoring the changes in absorbance at 467 nm for
complex 1 and 465 nm for complex 2 with increasing concentration of
DNA. The intrinsic binding constants Kb of complexes 1 and 2 were
1.81×105 M−1 and 0.89×105 M−1, respectively. The results indicate
that the binding strength of complex 1 is stronger than that of 2. Such
a small change in λmax is more in keeping with groove binding,
leading to small perturbations. The Kb value obtained here is lower
than that reported for classical intercalator (for ethidium bromide and
[Ru(phen)DPPZ] whose binding constants have been found to be in
the order of 106–107 M) [28,29]. The observed binding constant is
more in keeping with the groove binding with DNA, as observed in the
literature [14,30].
Hydrodynamic methods that are sensitive to length are regarded
as one of the least ambiguous and most critical tests of a binding mode
in solution in the absence of crystallographic structural data.
Intercalating agents are expected to elongate the double helix to
accommodate the ligands in between the base leading to an increase
in the viscosity of DNA. In contrast, complexes that bind 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 [37]. The values of
Fig. 1. ORTEP drawing of the compound 2 with thermal ellipsoid at 50% probability level, all hydrogen atoms are omitted for clarity.

Z. Xu et al. / Inorganic Chemistry Communications 14 (2011) 1569–1573
1571
Fig. 4. Emission spectra of the complex 1 (a) and 2 (b) in Tris–HCl buffer upon addition
of calf-thymus DNA. [complex]=1×10−5 M, [DNA]=(0–5)×10−5 M. Arrow shows
the intensity changing upon increasing DNA concentrations.
Fig. 2. Absorption spectra of the complex 1 (a) and 2 (b) in Tris–HCl buffer upon
addition of calf-thymus DNA. [complex]=1×10−5 M, [DNA]=(0–5)×10−5 M. Arrow
shows the absorbance changing upon increasing DNA concentrations. Inset: plots of
[DNA]/(εa −εf) versus [DNA] for the titration of DNA with the complex.
are 0.058 and −0.474 V, respectively. In the absence of DNA, the
separation of the anodic and cathodic peak potentials, ΔEp =1.985 V
for complex 1 and 0.714 V for complex 2. With the addition of DNA in
the solution, it caused a considerable difference in the voltammetric
current which changed from 1 to 0.59 for complex 1 and 1.16 for
complex 2, respectively. In addition, the peak potentials, Epc and Epa, as
well as E1/2 had a shift to more negative potential. The shift of the redox
potential of the complexes in the presence of DNA to more negative
values indicates a binding interaction between the complex and DNA
that makes the complexes less readily reducible. The decreased
extents of the peak currents observed for the complexes upon addition
of CT DNA may indicate that complex 1 possesses higher DNA-binding
affinity than complex 2 does. The results parallel the above
spectroscopic and viscosity data of Co and Ni complexes in the
presence of DNA.
(η/η0)1/3 were plotted against [complex]/[DNA] (see Fig. S2). The
results reveal that the complexes 1 and 2lead relatively to an
inapparent increase in DNA viscosity, which is consistent with DNA
groove binding suggested above, which is also known to enhance DNA
viscosity [38]. The increased degree of viscosity, which may depend
on its affinity to DNA follows the order of 1N2, which is consistent
with our foregoing hypothesis.
The cyclic voltammetric behavior of the Co and Ni complexes in the
presence of CT-DNA is shown in Table 1. In the absence of CT DNA, the
Epc and Epa are 1.050 and −0.935 V for 1, and −0.117 and −0.831 V
for 2. The half-wave potentials E1/2, taken as the average of Epc and Epa
Complexes 1, 2 and the ligand (H2L) had been tested for their in
vitro antibacterial activity against Escherichia coli, Staphylococcus
aureus, Bacillus subtilis, comparative with free ligand, using the paper
disc diffusion method [39,40] (for the qualitative determination) and
the serial dilutions in liquid broth method [41] for determination of
MIC. The antibacterial activity results of all of the tested compounds
and their MIC values are presented in Table 2. The results evidently
show that the ligand and its complexes possess antimicrobial
activities against the tested bacteria at MIC values between 93 and
567 μg/ml, and that complex 1 is more active than complex 2 which is
in agreement with their DNA-binding behaviors. The ligand exhibits
better activity than the complexes. The presence of electron densities
on aroylhydrazone group contributes positively to the increase of the
activity of compounds against bacteria [42]. But the decrease of the
electron densities by the coordination through the donor atoms
Fig. 3. CD spectra of CT-DNA (100 mM) in the absence (Solid line) and presence of
compound 1 (Dash line) and 2 (Dot line) (50 mM).

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Z. Xu et al. / Inorganic Chemistry Communications 14 (2011) 1569–1573
Table 2
The MICs values of ligand and its complexes (μg/ml).
Compounds
H2L
Escherichia coli
93
Staphylococcus aureus
123
Bacillus subtilis
74
1
2
170
283
–
170
212
–
140
213
–
DMSO
Appendix A. Supplementary material
Crystallographic data for the structure have been deposited with
the Cambridge Crystallographic Data Center as supplementary
material (CCDC-655710). These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by e-mailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallo-
graphic Data Center, 12, Union Road, Cambridge CB2 1EZ, UK;
fax: +44 1223 336033. Supplementary data to this article can be
found online at doi:10.1016/j.inoche.2011.06.005.
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Table 1
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Complex
1
R
Epa/V
Epc/V
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−0.117
−0.035
ΔEp/V
E1/2/V
ipa/ipa (R=0)
0
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7.34 (t, 1H, -2-hydroxyl-Ph), 7.52–7.69 (m, 5H, -quinoline), 7.87 (t, 1H, -2-
hydroxyl-Ph), 8.19 (d, 1H, -Ph), 8.27 (d, 1H, -Ph), 8.33–8.42 (m, 3H, -Ph), 8.62 (s,
1H, -CH), 11.08 (s, 1H, -NH-), 12.47 (s, 1H, ―OH). IR (KBr) (cm−1): νO―H 3436,
νN―H 3144, νC O 1676, νC N 1619, νC N―N 1564, δN―H 1492, νC―N 1357. UV λmax
(nm): 220, 261, 329
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