A new nickel(II) complex with the thiosemicarbazone of quinoline-2- carboxaldehyde: Structure, DNA-binding, cleavage, and cytotoxic activities

Article abstract of DOI:10.1080/00958972.2012.710842

[Ni(QTS)₂]ClCH₃OH, where QST=quinoline-2- carboxaldehyde thiosemicarbazone, has been synthesized and characterized. The complex crystallized in a monoclinic system with space group P2(1)/n. Nickel(II) is situated in a distorted octahedral geometry with two tridentate ligands and one ligand is mono-deprotonated to coordinate to nickel(II). Interaction of the nickel(II) complex with calf thymus DNA was investigated by electronic absorption, CD, and fluorescence spectra. The results suggest that nickel(II) complex binds to DNA through a groove binding mode. The nickel(II) complex exhibited efficient DNA cleavage at micromolar concentration in the presence of ascorbate with hydroxyl radicals as the active species. Invitro cytotoxicity assay showed that the nickel(II) complex was more potent against MCF-7 cell line but less active against A-549 cell line than cisplatin at the concentrations tested.

Full text of DOI:10.1080/00958972.2012.710842

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A new nickel(II) complex with the
thiosemicarbazone of quinoline-2-
carboxaldehyde: structure, DNA-
binding, cleavage, and cytotoxic
activities
a
a
a
b
Shouchun Zhang , Juanjuan Dong , Xiaorui Fan , Yun Chen
&
a
Jianliang Zhou
a
School of Chemistry and Chemical Engineering, Central South
University, Changsha 410083, P.R. China
b
School of Chemistry and Chemical Engineering, Hunan
University, Changsha 410082, P.R. China
Accepted author version posted online: 10 Jul 2012.Published
online: 24 Jul 2012.
To cite this article: Shouchun Zhang , Juanjuan Dong , Xiaorui Fan , Yun Chen & Jianliang Zhou
(
2012) A new nickel(II) complex with the thiosemicarbazone of quinoline-2-carboxaldehyde:
structure, DNA-binding, cleavage, and cytotoxic activities, Journal of Coordination Chemistry,
5:17, 3098-3110, DOI: 10.1080/00958972.2012.710842
6
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Journal of Coordination Chemistry
Vol. 65, No. 17, 10 September 2012, 3098–3110
A new nickel(II) complex with the thiosemicarbazone of
quinoline-2-carboxaldehyde: structure, DNA-binding,
cleavage, and cytotoxic activities
SHOUCHUN ZHANG*y, JUANJUAN DONGy, XIAORUI FANy,
YUN CHENz and JIANLIANG ZHOUy
ySchool of Chemistry and Chemical Engineering,
Central South University, Changsha 410083, P.R. China
zSchool of Chemistry and Chemical Engineering, Hunan University,
Changsha 410082, P.R. China
(
Received 22 February 2012; in final form 6 June 2012)
[
Ni(QTS)
2
] ꢀ Cl ꢀ CH OH, where QST ¼ quinoline-2-carboxaldehyde thiosemicarbazone, has
3
been synthesized and characterized. The complex crystallized in a monoclinic system with space
group P2(1)/n. Nickel(II) is situated in a distorted octahedral geometry with two tridentate
ligands and one ligand is mono-deprotonated to coordinate to nickel(II). Interaction of the
nickel(II) complex with calf thymus DNA was investigated by electronic absorption, CD, and
fluorescence spectra. The results suggest that nickel(II) complex binds to DNA through a
groove binding mode. The nickel(II) complex exhibited efficient DNA cleavage at micromolar
concentration in the presence of ascorbate with hydroxyl radicals as the active species. In vitro
cytotoxicity assay showed that the nickel(II) complex was more potent against MCF-7 cell line
but less active against A-549 cell line than cisplatin at the concentrations tested.
Keywords: Nickel complex; Thiosemicarbazone; DNA-binding; DNA cleavage; Cytotoxicity
1. Introduction
Thiosemicarbazones and their derivatives with wide pharmacological versatility, such as
anticancer, antibacterial, antiviral, antifungal, and antimicrobial activities, have
received attention [1–6]. These compounds have excellent capacity to coordinate with
many transition metal ions, and the generated complexes show significant biological
activities [7–13]. In many cases, the activities of these complexes are higher than that of
the thiosemicarbazone alone [14–17].
Many studies suggest that thiosemicarbazones and derivatives with biological
activities usually have a nitrogen-containing heterocycle (e.g., ꢀ-pyridyl moiety)
1
adjacent to the N position [18–22]. For example, 2-formylpyridine thiosemicarbazone
reported about 50 years again was the first ꢀ-(N)-heterocyclic carboxaldehyde
thiosemicarbazone with antitumor properties [23]. Later, 5-hydroxy-2-formylpyridine
*
Corresponding author. Email: zhang_shch@yahoo.com.cn
Journal of Coordination Chemistry
ISSN 0095-8972 print/ISSN 1029-0389 online ß 2012 Taylor & Francis
http://dx.doi.org/10.1080/00958972.2012.710842

Nickel(II) thiosemicarbazone
3099
thiosemicarbazone was developed as the first compound of this series that entered phase
I clinical trials. However, the high instability and severe side effects prevented its further
clinical development [24, 25]. Another representative example was 3-aminopyridine-2-
carboxaldehyde thiosemicarbazone, which entered clinical trials for the treatment of a
variety of malignancies [26–28]. Many other ꢀ-pyridylthiosemicarbazones and their
metal complexes with promising biological activities have been investigated [29–32].
We focus our research on the synthesis and antitumor activities of new ꢀ-(N)-
heterocyclic carboxaldehyde thiosemicarbazones and their metal complexes to find
more potent pharmacological agents. It is commonly thought that DNA is the primary
pharmacological target of many antitumor compounds, and interaction between metal
complexes and DNA is in close relationship with their potential biological and
pharmaceutical activities [33–35]. Binding modes to DNA would give insights into the
biochemical mechanism of action of the complexes, which have been of interest for
development of effective chemotherapeutic agents.
Here we report the synthesis and characterization of a new nickel(II) complex with
thiosemicarbazone of quinoline-2-carboxaldehyde. The DNA-binding, DNA cleavage
and in vitro cytotoxic activities of the nickel(II) complex are also studied.
2
2
. Experimental
.1. Chemicals and physical measurements
Common reagents such as NiCl ꢀ 6H O, thiosemicarbazide, ethanol, methanol, and
2
2
anhydrous ether are of analytical grade and used as received. Plasmid pBR322 and
disodium salt of calf thymus DNA (CT-DNA) were commercially available from MBI
Fermentas, while quinoline-2-carboxaldehyde, ascorbate, tris(hydroxy-methyl) amino-
methane (Tris), and ethidium bromide (EB) were from Sigma. Infrared spectra were
ꢁ
1
recorded on a Bruker VECTOR22 spectrometer as KBr pellets (4000–500 cm ) and
ꢂ
elemental analysis was performed on a Perkin-Elmer 240 C analytical instrument.
UV-Vis spectra were recorded on a UV-3100 spectrometer. CD spectra were obtained
by using a Jasco J-810 automatic recording spectropolarimeter. Fluorescence spectra
were recorded on an AMINCO Bowman Series 2 Luminescence Spectrometer.
2
2
.2. Synthesis
.2.1. Ligand. Quinoline-2-carboxaldehyde thiosemicarbazone (QTS) was obtained
following a similar procedure reported [30] by condensation of thiosemicarbazide and
quinoline-2-carboxaldehyde (1 : 1 ratio) in ethanol.
.
.
.2.2. [Ni(QTS) ] Cl CH OH.
2
(
(
A
methanol solution (20 mL) of NiCl ꢀ 6H O
2
3
2
2
0.5 mmol, 119 mg) was added dropwise to methanol solution (20 mL) of QTS
1 mmol, 230 mg), and the resulting solution was refluxed for 4 h. Dark red block
single crystals suitable for X-ray diffraction were obtained on slow evaporation of the
ꢁ
1
solution of the complex. IR/(KBr, ꢁ/cm ): 3269, 3149 ꢁ(N–H), 1567 ꢁ(C ¼ N), 1123
(C¼S). Anal. Calcd for C H N OS ClNi (%): C, 47.16; H, 3.95; N, 19.12. Found
ꢁ
2
3
23
8
2
(%): C, 47.08; H, 3.98; N, 19.19.

3
100
S. Zhang et al.
Table 1. Crystal data and structure refinement for [Ni(QTS)
2
] ꢀ Cl ꢀ CH
3
OH.
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
C
23
H
23
N
8
NiS ClO
2
585.77
273(2)
Monoclinic
P2(1)/n
˚
ꢂ
Unit cell dimensions (A, )
a
b
c
ꢀ
ꢃ
ꢄ
10.7127(6)
9.5598(5)
24.2734(13)
90
94.748(2)
90
2477.3(2), 4
1.571
1.095
3
˚
Volume (A ), Z
ꢁ
3
Calculated density (Mg m
Absorption coefficient (mm
F(000)
ꢂrange for data collection ( )
Limiting indices
)
ꢁ
1
)
1208
2.15–28.00
ꢁ12 ꢄ h ꢄ 14;
ꢂ
ꢁ
ꢁ
12 ꢄ k ꢄ 11;
31 ꢄ l ꢄ 29
Reflection collected/unique
Absorption correction
Refinement method
Data/restraints/parameters
Goodness-of-fit on F
17,318/5482 (Rint ¼ 0.1865)
Empirical
2
Full-matrix least-squares on F
5482/1/327
0.983
2
Final R indices [I 4 2ꢅ(I)]
R indices (all data)
R
R
1
1
¼ 0.0981; wR
2
2
¼ 0.2488
¼ 0.3705
a
¼ 0.2075; wR
ꢁ
3
2
˚
Largest difference peak and hole (e A
)
1.147 and ꢁ1.574
a
R
2
2
2
2 1/2
o
1
¼ ÆjjF
o
j ꢁ jF
c
jj/ÆjF
o
j; wR
2
¼ [Æw(Fo ꢁ F ) /Æw(F ) ]
c
.
2.3. X-ray crystallography
Details of the crystal parameters, data collection, and refinement are listed in table 1.
The structure was determined on a Siemens P4 four-circle diffractometer. Cell constants
and an orientation matrix for data collection were obtained by least-squares refinement
of diffraction data from 34 reflections in the range 1.84 5 ꢂ5 24.97. Data were
collected at 293 K using monochromated Mo-Ka radiation and the !–2ꢂscan technique
ꢁ1
with a variable scan speed of 5.0–50.0 min in ! and corrected for Lorentz and
polarization effects. An empirical absorption correction was made ( -scan). The
structure was solved by Patterson methods and completed by iterative cycles of least-
squares refinement and DF-syntheses. Hydrogen atoms were located in their calculated
positions and treated as riding on the atoms to which they are attached. All non-
hydrogen atoms were refined anisotropically. All calculations were carried out using
SHELXTL [36].
2
2
.4. Spectroscopic studies on DNA interaction
.4.1. Electronic absorption spectra. The UV absorbances at 260 and 280 nm of
ꢁ1
ꢁ1
CT-DNA solution in 50 mmol L NaCl/5 mmol L Tris-HCl buffer (pH 7.2) give a
ratio of ꢃ1.9, indicating that the DNA was sufficiently free of protein [37]. The DNA
concentration was determined by measuring the UV absorption at 260 nm, taking the

Nickel(II) thiosemicarbazone
3101
[38].
ꢁ
1 ꢁ1
ꢁ1
cm
molar absorption coefficient ("260) of CT-DNA as 6600 (mol L
ꢁ
)
5
ꢁ1
Electronic spectra of [Ni(QTS) ] ꢀ Cl ꢀ CH OH (5.0 ꢅ 10 mol L ) were recorded
2
3
before and after addition of CT-DNA (r ¼ 0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, where r is
ꢁ
1
the molar ratio of DNA and complex) in the 5 mmol L Tris-HCl/50 mmol L NaCl
ꢁ1
buffer. The intrinsic binding constant K for interaction of the studied complexes with
b
CT-DNA was calculated by electronic absorption spectral titration data using the
following equation [39]:
½DNAꢆ=ð"a ꢁ "fÞ ¼ ½DNAꢆ=ð"b ꢁ "fÞ þ 1=Kbð"b ꢁ "fÞ,
where ", ", and " correspond to Aobsd/[Ni], the extinction coefficient for the free nickel
a
f
b
complex, and the extinction coefficient for the nickel(II) complex in the fully bound
form, respectively. In the plot of [DNA]/(" ꢁ ") versus [DNA], K is then given by the
a
f
b
ratio of the slope to intercept.
2
.4.2. CD spectra. CD spectra of in the absence and presence of nickel(II) complex
were collected. All CD experiments were carried out on a Jasco-810 spectropolarimeter
at room temperature from 220 to 320 nm.
2.4.3. Fluorescence spectra. Fluorescence spectra were recorded at room temperature
with excitation at 520 nm and emission at 600 nm. The experiment was carried out
ꢁ
5
ꢁ1
ꢁ1
in 5 mmol L Tris-HCl/
by titrating [Ni(QTS) ] ꢀ Cl ꢀ CH OH (5.0 ꢅ 10 mol L
2
3
ꢁ1
ꢁ4
ꢁ1
5
0 mmol L
NaCl buffer) into samples containing 1.0 ꢅ 10 mol L
DNA and
.0 ꢅ 10ꢁ mol L EB.
5
ꢁ1
1
2
.5. DNA cleavage
A typical reaction was carried out by mixing 1 mL of pBR322 DNA (0.1 mg/mL), 4.2 mL
of Tris-HCl/NaCl buffer (pH 7.4) and 2.4 mL [Ni(QTS) ] ꢀ Cl ꢀ CH OH solution in
2
3
ꢁ1
4
0 mmol L Tris-HCl/NaCl buffer (pH 7.4, 4% DMF) with 2.4 mL of ascorbate (H A)
2
at a 50-fold molar excess relative to the complex to yield a total volume of 10 mL. After
mixing, the sample was incubated at 310 K for 1 h. The reactions were quenched by
addition of ethylenediaminetetraacetic acid (EDTA) and loading buffer (0.25%
bromphenol blue, 50% glycerol). Then, the solution was subjected to electrophoresis
ꢁ
1
ꢁ1
on 0.7% agarose gel in TBE buffer (89 mmol L
Tris, 89 mmol L
mmol L EDTA) at 100 V and visualized by EB staining. The Gel Imaging and
H BO and
3
3
ꢁ1
2
Documentation DigiDoc-It
TM
System (Version 1.1.23, UVP, Inc. Unpland, CA) was
assessed using Labworks Imaging and Analysis Software (UVP, Inc. Unpland, CA).
In the inhibition reactions, scavengers of reactive oxygen intermediates, DMSO, tert-
butyl alcohol, and histidine (2 mL) were added initially to pBR322 DNA (100 ng) in the
Tris-HCl/NaCl buffer and incubated for 15 min at 310 K prior to addition of
[
Ni(QTS) ] ꢀ Cl ꢀ CH OH and H A. The mixture was diluted with the buffer to a total
2
3
2
volume of 10 mL. After a further incubation of 3 h at 310 K the sample was subjected to
gel electrophoresis using the described procedures.

3
102
.6. Cytotoxicity assay
S. Zhang et al.
2
The in vitro cytotoxicity of [Ni(QTS) ] ꢀ Cl ꢀ CH OH was tested against the human breast
2
3
cancer cell line MCF-7 by 3-(4,5-dimethylthiazole-2-yl)-2,5-diphenyltetrazolium bro-
mide (MTT) assay [40] and against the human non-small-cell lung cancer cell line A-549
by sulforhodamine B (SRB) assay [41]. Cisplatin was chosen as a positive reference.
The detailed procedure of the assay has been described in our previous work [42].
3
3
. Results and discussion
.1. Crystal structure
The molecular structure and atom-numbering schemes for [Ni(QTS) ] ꢀ Cl ꢀ CH OH are
2
3
shown in figure 1. Selected bond lengths and angles are summarized in table 2.
Nickel(II) is coordinated by two QST ligands in a distorted octahedral geometry
through two cis quinolinyl nitrogen atoms (N(1) and N(5)) and two cis thiolato sulfurs
(S(1) and S(2)) in the square plane, and two trans imine nitrogen atoms (N(2) and N(6))
in axial positions. Both QTS ligands are tridentate and the ligands show a (Z, E, Z)-
conEguration for these three donors, respectively. Similar meridional conEgurations of
tridentate NNS ligands around nickel(II) have also been observed in other bischelated
˚
nickel(II) complexes of NNS thiosemicarbazones [43]. The C(22)–S(2) (1.690(8) A)
˚
bond length is shorter than C(11)–S(1) (1.716(7) A). Bond angles of N(6)–N(7)–C(22)
Figure 1. ORTEP diagram for [Ni(QTS)
2
3
] ꢀ Cl ꢀ CH OH.

Nickel(II) thiosemicarbazone
3103
˚
Table 2. Selected bond lengths (A) and angles ( ) of [Ni(QTS)
ꢂ
2
] ꢀ Cl ꢀ CH
3
OH.
Ni(1)–N(1)
Ni(1)–N(2)
S(1)–C(11)
2.180(7)
2.024(6)
1.716(7)
Ni(1)–N(5)
Ni(1)–N(6)
S(2)–C(22)
2.175(6)
2.036(6)
1.690(8)
Ni(1)–S(1)
Ni(1)–S(2)
2.409(3)
2.429(2)
N(1)–Ni(1)–N(2)
N(1)–Ni(1)–N(5)
N(1)–Ni(1)–N(6)
N(5)–Ni(1)–N(6)
N(2)–Ni(1)–N(5)
N(2)–N(3)–C(11)
S(2)–C(22)–N(7)
77.6(3)
94.9(3)
107.8(3)
76.9(2)
111.9(2)
111.8(6)
122.3(5)
N(2)–Ni(1)–N(6)
N(1)–Ni(1)–S(1)
N(2)–Ni(1)–S(1)
N(5)–Ni(1)–S(1)
N(6)–Ni(1)–S(1)
N(6)–N(7)–C(22)
S(1)–C(11)–N(4)
169.6(3)
N(1)–Ni(1)–S(2)
N(2)–Ni(1)–S(2)
N(5)–Ni(1)–S(2)
N(6)–Ni(1)–S(2)
S(1)–Ni(1)–S(2)
S(1)–C(11)–N(3)
S(2)–C(22)–N(8)
91.63(18)
90.36(17)
157.62(16)
80.73(17)
98.04(10)
128.5(7)
156.49(16)
81.0(2)
84.2(2)
94.9(2)
118.8(6)
117.0(7)
121.8(7)
˚
D–H (A)
˚
˚
D–Hꢀ ꢀ ꢀA (ꢂ)
Hꢀ ꢀ ꢀA (A)
Dꢀ ꢀ ꢀA (A)
Hydrogen-bonding contacts D–Hꢀ ꢀ ꢀA
i
N(4)–H(4 A)ꢀ ꢀ ꢀCl(1)
0.860
0.860
0.860
0.860
2.462
3.272
3.345
3.247
3.573
157.32
152.52
175.53
138.00
ii
N(4)–H(4B)ꢀ ꢀ ꢀCl(1)
2.558
2.389
2.887
iii
N(8)–H(8 A)ꢀ ꢀ ꢀCl(1)
N(8)–H(8B)ꢀ ꢀ ꢀCl(1)
i
ii
iii
¼
x ꢁ 1, y, z; ¼ꢁx þ 1/2, y ꢁ 1/2, ꢁz þ 1/2; ¼ꢁx þ 2/3, y ꢁ 1/2, ꢁz þ 1/2.
ꢂ ꢂ
118.8(6) ) and N(8)–C(22)–S(2) (121.8(7) ) are bigger than that of N(2)–N(3)–C(11)
(
(
(
ꢂ
ꢂ
111.8(6) ) and N(4)–C(11)–S(1) (117.0(7) ), while the N(7)–C(22)–S(2) bond angle
ꢂ
ꢂ
122.3(5) ) is smaller than that of N(3)–C(11)–S(1) (128.5(7) ). These bond lengths and
a-Tojal et al. to
angles show excellent agreement with those proposed by Garcı
´
distinguish neutral or anionic form in ligands based on pyridine-2-carbaldehyde
thiosemicarbazone [44] and ratify the coexistence of both forms of QTS in the same
nickel(II) complex. Chloride is a counterion in this complex. The Ni–S [2.409(3) and
˚
˚
˚
2
2
.429(2) A], Ni–N
[2.024(6) and 2.036(6) A], and Ni–N
[2.180(7) and
.175(6) A] bond lengths are similar to those of other six-coordinate distorted
imine
quinoline
octahedral nickel(II) complexes of tridentate sulfur–nitrogen chelating agents [45, 46].
The two Ni–Nquinoline bond lengths are very close and longer than Ni–Nimine bond
lengths, indicative of weak coordination by quinoline. The bond angles of N(1)–Ni(1)–
ꢂ
ꢂ
S(1) (156.49(16) ) and N(5)–Ni(1)–S(2) (157.62(16) ) are contracted from the ideal value
ꢂ
of 180 for a regular square-planar structure, and the bond angle of N(2)–Ni(1)–N(6)
ꢂ ꢂ
(
169.6(3) ) is also contracted from 180 , suggesting distorted octahedral geometry.
Several hydrogen-bonding interactions are established in the lattice. The most relevant
are given in table 2.
3
3
.2. Spectral studies of the interactions with DNA
.2.1. Electronic absorption spectra. It is very common to apply electronic absorption
spectroscopy in DNA-binding studies [47]. Electronic absorption spectra of
[
Ni(QTS) ] ꢀ Cl ꢀ CH OH in the absence and presence of CT-DNA are shown in
2
3
figure 2. The bands at 238 and 293 nm can be assigned to ꢆ!ꢆ* transitions of the
coordinated thiosemicarbazone. Band at 339 nm and weak shoulder around 350 nm can
be attributed to n ! ꢆ* transitions of the coordinated thiosemicarbazone. Bands at 383
and 423 nm must be due to L ! M transfer from sulfur to nickel. Occurrence of S ! M
(LMCT) bands is quite common in electronic spectra of metal complexes of
thiosemicarbazones [48, 49]. Upon increasing the concentration of DNA, moderate

3
104
S. Zhang et al.
Figure 2. Electronic absorption spectra of 5.0 ꢅ 10ꢁ mol L
5
ꢁ1
[Ni(QTS)
] ꢀ Cl ꢀ CH
2
3
OH in the absence
(
a) and presence (b ! g) of increasing amounts of CT-DNA at the ratio r ¼ 0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2.
hypochromism (22.4% and 23.8% at 238 and 293 nm, respectively) but no apparent
bathochromism were observed, suggesting interactions between DNA and the nickel(II)
complex via a non-classical intercalation, such as groove binding. Complexes binding
with DNA through intercalation result in distinct hypochromism and some bath-
ochromism [50, 51]. The intrinsic binding constant (K ) obtained for the nickel(II)
b
3
ꢁ1 ꢁ1
complex is 3.27 ꢅ 10 (mol L
)
classical intercalator EB whose binding constant has been found to be in the order of
using the absorption at 293 nm, lower than that of
6
0 (mol L
ꢁ1 ꢁ1
1 ) [52]. These results suggest that the nickel(II) complex has weaker
binding of DNA than the classical intercalator, and it is likely that the nickel(II)
complex binds to CT-DNA via groove binding.
3
.2.2. CD spectra. CD spectra are useful in monitoring the conformational variations
of DNA in solution. The CD spectrum of free helix DNA exhibits a positive band at
75 nm due to base stacking and a negative band at 245 nm due to the helicity of B-type
DNA [53]. Figure 3 displays the CD spectra of CT-DNA in the absence and presence of
2
[
Ni(QTS) ] ꢀ Cl ꢀ CH OH. The intensities of both the positive and negative bands
2
3
decrease significantly upon addition of the complex to DNA, which implies that the
nickel(II) complex can unwind the DNA helix and lead to the loss of helicity [54, 55].
These changes are indicative of a non-intercalative binding mode and support the
groove binding nature [56, 57]; intercalative DNA-binding generally results in an
increase of the intensities of both the positive and negative bands in CD spectra.
3
.2.3. Fluorescence spectra. Fluorescence spectra were also used to study the
interaction between [Ni(QTS) ] ꢀ Cl ꢀ CH OH and DNA by measuring the emission
2
3

Nickel(II) thiosemicarbazone
3105
ꢁ4
ꢁ1
Figure 3. CD spectra of 1.0 ꢅ 10 mol L CT-DNA in the absence (ꢀ ꢀ ꢀ) and presence (–) of increasing
amounts of [Ni(QTS)
2
] ꢀ Cl ꢀ CH OH at the ratio r ¼ 0, 0.076, 0.13.
3
intensity of EB bound to CT-DNA. EB is weakly fluorescent, but can emit intense
fluorescence in the presence of DNA because of its intercalative binding to DNA.
However, this enhanced fluorescence could be quenched or partly quenched by addition
of a second molecule that can replace bound EB or break the secondary structure of
DNA [58]. So EB can be used as a probe for determination of DNA structure. In this
study, the emission spectra of EB bound to DNA in the absence and presence of
[
Ni(QTS) ] ꢀ Cl ꢀ CH OH are shown in figure 4. The emission intensity of the CT-DNA-
2
3
EB system (ꢇ ¼ 599 nm) decreases with addition of the nickel(II) complex, indicating
em
that the complex can bind to DNA and replace EB from the DNA structure. This
behavior suggests that groove binding interactions would occur between the complex
and DNA [59, 60], and the electronic absorption spectra and CD spectral analysis also
exclude intercalation.
3
.3. DNA-cleavage activity
Many nickel(II) complexes that show efficient metallonuclease activity by oxidative or
hydrolytic pathways have been reported recently [61–64]. We also studied here the
DNA-cleavage activity of [Ni(QTS) ] ꢀ Cl ꢀ CH OH by gel electrophoresis using plasmid
2
3
pBR322 DNA. Figure 5 shows the cleavage of DNA at different concentration of the
nickel(II) complex for 1 h reaction time (pH 7.4, 310 K). Control experiments using only
ascorbate (H A) or nickel(II) complex failed to show apparent cleavage of DNA
2
(figure 5, lanes 2 and 3). With increasing concentration of the nickel(II) complex, the
supercoiled DNA (Form I) decreased gradually and was finally converted almost
completely to nicked DNA (Form II) in the presence of ascorbate as a reductant agent.

3
106
S. Zhang et al.
ꢁ
ꢁ5
5
ꢁ1
ꢁ1
Figure 4. Fluorescence emission spectra (excited at 520 nm) of the CT-DNA-EB system (1.0 ꢅ 10 mol L
ꢁ4
ꢁ1
EB, 1.0 ꢅ 10
mol L
CT-DNA) in the absence (a) and presence (b ! j) of 5.0 ꢅ 10 mol L
[
Ni(QTS)
2
] ꢀ Cl ꢀ CH
3
OH (20 mL per scan).
Form I Form II
100
9
9
1
1
9
9
3
3
99 22
8 89 9
88 00
8
6
4
2
0
0
0
0
0
77 33
77 00
55 33
44 77
3 03 0
2 27 7
22 00
11 11
99
88
77
Line
1
2
3
4
5
6
7
8
Form II
Form I
Figure 5. Cleavage of supercoiled pBR322 DNA by [Ni(QTS)
2
] ꢀ Cl ꢀ CH
3
OH at different concentrations in
ꢁ
1
ꢁ1
the presence of ascorbate (H
DNA control; lane 2, DNA þ H
represent the DNA-cleavage status at 2, 6, 10, 16, and 24 mmol L of [Ni(QTS)
of 50-fold excess of H A, respectively.
2
A) in 5 mmol L Tris-HCl/50 mmol L NaCl buffer (pH 7.4) at 310 K. Lane 1,
ꢁ1
2
A; lane 3, DNA þ [Ni(QTS)
2
] ꢀ Cl ꢀ CH
3
OH (10 mmol L ); lanes 4–8
] ꢀ Cl ꢀ CH OH in the presence
ꢁ
1
2
3
2
ꢁ1
At 24 mmol L , the nickel(II) complex was able to convert 92% of Form I to Form II,
which indicated that the nickel(II) complex was efficient in promoting cleavage of
plasmid DNA via oxidative pathway under the present experimental conditions.
The preliminary mechanism of pBR322 DNA cleavage by [Ni(QTS) ] ꢀ Cl ꢀ CH OH
2
3
was also studied using inhibiting reagents, such as DMSO, tert-butyl alcohol, and

Nickel(II) thiosemicarbazone
3107
histidine (figure 6). It was obviously that the hydroxyl radical scavenger DMSO and
tert-butyl alcohol diminished the DNA-cleavage activity of the nickel(II) complex, and
the singlet oxygen scavenger histidine failed to inhibit cleavage, which indicated that the
hydroxyl radical was involved in this cleavage process.
3
.4. Cytotoxicity assay
The in vitro cytotoxicity of [Ni(QTS) ] ꢀ Cl ꢀ CH OH against the human breast cancer
2
3
cell line MCF-7 and the human non-small-cell lung cancer cell line A-549 were tested
using cisplatin as a positive control. The results shown in figure 7 indicate that the
nickel(II) complex was more potent against MCF-7 cell line than cisplatin at all
Form I
89
Form II
100
90
8
5
8
6
4
2
0
0
0
0
0
5
7
49 51
4
3
1
5
10
1
1
Line
1
2
3
4
5
Form II
Form I
ꢁ
1
Figure 6. Cleavage of supercoiled pBR322 DNA by [Ni(QTS)
of inhibiting reagents in 5 mmol L Tris-HCl/50 mmol L NaCl buffer (pH 7.4) at 310 K. Lane 1, DNA
2
] ꢀ Cl ꢀ CH
3
OH (24 mmol L ) in the presence
ꢁ1
ꢁ1
ꢁ
1
control; lane 2, DNA þ H
2
A þ [Ni(QTS)
OH (24 mmol L ) þ DMSO; lane 4, DNA þ H
alcohol; lane 5, DNA þ H A þ [Ni(QTS) ] ꢀ Cl ꢀ CH OH (24 mmol L ) þ histidine.
2
] ꢀ Cl ꢀ CH
3
OH (24 mmol L ); lane 3, DNA þ H
A þ [Ni(QTS) ] ꢀ Cl ꢀ CH
3
2
A þ [Ni(QTS)
2
] ꢀ
ꢁ1
ꢁ1
Cl ꢀ CH
3
2
2
3
OH (24 mmol L ) þ t-butyl
ꢁ1
2
2
Ni complex cisplatin
100
8
6
4
2
0
0
0
0
0
2
5
10
20
30
50
2
5
10
20
30
50
Concentration (µM)
MCF-7
A-549
Figure 7. Cytotoxic activity of [Ni(QTS)
and the human non-small-cell lung cancer cell line A-549 with cisplatin as a positive control.
2
] ꢀ Cl ꢀ CH
3
OH against the human breast cancer cell line MCF-7

3
108
S. Zhang et al.
concentrations tested (2 ꢃ 50 mmol Lꢁ1). For example, the inhibition rates of the nickel
complex were 47.4% and 55.7% against MCF-7 cell line at the concentration of 2 and
5
ꢁ
1
mmol L , and were about three to four times higher than that of cisplatin at the same
1
ꢁ
concentration. At a compound concentration of 10 mmol L , the inhibition rate of the
nickel complex reached 67.8% against MCF-7 cell line, while the inhibition rate of
cisplatin was still only 29.8%. The maximum cell growth inhibition of the nickel
complex was similar to that of cisplatin (85.9% and 81.7%, respectively) at
ꢁ
1
5
compared with cisplatin and is only active with the inhibition rate of 74.1% at
0 mmol L . The nickel complex showed lower cytotoxicity against A-549 cell line
ꢁ
1
.
concentration of 50 mmol L
4
. Conclusion
We have synthesized and structurally characterized a new nickel(II) complex with
thiosemicarbazone of quinoline-2-carboxaldehyde. The complex crystallized in a
monoclinic system and nickel(II) has a distorted octahedral geometry. The nickel(II)
complex binds with DNA through groove mode and exhibits efficient oxidative DNA
cleavage in the presence of ascorbate as reductant. The involvement of hydroxyl radical
in the cleavage process was evidenced from inhibition in the presence of DMSO and
tert-butyl alcohol. In vitro cytotoxicity assays demonstrated that the nickel complex was
much more active than cisplatin against MCF-7 cell line, which implied that this new
nickel(II) complex may have potential as a new antitumor agent.
Supplementary material
Crystallographic data of [Ni(QTS) ] ꢀ Cl ꢀ CH OH have been deposited at the
2
3
Cambridge Crystallographic Data Centre with CCDC No. 854 025. Copies of this
information may be obtained free of charge from The Director, CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK (Fax: þ44-1223-336033; E-mail: deposit@ccdc.
cam.ac.uk or http://www.ccdc.cam.ac.uk).
Acknowledgments
Prof. Guo Zijian (Nanjing University) and Prof. Tan Lifeng (Xiangtan University) are
gratefully acknowledged for providing the laboratory facilities. Dr Wang Xiaoyong is
gratefully acknowledged for the technical assistance and helpful suggestions.
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