Bimetallic complexes constructed from asymmetrical N,N'-bis(substituted)- oxamide: Cytotoxicities, and reactivities towards DNA and protein

Article abstract of DOI:10.1016/j.ejmech.2012.06.022

A new asymmetrical N,N'-bis(substituted)oxamide ligand, N-(5-chloro-2-hydroxyphenyl)-N'-[3-(dimethylamino)propyl]oxamide (C ₁₃H₁₈N₃O₃Cl·H₂O, H₃L) and its two binuclear complexes [Cu₂

Full text of DOI:10.1016/j.ejmech.2012.06.022

European Journal of Medicinal Chemistry 54 (2012) 697e708
Contents lists available at SciVerse ScienceDirect
European Journal of Medicinal Chemistry
journal homepage: http://www.elsevier.com/locate/ejmech
Original article
Bimetallic complexes constructed from asymmetrical N,N0-bis
(substituted)-oxamide: Cytotoxicities, and reactivities towards
DNA and protein
,
b
c,
Xiao-Wen Li a, Lin Tao b, Yan-Tuan Li a , Zhi-Yong Wu , Cui-Wei Yan
*
*
a Marine Drug & Food Institute, Ocean University of China, 5 Yu shan Road, Qingdao 266003, PR China
b Key Lab. Marine Drug, Chinese Minist. Educ., Ocean University of China, Qingdao, PR China
c College of Marine Life Science, Ocean University of China, Qingdao 266003, PR China
a r t i c l e i n f o
a b s t r a c t
A new asymmetrical N,N0-bis(substituted)oxamide ligand, N-(5-chloro-2-hydroxyphenyl)-N0-[3-(dime-
thylamino)propyl]oxamide (C13H18N3O3Cl$H2O, H3L) and its two binuclear complexes [Cu2L
(H2O)(bpy)](ClO4)$CH3OH (1) and [Ni2L(bpy)2](ClO4) (2) [bpy ¼ 2,20-bipyridine] have been synthesized
and characterized by X-ray single-crystal diffraction. In the crystal structure, H3L adopting a transoid
conformation occurs as a neutral molecule linked with a water molecule by an intermolecular hydrogen
bond. In the two complexes, the cis-L3ꢀ ligand bridges two metal ions with the corresponding separa-
tions of 5.2032(15) Å (1) and 5.2466(7) Å (2), respectively. In vitro cytotoxic activities, and the reactivities
of the three compounds towards DNA and protein are investigated.
Article history:
Received 18 April 2012
Received in revised form
10 June 2012
Accepted 12 June 2012
Available online 19 June 2012
Keywords:
Crystal structure
Binuclear complex
Cytotoxicity
Ó 2012 Elsevier Masson SAS. All rights reserved.
DNA interaction
Protein binding
1. Introduction
the structures of BSA are similar to human serum albumin (HSA) in
76% [14]. Compared to the number of organic molecules [15e18],
Investigations on transition-metal complexes towards DNA and
protein are of current interest in connection with information
about drug design and tools of molecular biology [1e9]. Modes of
DNA non-covalent interaction with metal complexes include elec-
trostatic effect, groove binding, and intercalation. Many important
applications of these metal complexes require that they can bind to
DNA in an intercalative mode [10]. Both the planarity of ligand and
the coordination geometry of the metal ion play important roles in
deciding the intercalating ability of complexes to DNA [11,12]. By
varying the metal center, it is possible to modify the mode as well
as the extent of interaction of the complex with nucleic acids and
facilitate individual applications [13]. On the other hand, serum
albumins play an important and efficient role in drug delivery due
to their remarkable binding properties. With the advantages of low
cost and ready availability, bovine serum albumin (BSA) is a widely
used serum albumin. Many studies dealing with the interactions
between organic molecules and BSA have been carried out because
relatively few metal complexes [19,20] especially binuclear
complexes [21] have been investigated the reactivities towards BSA.
With the facts that copper and nickel elements are essential
elements to the life process, and binuclear copper and nickel
complexes have been found to inhibit DNA replication and tumor
cell proliferation [22e26], it is necessary to design and synthesize
new bicopper and binickel complexes to evaluate their cytotoxic-
ities, and reactivities towards DNA and protein.
A
promising strategy to design and synthesize binuclear
complexes is the use of bridging ligand to react with metal salts and
terminal ligands. N,N0-bis(substituted)oxamides are well-known to
be good candidates as the binucleating bridging ligand in forming
binuclear complexes. Comparatively, due to the synthesis difficulty,
the studies on the binuclear complexes involving the bridging
asymmetrical N,N0-bis(substituted)oxamides are very limited
[27,28]. However, the fact that those complexes bridged by asym-
metrical N,N0-bis(substituted)oxamides have shown predominant
properties [29e33] stimulate us to design and synthesize new
binuclear complexes with asymmetrical N,N0-bis(substituted)oxa-
mides to gain some insight into the influence of metal ions in this
kind of complexes on structure, cytotoxic activities and reactivities
towards DNA and protein.
* Corresponding authors. Fax: þ86 532 82033054.
E-mail addresses: yantuanli@ouc.edu.cn (Y.-T. Li), cuiweiyan@ouc.edu.cn
(C.-W. Yan).
0223-5234/$ e see front matter Ó 2012 Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.ejmech.2012.06.022

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X.-W. Li et al. / European Journal of Medicinal Chemistry 54 (2012) 697e708
Recently, we reported several transition-metal complexes bridged
DMF), lmax(nm) [
300.2 ([M þ H]þ).
3
(L molꢀ1 cmꢀ1)]: 301(22 550). ES-MS, m/z:
max
by asymmetrical N,N0-bis(substituted)oxamides and found they have
significant DNA-binding and cytotoxic activities [34,35]. Enlightened
by the above-mentioned facts, and as a continuation of our on going
program, in this paper, a new asymmetrical N,N0-bis(substituted)
oxamide ligand, N-(5-chloro-2-hydroxyphenyl)-N0-[3-(dimethyla-
mino)propyl]oxalamide (H3L), and its two binuclear transition metal
complexes, [Cu2L(H2O)(bpy)](ClO4)$CH3OH (1) and [Ni2L(bpy)2]
(ClO4) (2), were synthesized and structurally characterized by X-ray
single-crystal diffraction. In vitro cytotoxic activities of the three
compounds were tested by Sulforhodamine B (SRB) assays against
human hepatocellular carcinoma cell SMMC-7721 and human lung
adenocarcinoma cell A549. The interactions of the three compounds
with herring sperm DNA (HS-DNA) are investigated by using UV
absorption and fluorescence spectra and viscometry. Furthermore,
the protein binding ability has been monitored by using UV absorp-
tion and tryptophan fluorescence quenching experiment in the
presence of the compounds using bovine serum albumin (BSA) as
model protein. To the best of our knowledge, the binding of the
asymmetrical N,N0-bis(substituted)oxamide bridged binuclear
complexes to proteins has seldom been investigated.
2.2.2. Synthesis of [Cu2L(H2O)(bpy)](ClO4)$CH3OH (1)
To a stirred methanol solution (5 mL) containing Cu(ClO4)2$6H2O
(0.0742 g, 0.2 mmol) was added dropwise a methanol solution
(10 mL) of H3L (0.0299 g, 0.1 mmol) and piperidine (0.0256 g,
0.3 mmol) at roomtemperature. After stirring for 30 min, a methanol
solution (5 mL) of bpy (0.0156 g, 0.1 mmol) was added dropwise. The
mixture was stirred quickly at 333 K for 6 h, the resulting brown
solution was filtered and brown cube crystals of the complex suit-
able for X-ray analysis were obtained by slow evaporation at room
temperature. Yield: 82%. Analysis; Calc for Cu2C24H29N5O9Cl2: C,
39.51; H, 4.01; N, 9.60. Found: C, 39.53; H, 3.98; N, 9.67. IR (KBr,
cmꢀ1): 1632 [
conductance, LM (DMF solution): 79
n
(C]O)]; 1442 [
n
(C]N)]; 1104, 622 [
n(ClO4)]. Molar
U
ꢀ1 cm2 molꢀ1. UVevisible (in
DMF), lmax(nm) [
max(L molꢀ1 cmꢀ1)]: 612(800), 311(40 400),
3
299(44 300), 243(75 700). ES-MS, m/z: 598.0 ([MeClO4eCH3OH]þ).
2.2.3. Synthesis of [Ni2L(bpy)2](ClO4) (2)
Deep reddish purple crystals suitable for X-ray single-crystal
analysis were obtained by the same method of preparing complex 1
except using Ni(ClO4)2$6H2O instead of Cu(ClO4)2$6H2O. Yield: 58%.
Analysis; Calc for Ni2C33H31N7O7Cl2: C, 47.99; H, 3.78; N,11.87. Found:
2. Experimental
2.1. Materials and physical measurements
C, 47.92; H, 3.75; N, 11.81. IR (KBr, cmꢀ1): 1633 [
(C]N)]; 1094, 623 [
n(C]O)]; 1472, 1443
[n
n(ClO4)]. Molar conductance, LM (DMF solu-
DNA and BSA were purchased from Sigma and were used as
supplied. Other chemicals were of reagent grade. Carbon, hydrogen
and nitrogen elemental analyses were performed with a Per-
kineElmer elemental analyzer Model 240. Molar conductance
was measured with a Shanghai DDS-11A conductometer. The
infrared spectra of the samples were recorded as KBr pellets on
a Nicolet model Impact 470 FTIR spectrophotometer in the spectral
range 4000e400 cmꢀ1. The mass spectra (ES-MS) were measured
with a Waters Q-TOF GLOBLE mass spectrometer. The UVevisible
spectrum was recorded in a 1-cm-path length quartz cell on
a Cary 300 spectrophotometer. Fluorescence was tested on an Fp-
750w fluorometer. Viscosity measurement was carried out using
an Ubbelohde viscometer immersed in a thermostatic water bath
maintained at 289(ꢁ0.1) K.
tion): 87 Uꢀ1 cm2 molꢀ1. UVevisible (in DMF), lmax(nm) [
3
max
(L molꢀ1 cmꢀ1)]: 605(912), 420(623), 306(30 300), 295(29 500),
241(30 500). ES-MS, m/z: 724.1 ([M ꢀ ClO4]þ).
2.3. X-ray crystallography
The X-ray diffraction experiments for three compounds were
made on a Bruker APEX area-detector diffractometer with graphite
monochromatic Mo K
structures were solved by the directed method followed by Fourier
syntheses. Structure refinements were performed by full matrix
least-squares procedures using SHELXL-97 on F2 [36].
a
radiation (
l
¼ 0.71073 Å). The crystal
2.4. In vitro cytotoxic activity evaluation by SRB assays
2.2. Synthesis
In vitro cytotoxic activities of the three compounds and cis-
platin were evaluated against two cancer cell lines including
SMMC-7721 and A549 by using the Sulforhodamine B (SRB) assay.
All cells were cultured in RPMI 1640 supplemented with 10% (v/v)
fetal bovine serum, 1% (w/v) penicillin (104 U/mL) and 10 mg/mL
streptomycin. Cell lines are maintained at 310 K in a 5% (v/v) CO2
atmosphere with 95% (v/v) humidity. Cultures were passaged
weekly using trypsineEDTA to detach the cells from their culture
flasks. The three compounds and cis-platin were dissolved in DMSO
and diluted to the required concentration with culture medium
when used. The content of DMSO in the final concentrations did not
exceed 0.1%. At this concentration, DMSO was found to be non-toxic
to the cells tested. Rapidly growing cells were harvested, counted,
and incubated at the appropriate concentration in 96-well micro
plates for 24 h. The three compounds and cis-platin dissolved in
culture medium were then applied to the culture wells to achieve
2.2.1. Synthesis of the ligand (C13H18N3O3Cl$H2O, H3L)
The synthesis of N-(5-chloro-2-hydroxyphenyl)-N0-[3-(dime-
thylamino)propyl]oxalamide (H3L), consists of two steps. The first
step was the preparation of oxamido-2-amino-4-chlorophenol-
ethyloxamate (H3oxch) according to the reported method [27]. For
that, a 40 mmol (5.49 g) portion of ethyl oxalyl chloride in 10 mL of
tetrahydrofuran (THF) was added dropwise into a 40 mL THF solu-
tion of 40 mmol (5.92 g) of 2-amino-4-chlorophenol (97%). The clear
solution was concentrated under vacuum and the H3oxch was
precipitated as a white powder then washed with ethanol and dried
under vacuum. The second step was the synthesis of N-(5-chloro-2-
hydroxyphenyl)-N0-[3-(dimethylamino)propyl]oxalamide (H3L). A
5 mmol (1.22 g) amount of H3oxch in 20 mL absolute ethanol was
added dropwise into 20 mL absolute ethanol solution containing
6 mmol (0.76 mL) 3-(dimethylamino)propyl at 273 K. The resulting
solution was stirred for 2 h, and H3L was precipitated as a white
powder and then recrystallized in ethanol and dried under vacuum.
Well-shaped colorless single crystals of the ligand were obtained by
slow evaporation of the ethanol solution of the recrystallized
product. Yield: 72%. Analysis; Calc for C13H18N3O3Cl$H2O: C, 49.14;
H, 6.34; N, 13.22. Found: C, 49.10; H, 6.32; N, 13.19. IR (KBr, cmꢀ1):
final concentrations ranging from 10ꢀ4 to 102
mg/mL. Control wells
were prepared by addition of culture medium without cells. The
plates were incubated at 310 K in a 5% CO2 atmosphere for 48 h.
Upon completion of the incubation, the cells were fixed with ice-
cold 10% trichloroacetic acid (100 mL) for 1 h at 277 K, washed
five times in distilled water and allowed to dry in the air and
stained with 0.4% SRB in 1% acetic acid (100 mL) for 15 min.
The cells were washed four times in 1% acetic acid and air-dried.
3356 [n(NH)]; 1686 [n(C]O)] (oxamidate group). UVevisible (in

X.-W. Li et al. / European Journal of Medicinal Chemistry 54 (2012) 697e708
699
The stain was solubilized in 10 mM unbuffered Tris base (100 mL)
and the OD of each well was measured at 540 nm on a microplate
spectrophotometer. The IC50 values were calculated from the
curves constructed by plotting cell survival (%) versus the
as (s/s )1/3 versus binding ratio [41], where s is the viscosity of DNA
0
in the presence of complex and s is the viscosity of DNA alone.
0
2.6. BSA-binding studies
compounds concentration (mg/mL).
All experiments involving bovine serum albumin (BSA) were
performed in 50 mM NaCl/TriseHCl buffer solution (pH ¼ 7.12).
And solutions of BSA and the three compounds were prepared by
dissolving them in the NaCl/TriseHCl buffer solution to required
concentrations, respectively. For UV absorption experiment, a 5 mL
2.5. DNA-binding studies
All experiments involving herring sperm DNA (HS-DNA) were
performed in TriseHCl buffer solution (pH ¼ 7.12). Solutions of HS-
DNA in TriseHCl buffer gave a ratio of UV absorbance at 260 and
280 nm, A260/A280, of ca.1.9, indicating that the DNA was sufficiently
free of protein [37]. The concentration of DNA was determined by
solution of BSA (10 mM) was titrated with various concentrations of
the compounds. Equal solutions of compounds were added to the
reference solutions to eliminate the absorbance of the compounds
themselves. In the tryptophan fluorescence quenching experiment,
quenching of the tryptophan residues of BSA [42] was done by
keeping the concentration of the BSA constant while varying the
compounds (quenchers) concentration, producing the solutions
with the varied mole ratio of the quenchers to BSA. The fluores-
cence spectra were recorded at an excitation wavelength of 295 nm
and an emission wavelength of 347 nm in the Fluorometer after
each addition of the quencher.
UV absorbance at 260 nm. The molar absorption coefficient,
3
260
,
was taken as 6600 L molꢀ1 cmꢀ1 [38]. Stock solution of DNA was
stored at 277 K and used after no more than 4 days. Concentrated
stock solution of the two complexes and the free ligand were
prepared by dissolving the compounds in the TriseHCl buffer
solution to required concentrations for all the experiments,
respectively. Absorption spectral titration experiment was per-
formed by keeping the concentration of the compounds constant
while varying the HS-DNA concentration. Equal solution of HS-DNA
was added to the compounds solution and reference solution to
eliminate the absorbance of DNA itself. In the ethidium bromide
3. Results and discussion
(EB) fluorescence displacement experiment, 5
mL of the EB TriseHCl
3.1. Synthesis and general property of the binuclear complexes
solution (1 mmol Lꢀ1) was added to 1 mL of DNA solution (at
saturated binding levels) [39], stored in the dark for 2 h. Then the
solution of the compounds was titrated into the DNA/EB mixture
and then diluted in TriseHCl buffer to 5 mL, producing the solutions
with the varied mole ratio of the compound to HS-DNA. Before
measurements, the mixture was shaken up and incubated at room
temperature for 30 min. The fluorescence spectra were obtained at
an excitation wavelength of 522 nm and an emission wavelength of
584 nm in the Fluorometer. In viscosity measurement, DNA
samples approximately 200 base pairs in length were prepared by
sonication in order to minimize complexities arising from DNA
flexibility [40]. Flow time was measured with a digital stopwatch,
and each sample was measured three times, and an average flow
time was calculated. Relative viscosities for DNA in the presence
and absence of the compound were calculated from the relation
s ¼ (t ꢀ t0)/t0, where t is the observed flow time of DNA-containing
solution and t0 is that of TriseHCl buffer alone. Data were presented
The aim of this study was to obtain binuclear complexes with
different metal ions. Therefore, heterobinucleating ligand, N-(5-
chloro-2-hydroxyphenyl)-N0-[3-(dimethylamino)propyl]oxalamide
(H3L), was chosen as bridging ligand, which can coordinate to metal
ions through not only the oxamido oxygens and nitrogens but also
phenolate oxygens [43]. Simultaneously, 2,20-bipyridine (bpy) was
used as the terminal ligands. In the course of preparing the
complexes the use of piperidine as base makes the bridging ligand
(H3L) coordinate to copper(II)/nickel(II) center through the depro-
tonated oxamido nitrogen and phenolate oxygen atoms. Indeed,
elemental analyses indicate that the reaction of H3L with
Cu(ClO4)2$6H2O/Ni(ClO4)2$6H2O and bpy in 1:2:1 mol ratio yielded
the binuclear complexes, [Cu2L(H2O)(bpy)](ClO4)$CH3OH (1) and
[Ni2L(bpy)2](ClO4) (2). The synthetic pathway for the bridging
ligand (H3L) and the two complexes 1 and 2 may be represented by
Scheme 1. For the two binuclear complexes, the molar conductance
Scheme 1.

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X.-W. Li et al. / European Journal of Medicinal Chemistry 54 (2012) 697e708
values (79 and 87 Uꢀ1 cm2 molꢀ1) in DMF solution fall in the
expected range for 1:1 electrolytes [44]. The results suggest that the
two complexes both consist of a binuclear complex cation and an
uncoordinated perchlorate anion in solutions. The stability of
complexes 1 and 2 existed as a whole binuclear entity in solution
was supported by the ES-MS results which can be found in
synthesis part.
3.2. IR and electronic spectra
The IR spectra of the free ligand and the two binuclear
complexes taken in the region of 4000e400 cmꢀ1 provide some
information regarding the mode of coordination in the binuclear
complexes and are analyzed in a careful comparison with that of
the free ligand (H3L). In the IR spectrum of the free ligand, a sharp
strong band observed at 3286 cmꢀ1 is the result of overlapping
between n(OeH) and n(NeH), which is absent in the spectra of the
two complexes, indicating that the phenolic oxygen atom and
amido nitrogen atom take part in the coordination. In addition, the
carbonyl (C]O) stretching vibration of H3L (1686 cmꢀ1) is found to
be shifted toward lower wave-numbers of 1632 and 1633 cmꢀ1
,
respectively. This shift has often been used as a diagnostic indicator
for oxamido-bridged structures [45]. Besides, the eC]Ne
stretching vibration for the terminal ligands (bpy) is observed at
about 1450 cmꢀ1, indicating that these terminal ligands are
involved in the coordination in the two binuclear complexes [46].
Fig. 1. An ORTEP view of H3L with the thermal ellipsoids at 30% probability level. H
atoms are shown as small spheres of arbitrary radii. Hydrogen bonds are shown as
dotted lines.
Furthermore, a broad and intense band centered at ca. 1100 cmꢀ1
,
and a strong sharp band at about 620 cmꢀ1, typical for a non-
coordinated perchlorate group [46,47], are observed for the two
binuclear complexes. This is consistent with the conductance data
of the binuclear complexes.
b axis (Fig. S1). These chains are linked by water molecules (O4) and
oxamido oxygen atoms (O31/2þx, ꢀ1/2þy, z) into a two-dimensional
hydrogen bonding network extending along (0 0 1) plane.
In order to obtain further structural information, the electronic
spectra of the two complexes were recorded in the UVevisible
region (200e800 nm) using DMF as solvent. For both complexes
1 and 2, four main absorption bands with varied intensities can be
observed. The intense bands about 240 nm are assignable to inter-
3.3.2. Crystal structure of [Cu2L(H2O)(bpy)](ClO4)$CH3OH (1)
As illustrated in Fig. 2a, the structure of 1 consists of a binuclear
copper(II) complex cation [Cu2L(H2O)(bpy)]þ, one uncoordinated
perchlorate anion and one solvent methanol molecule. The Cu/Cu
distance through the oxamide-bridge is 5.2032(15) Å. The cis-
oxamide coordinates to Cu1 and Cu2 in the usual chelating mode
with bite angles of 82.27(13) and 86.08(11)ꢂ, respectively.
The environment around Cu1 atom can be best described as
or intra-ligand (pep*) transition. Meanwhile the bands around
300 nm in the complexes can be assigned to the charge transfer
transition between ligand and metal. Besides, broad bands are
observed at the lower frequency in the spectra, corresponding to
the ded transition of copper(II) or nickel(II) [48]. The structures of
the two binuclear complexes are further supported by the
following determination of the crystal structures.
a distorted square-pyramidal geometry with the
s value of 0.05
[50]. Atoms N1, N2, N3 and O1 from the bridging ligand (L) form the
basal coordination plane, while an oxygen atom (O4) from the
coordinate water molecular occupied the apical position with
3.3. Crystal structures
The ORTEP views of the free ligand (H3L) and complexes 1 and 2
were displayed in Figs. 1e3, respectively. Table 1 summarizes the
crystal data and structural refinement parameters for the three
compounds and their selected bond lengths and angles are listed in
Table 2. Non-covalent interactions found in the crystal structures
are described in supporting information (Table S1, Figs. S1-S3).
3.3.1. Crystal structure of the ligand (C13H18N3O3Cl$H2O, H3L)
As shown in Fig. 1a, the crystal structure of the ligand occurs as
a neutral molecule linked with a solvent water molecule by an
intermolecular hydrogen bond between the phenolic hydroxyl
oxygen atom (O1) and the water molecule (O4). The ligand has
a transoid conformation, and the six non-H atoms of oxamido group
are almost coplanar, which is similar to other oxamido compounds
[49]. The oxamido plane (r.m.s. deviation 0.0688 Å) is nearly
parallel to the benzene ring, with a dihedral angle of 7.18(10)ꢂ.
Hydrogen bonds dominate the crystal structure. The molecule of
the ligand and water molecule are connected by classical hydrogen
bonds listed in Table S1 into a one-dimensional chain parallel to
Fig. 2. An ORTEP view of complex 1 with the thermal ellipsoids at 30% probability
level. H atoms are shown as small spheres of arbitrary radii. Hydrogen bonds are
shown as dotted lines.

X.-W. Li et al. / European Journal of Medicinal Chemistry 54 (2012) 697e708
701
Fig. 3. An ORTEP view of complex 2 with the thermal ellipsoids at 30% probability level. H atoms are shown as small spheres of arbitrary radii. Hydrogen bonds are shown as dotted
lines.
Cu1eO4 bond length of 2.423(3) Å. The maximum displacement of
them from the coordination plane is 0.1138(17) Å (N1). Cu1 atom
displaces 0.2303(15) Å out of the plane. The behavior of L3ꢀ results
in the formation of two five- and one six-membered rings around
Cu1 atom. The two five-membered rings Cu1eO1eC1eC2eN1 and
Cu1eN1eC7eC8eN2 are almost planar. The puckering parameters
[51] of the six-membered ring Cu1eN2eC9eC10eC11eN3 are
planar geometry, which coordinates to the exo-O atoms of the
cis-L3ꢀ ligand (O2 and O3). The other two atoms of the plane (N4,
N5) are both from the bpy molecule. The deviations of four defined
atoms (N4, N5, O2, O3) from the coordinated plane are in the range
of 0.0589 (14)e0.0630(16) Å, and Cu2 atom displaces 0.0556(16) Å
out of the plane.
The complex cations and solvent methanol molecules are con-
nected by hydrogen bonds listed in Table S1 into a one-dimensional
chain parallel to the a axis. These chains are linked by perchlorate
Q ¼ 0.508(10) Å,
q
¼ 33.4(9)ꢂ and 4 ¼ 210.4(14)ꢂ. The Cu1eN3 bond
[2.042(3) Å] is longer than the Cu1eN1 bond [1.939(3) Å] and
Cu1eN2 bond [1.991(4) Å], which is consistent with the stronger
donor ability of the deprotonated amido nitrogen compared with
the amino nitrogen atoms [52]. Cu2 atom has a distorted square-
anions into
a two-dimensional hydrogen bonding network
extending along (0 0 1) plane (Fig. S2). Furthermore, as shown in
Table 2
Selected bond distances (Å) and bond angles (ꢂ) for the ligand H3L and complexes 1
and 2.
Table 1
Crystal data and details of the structure determination for the ligand H3L and
complexes 1 and 2.
Bond lengths
Bond angles
H3L
Cl1eC4
O1eC1
N1eC2
N1eC7
O2eC7
C7eC8
O3eC8
N2eC8
Complex 1
Cu1eN1
Cu1eN2
Cu1eN3
Cu2eN4
Cu2eN5
Cu1eO1
Cu2eO2
Cu2eO3
Complex 2
Ni1eN1
Ni1eN2
Ni1eN3
Ni1eO1
Ni2eN4
Ni2eN5
Ni2eN6
Ni2eN7
Ni2eO2
Ni2eO3
Compounds
Formula
H3L
1
2
1.743(2)
1.354(2)
1.408(3)
1.339(3)
1.218(2)
1.531(3)
1.231(2)
1.315(3)
O1eC1eC2
O1eC1eC6
Cl1eC4eC3
Cl1eC4eC5
N1eC2eC1
N1eC2eC3
C2eN1eC7
O3eC8eN2
116.59(17)
124.44(18)
118.37(17)
119.56(17)
116.26(17)
123.67(18)
128.05(17)
126.10(18)
C13H20ClN3O4
C24H29Cl2
Cu2N5O9
729.52
Triclinic
P-1
8.369(4)
12.302(5)
14.606(6)
93.333(5)
102.715(6)
100.987(3)
1432.1(11)
2
C33H31Cl2
Ni2N7O7
825.93
Monoclinic
C2
21.6780(14)
12.1165(8)
15.1668(10)
90
114.7520(10)
90
3617.7(4)
4
M
317.77
Monoclinic
C2/c
12.2386(18)
11.1168(18)
23.919(4)
90
98.898(3)
90
3215.1(9)
8
1.313
0.256
1344
296
Mo K
0.71073
9338, 3703,
0.024
1.8 to 27.7
Crystal System
Space group
a (Å)
b (Å)
c (Å)
a
b
g
(ꢂ)
(ꢂ)
(ꢂ)
1.939(3)
1.991(4)
2.042(3)
1.972(3)
1.960(4)
1.974(3)
1.940(3)
1.944(3)
N1eCu1eN2
N1eCu1eO1
N2eCu1eN3
N3eCu1eO1
N4eCu2eO3
N4eCu2eN5
N5eCu2eO2
O2eCu2eO3
82.27(13)
82.41(13)
97.11(14)
95.72(12)
179.29(13)
82.01(13)
171.69(14)
86.08(11)
V (Å3)
Z
D(calc)(g cm3)
1.692
1.732
744
296
1.516
1.245
1696
296
m
(Mo K
a
)(mmꢀ1
)
F(000)
Temperature (K)
Radiation (Å)
a
Mo Ka 0.71073 Mo Ka 0.71073
1.846(4)
1.902(4)
1.939(4)
1.865(3)
2.090(4)
2.052(4)
2.069(4)
2.084(3)
2.061(3)
2.087(3)
N1eNi1eN2
N1eNi1eO1
N2eNi1eN3
N3eNi1eO1
N4eNi2eN5
N4eNi2eN6
N4eNi2eO2
N4eNi2eN7
N5eNi2eO3
N5eNi2eO2
84.49(14)
85.33(13)
98.87(16)
91.41(15)
78.73(15)
98.47(16)
90.20(13)
177.12(16)
90.74(15)
166.91(15)
Tot., uniq. data, R(int)
10 966, 6356,
0.036
2.1 to 27.5
4702
10 851, 7199,
0.028
2.0 to 27.6
5209
q
range
Observed data [I > 2
R, wR2, S
s(I)] 2651
0.0470, 0.1425, 0.0445, 0.1693, 0.0422, 0.0847,
1.046
1.046
0.00, 0.00
1.088
1.088
0.00, 0.00
0.968
0.968
0.00, 0.00
Goodness-of-fit
Max., av. shift/error

702
X.-W. Li et al. / European Journal of Medicinal Chemistry 54 (2012) 697e708
a
100
80
60
40
20
0
88
SMMC-7721
A549
0.4
0.3
0.2
0.1
0.0
75
4.9
4.5
4.1
3.7
ε
45
ε
38
19
1
2
3
4
5
6
17
[DNA]×105 mol/L
7.6
5.4
H3L
1
2
cis-platin
(ng/mL)
Compounds
Fig. 4. In vitro cytotoxic activities of the three compounds and cis-platin against
human hepato-cellular carcinoma cell line SMMC-7721 and human lung adenocarci-
noma cell A549.
280
320
360
400
440
Wavelength(nm)
Fig. S3, there are two types of offset pep stackings in complex 1. One
b
is observed between the benzene ring and the N5 pyridine ring at
(ꢀ1 þ x, y, z) [symmetry operation (iii)]; the closest distance
between atom C23iii and the benzene ring is 3.307(5) Å. Another one
is between the pyridine ring containing atoms N5 and the benzene
ring at (1 þ x, y, z), with a closest separation of 3.389(4) Å (C2v).
0.7
1.1
0.9
ε
ε
0.6
0.5
0.4
0.3
0.7
3.3.3. Crystal structure of [Ni2L(bpy)2](ClO4) (2)
As illustrated in Fig. 3, complex 2 consists of a [Ni2L(bpy)2]þ
cation and two halves of uncoordinated perchlorate anions of
which the occupancies are constrained to 0.5 because of the
disordered. Some atoms of the two perchlorate anions are disor-
dered of higher inversion symmetry [symmetry code: (i) ꢀx, y, ꢀz,
O4i, O5i, O6i, O7i; (ii) ꢀx þ 1, y, ꢀz þ 1, Cl3ii] than the cations. The
deprotonated L3ꢀ ligand exhibits a cisoid conformation and bridges
two Ni atoms, with a Ni/Ni distance of 5.2466 (7) Å.
0.5
1
2
3
4
5
6
7
[DNA]×105 mol/L
Ni1 atom has a distorted square-planar geometry, formed by N1,
N2, N3 and O1 from the L3ꢀ. The maximum displacement from the
coordination plane is 0.040(2) Å (N1). Ni1 lies exactly in the plane
[0.0059(18) Å]. The behavior of L3ꢀ results in the formation of two
five- and one six-membered rings around Ni1 atom. The five-
membered ring Ni1eN1eC7eC8eN2 has a twist conformation
with puckering parameters of Q ¼ 0.083(4) Å and 4 ¼ 230(2)ꢂ,
while the other five-membered rings Ni1eO1eC1eC2eN1 is
almost planar. The puckering parameters [51] of the six-membered
220
240
260
280
300
320
Wavelength(nm)
c
0.35
0.30
0.25
0.20
0.15
0.10
1.1
1
ring Ni1eN2eC9eC10AeC11AeN3 are
q
Q
¼
0.618(12) Å,
¼ 121.4(8)ꢂ and 4 ¼ 27.8(9)ꢂ. Similar to complex 1, complex 2 has
0.9
a longer Ni1eN3 bond [1.939(4) Å] than Ni1eN1 [1.846(4) Å] and
Ni1eN2 [1.902(4) Å] [52]. Ni2 atom is coordinated by four N-atom
donors of two bpy molecules and two O atoms of the L3ꢀ ligand. The
hexa-coordinated atom Ni2 has a distorted octahedral geometry.
Atoms N4 and N7 are axially coordinated, with an approximately
linear NeNieN angle [177.12(16)ꢂ]. The equatorial plane is defined
by the other four atoms and the mean displacement from this plane
is 0.1815 Å. Atom Ni2 lies exactly in the plane [0.0087(18) Å]. The
bpy terminal ligands are present in the usual chelated ring on Ni2
with bite angles of 78.73(15) and 78.92(16)ꢂ, respectively. The
intermolecular interactions in the structure of complex 2 are not
studied, since there are no classic hydrogen bonds, no significant
ε
0.8
ε
0.7
0.6
0.5
1
2
3
4
5
6
[DNA]×105 mol/L
pep stacking interactions are found in its three-dimensional
supramolecular structure.
Comparing the crystal structures of the two binuclear
complexes, we find that the two complexes have the same bridging
ligand (L3ꢀ) and terminal ligand (bpy). The main difference
between them is the metal ion which affects their coordinated
240
270
300
330
360
390
Wavelength(nm)
Fig. 5. Absorption spectra of compounds (a for H3L, b for 1 and c for 2) upon the titration of
HS-DNA. Arrow indicates the change upon increasing the DNA concentration. Inset: Plot of
[DNA]/( 3 ꢀ 3 ) vs. [DNA] for the absorption titration of HS-DNA with the compound.
a
f

X.-W. Li et al. / European Journal of Medicinal Chemistry 54 (2012) 697e708
703
a
120
Table 3
The Kb and Ksv values for the free ligand H3L and complexes 1 and 2.
1.3
1.2
1.1
1
H3L
0.73
2.6
4.2
4.2
1
2
100
80
60
40
20
0
Ι
HS-DNA
Kb ꢄ 10ꢀ4 (Mꢀ1
)
)
)
3.2
5.1
2.1
2.1
0.72
0.96
2.6
3.8
2.5
2.5
0.94
1.02
Ι
Ksv ꢄ 10ꢀ4 (Mꢀ1
Ksv ꢄ 10ꢀ4 (Mꢀ1
BSA
Kq ꢄ 10ꢀ12 (Mꢀ1 sꢀ1
)
n
1.09
11.0
K ꢄ 10ꢀ4 (Mꢀ1
)
0
2
4
6
8 10 12
[compound]×106mol/L
environment. Furthermore, they affect the cytotoxic activities and
DNA binding properties of the binuclear complexes (vide infra).
3.4. In vitro cytotoxic activities
In vitro cytotoxic activities of the three compounds and cis-
platin against two cancer cell lines human hepatocellular carci-
noma cell line SMMC-7721 and human lung adenocarcinoma cell
line A549 were conducted in our study. The results, shown in Fig. 4,
indicated that all of the three compounds have rather cytotoxicities
against the two cancer cell lines, and complex 1 demonstrated the
best activities among the three compounds. Although the
measured cytotoxic activities are less than those of cis-platin (IC50
values of 5.4 and 7.6 ng/mL, respectively), inhibition of cell prolif-
eration produced by the three compounds on the same batch of cell
lines and under identical experimental conditions is still rather
active. As it has been indicated that the cytotoxicities of the three
compounds would vary with the metal ions and the extent of their
intercalation into DNA base pairs [13], the significant cytotoxicity of
complex 1 may be partially attributed to its strong intercalative-
property (vide infra). These findings of the cytotoxic activities
prompt us to explore the DNA and protein binding properties of the
three compounds.
560
580
600
620
640
Wavelength(nm)
b
1.6
80
60
40
20
0
Ι
1.4
Ι
1.2
1
0
2
4
6
8
10 12
[compound]×106mol/L
3.5. DNA-binding properties
3.5.1. Electronic absorption titration
The application of electronic absorption spectroscopy is one of
the most useful techniques for DNA-binding properties [53]. The
absorption spectra of the free ligand H3L and the two binuclear
complexes in the absence and presence of herring sperm DNA (HS-
DNA) are recorded, and the absorption spectra of the three
compounds are given in Fig. 5. As shown in these figures, when
titrated by HS-DNA, all of them presented significant hypochrom-
ism, accompanied the slightly red-shifts in the absorbance maxima.
In general, the hypochromism and red-shift are associated with the
binding of the metal complexes to the DNA helix, due to the
intercalative mode involving a strong stacking interaction between
the aromatic chromophore of compounds and the base pairs of
DNA [53]. These results suggest that all of the three compounds
interact with HS-DNA through the intercalation mode.
To compare quantitatively the binding affinity of the three
compounds toward HS-DNA, the intrinsic binding constants Kb of
the three compounds with HS-DNA were determined by moni-
toring the changes in absorbance at 301 nm (H3L), 299 nm (1) and
295 nm (2) with increasing concentration of HS-DNA according to
the following equation [53]:
560
580
600
Wavelength(nm)
620
640
120
100
80
60
40
20
0
c
1.5
1.4
1.3
1.2
1.1
1
Ι
Ι
0
2
4
6
8
10 12
[compound]×106mol/L
ꢀ
ꢁ
ꢀ
ꢁ
ꢀ
ꢁ
560
580
600
Wavelength(nm)
620
640
½DNAꢃ=
3
a ꢀ
3
¼ ½DNAꢃ=
3
b ꢀ
3
þ 1=Kb
3
b ꢀ
3
(1)
f
f
f
where
3
,
3
f
and
3
b
correspond to the extinction coefficient,
a
respectively, for each addition of DNA to the tested compounds, for
the free tested compounds and for the tested compounds in the
fully bound form. The binding constants for three compounds can
Fig. 6. Emission spectra of the HS-DNA-EB system upon the titration of compounds (a for H3L,
b for 1 and c for 2). Arrow shows the change upon the increasing compound concentration.
Inset: Plot of I0/I vs. [compound] for the titration of the compound to HS-DNA-EB system.

704
X.-W. Li et al. / European Journal of Medicinal Chemistry 54 (2012) 697e708
be estimated from the plots of [DNA]/(
3
ꢀ
3
f) vs. [DNA] (inset in
increasing steadily, which is proved that the compounds bound to
DNA by intercalation. The increased degree of viscosity, which may
depend on their affinity to DNA follows the order of 1 > 2 > H3L.
a
Fig. 5). As shown in Table 3, the intrinsic binding constants Kb of the
three compounds with HS-DNA follow the order 1 > 2 > H3L. The
higher Kb value observed for the two complexes may be due to the
complexation of the oxamido-bridge and the terminal ligands
providing more planar structures than the free ligand, which could
increase the degree of intercalation. Besides, electrostatic interac-
tion may be another factor for complexes 1 and 2 to enhance the
binding strength to DNA, which is lack in H3L. The lower binding
affinity of complex 2 to HS-DNA than that of complex 1 may be due
to the type of metal ions, which are responsible for the coordinated
geometry of the complexes and have a profound effect on the DNA-
binding properties, as revealed by the different binding affinities.
3.6. BSA-binding properties
3.6.1. UV absorption spectra of BSA in the presence of the
compounds
UV absorption spectrum is a very simple and applicable method
to explore the structural change and to know the complex forma-
tion in solution [59]. Fig. 8 shows the UV absorption spectra of BSA
in the presence of different concentrations of the three compounds.
As can be seen from this figure, free BSA has maximum absorption
at 278 nm and it increases with the addition of the three
compounds. This phenomenon indicates the interaction of BSA
with the compounds [60].
3.5.2. Fluorescence titration
The ethidium bromide (EB) fluorescence displacement experi-
ment has been widely used to investigate the interaction of small
molecule with DNA. In order to further investigate the interaction
modes of the three compounds with HS-DNA, the EB fluorescence
displacement experiments were used, and the principle can be
described as follows [54].
3.6.2. Tryptophan quenching experiment
In order to further investigate the interaction of the three
compounds with protein, the tryptophan emissionequenching
experiments were carried out using bovine serum albumin (BSA)
DNAðlow fluorescence intensityÞ þ EBðlow fluorescence intensityÞ#EB ꢀ DNAðenhanced fluorescence intensityÞ#
Fluorescence intensity of EB can be quenched
It can be seen from Fig. 6, the fluorescence emission intensities
of EB bound to HS-DNA at 584 nm show remarkable decreasing
trend with increasing concentration of the ligand and the two
complexes, indicating some EB molecules were released into
solution after an exchange with the three compounds which result
in the fluorescence quenching of EB. This observation is often
considered as the characteristic of intercalation [55].
In order to understand quantitatively the magnitude of the
binding strength of the three compounds with HS-DNA, the linear
SterneVolmer equation is employed [56]:
in the presence of the three compounds. The emission intensity
depends on the degree of exposure of the two tryptophan side
chains [61], 134 and 212, to polar solvent. It can be seen from Fig. 9,
the fluorescence emission intensities of BSA at 347 nm show
remarkable decreasing trend with increasing concentration of the
three compounds, indicating that the interaction of the compounds
with BSA could cause changes in protein secondary structure
leading to changes in tryptophan environment of BSA [62]. In order
to understand quantitatively the magnitude of the compounds to
quench the emission intensity of BSA, the linear SterneVolmer
equation (3) is also employed:
I0=I ¼ 1 þ Ksv½Qꢃ
(2)
where I0 and I represent the fluorescence intensities in the absence
and presence of quencher, respectively. Q is the concentration of
quencher. Ksv is a linear SterneVolmer quenching constant. In the
quenching plot of I0/I versus [compound] (Fig. 6), Ksv of the three
compounds are given by the slope. The results, shown in Table 3,
support the fact that the Ksv values for the ligand and the two
complexes follow the order 1 > 2 > H3L, which is in agreement with
that derived by UV absorption spectra measurements.
1.7
1.6
1.5
1.4
3.5.3. Viscosity measurements
η
To further clarify the interaction mode of the three compounds
and HS-DNA, the relative specific viscosity measurements were
carried out by varying the concentration of the added compounds.
Viscosity measurement is a classical means to study the binding
mode of small molecule with DNA in solution and provides stronger
arguments for intercalative binding mode [57,58]. In classical
intercalation, the DNA helix lengthens as base pairs are separated to
accommodate the bound compound leading to increased DNA
viscosity whereas a partial, non-classical compound intercalation
causes a bend in DNA helix reducing its effective length and thereby
its viscosity. The effects of the three compounds on the viscosity of
HS-DNA are shown in Fig. 7. As illustrated in this figure, on increasing
the amountof the three compounds, the relative viscosityof HS-DNA
1.3
η
1.2
1.1
1
0
0 .0 2
0 .0 4
0 .0 6
0 .0 8
0 .1
[complex]/[DNA]
Fig. 7. Effect of the increasing amount of the compounds (- for H3L, A for 1 and : for
2) on the relative viscosity of herring sperm DNA at 289(ꢁ0.1) K, [DNA] ¼ 0.1 mM.

X.-W. Li et al. / European Journal of Medicinal Chemistry 54 (2012) 697e708
705
a
0.5
0.4
0.3
0.2
250
260
270
280
290
300
Wavelength(nm)
b
0.40
0.35
0.30
0.25
0.20
250
260
270
280
290
300
Wavelength(nm)
c
0.40
0.35
0.30
0.25
0.20
250
260
270
280
290
300
Wavelength(nm)
Fig. 8. Absorption spectra of BSA upon the titration of H3L (a), 1 (b) and 2 (c). Arrow indicates the change upon increasing the compound concentration.
of the fluorophore in absence of the quencher as 10ꢀ8 S. From the
quenching plot of I0/I versus [compound] (inset in Fig. 9), Ksv and Kq
of the three compounds are obtained from the slope
I0=I ¼ 1 þ Ksv½Qꢃ ¼ 1 þ Kqs0½Qꢃ
I0 and I represent the fluorescence intensities in the absence and
presence of quencher, respectively. Q is the concentration of
quencher. Ksv is a linear SterneVolmer quenching constant. Kq is
(3)
(Kq ¼ w1012 Mꢀ1
s
ꢀ1, Table 3). However, maximum collision
quenching constant of various kinds of quenchers to bio-
macromolecule is 2.0 ꢄ 1010 Mꢀ1 sꢀ1 [63]. Obviously, the rate
the biomolecular quenching rate constant, s0 is the average lifetime

706
X.-W. Li et al. / European Journal of Medicinal Chemistry 54 (2012) 697e708
a
300
250
200
150
100
50
1.4
-0.41
1.3
-0.51
/Ι
Ι
1.2
1.1
1
-0.61
-0.71
-0.81
-0.91
-1.01
-1.11
-1.21
1
3
5
7
9
11
[compound]×106mol/L
0
-5.7
-5.5
-5.3
-5.1
-4.9
320
340
360
380
400
420
Log[Q]
Wavelength(nm)
b
300
250
200
150
100
50
1.2
1.15
-0.7
-0.9
-1.1
-1.3
-1.5
-1.7
/Ι
1.1
1.05
1
Ι
0
2
4
6
8
[compound]×106mol/L
0
-6
-5.8
-5.6
-5.4
-5.2
-5
320
340
360
380
400
420
Log[Q]
Wavelength(nm)
c
300
250
200
150
100
50
-0.5
-0.6
-0.7
-0.8
-0.9
-1
1.3
1.2
Ι
Ι
1.1
1
0
2
4
6
8 10 12
[compound]×106mol/L
-1.1
-5.7
-5.5
-5.3
-5.1
-4.9
0
320
340
360
380
400
420
Log[Q]
Wavelength(nm)
Fig. 9. Emission spectra of BSA upon the titration of H3L (a), 1 (b) and 2 (c). Arrow shows the change upon the increasing compound concentration. Inset: Plot of I0/I vs. [compound]
for the titration of the compound to BSA; Right: Plot of log [(F0 ꢀ F)/F] vs. log [Q] for the titration of the compound to BSA.
constant of protein quenching initiated by the three compounds is
greater than 2.0 ꢄ 1010 Mꢀ1 sꢀ1. This shows that above quenching is
not initiated by dynamic collision but via the combination of the
compound and BSA. Therefore, the quenching of BSA fluorescence
by the three compounds depended on the formation of the
complex between the three compounds and BSA. The binding
constant (K) was calculated by the method given in the following
section.
3.6.3. Binding constant and number of binding sites
When small molecules bind independently to a set of equiv-
alent sites on a macromolecule, then the equilibrium between the

X.-W. Li et al. / European Journal of Medicinal Chemistry 54 (2012) 697e708
707
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M.A. Akbarsha, B.S. Srinag, H. Krishnamurthy, Inorg. Chem. 46 (2007)
8208e8221.
free and bound molecule is given by the following equation
[64,65]:
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(2003) 133e140.
[8] K. Moebus, J. Siepmann, R. Bodmeier, Eur. J. Pharm. Biopharm. 72 (2009)
42e53.
log½ðF0 ꢀ FÞ=Fꢃ ¼ logK þ nlog½Qꢃ
(4)
[9] P. Sevilla, J.M. Rivas, F. García-Blanco, J.V. García-Ramos, S. Sánchez-Cortés,
Biochim. Biophys. Acta 1774 (2007) 1359e1369.
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Dalton Trans. 11 (2003) 2260e2268.
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1255e1263.
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9e14.
F0 and F represent the fluorescence intensities in the absence and
presence of quencher, respectively. K is the binding constant of the
three compounds with BSA, n is the binding sites. From the
quenching plot of log [(F0 ꢀ F)/F] versus log [Q] (right in Fig. 9), K and
n of the three compounds can be obtained. The K and n values are
listed in Table 3. As shown in this table, the binding constant
between the three compounds and BSA follows the order
H3L > 2 z1, and the binding sites are all about 1. The higher binding
constant for the free ligand H3L reveals the stronger protein-binding
ability of the neutral ligand with better hydrophobicity.
[18] M. Obata, S. Hirohara, K. Sharyo, H. Alitomo, K. Kajiwara, S. Ogata, M. Tanihara,
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New York, 1993.
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Chem. 44 (2009) 3552e3559.
4. Conclusion
In order to examine the effect of the metal ions in binuclear
complexes upon the structure, cytotoxic activities and reactivities
towards DNA and protein, in this paper, an asymmetrical N,N0-
bis(substituted)oxamide ligand, N-(5-chloro-2-hydroxyphenyl)-N0-
[3-(dimethylamino)propyl]-oxalamide (H3L) and its two binuclear
complexes with formulas of [Cu2L(H2O)(bpy)](ClO4)$CH3OH (1) and
[Ni2L(bpy)2](ClO4) (2) were synthesized and characterized by X-ray
single-crystal diffraction. In vitro cytotoxic activities of the three
compounds were investigated, and complex 1 displayed the best
cytotoxic activities, which are consistent with DNA-binding abili-
ties. In agreement with the electronic absorption spectroscopy,
fluorescence spectroscopy and viscosity measurement, all of the
three compounds, especially the two binuclear complexes could
interact with HS-DNA through the intercalation mode and both
follow the binding affinity order of 1 > 2 > H3L. As for the protein
binding abilities, the free ligand H3L exhibits a higher ability than
that of complexes 1 and 2. The results implied that the cytotoxic
activities of the compounds may be associated with or originate
from their ability to intercalate the base pairs of DNA and that
through modifying the nature of the metal ions the DNA-binding
and cytotoxic activities could possibly be tuned. This strategy
should be valuable in understanding the cytotoxic activities of
transition metal complexes as well as laying a foundation for the
rational design of new, powerful agents for probing and targeting
nucleic acids.
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Acknowledgments
This project was supposed by the National Natural Science
Foundation of China (No. 21071133), the Program for Science and
Technology of Shandong Province (2011GHY11521), and the
Natural Science Foundation of Qingdao City [No. 11-2-4-1-(9)gch
and 12-1-3-52-(1)-nsh].
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
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