Synthesis, crystal structure, antioxidation and DNA-binding properties of a dinuclear copper(II) complex with bis(N-salicylidene)-3-oxapentane-1,5-diamine

Article abstract of DOI:10.1080/00958972.2014.959507

A Schiff base bis(N-salicylidene)-3-oxapentane-1,5-diamine (H₂L) and its Cu(II) complex, [Cu₂(L)₂]·CHCl₃, have been synthesized and characterized by physicochemical and spectroscopic methods. Single-crystal X-ray analysis revealed that the complex is a centrosymmetric binuclear neutral entity, in which Cu(II) is a five-coordinate in a distorted trigonal bipyramidal geometry. The DNA-binding properties of the free ligand and the complex have been investigated by electronic absorption, fluorescence, and viscosity measurements. The results suggest that the H₂L and the complex to DNA via the intercalation mode and the binding affinity of the complex were higher than that of the H₂L. The intrinsic binding constants K_b of the ligand and the complex are 2.2 × 10⁴ and 2.7 × 10⁴ M^-1, respectively. Antioxidant assay in vitro shows the Cu(II) complex possesses significant antioxidant activities and better scavenging activity than the H₂L and other antioxidants.

Full text of DOI:10.1080/00958972.2014.959507

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Synthesis, crystal structure,
antioxidation and DNA-binding
properties of a dinuclear copper(II)
complex with bis(N-salicylidene)-3-
oxapentane-1,5-diamine
Huilu Wua, Yuchen Baia, Yanhui Zhanga, Zhen Lia, Mingchang Wua,
Chengyong Chena & Jiawen Zhanga
a School of Chemical and Biological Engineering, Lanzhou Jiaotong
University, Lanzhou, PR China
Accepted author version posted online: 28 Aug 2014.Published
online: 23 Sep 2014.
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To cite this article: Huilu Wu, Yuchen Bai, Yanhui Zhang, Zhen Li, Mingchang Wu, Chengyong Chen
& Jiawen Zhang (2014) Synthesis, crystal structure, antioxidation and DNA-binding properties of
a dinuclear copper(II) complex with bis(N-salicylidene)-3-oxapentane-1,5-diamine, Journal of
Coordination Chemistry, 67:18, 3054-3066, DOI: 10.1080/00958972.2014.959507
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Journal of Coordination Chemistry, 2014
Vol. 67, No. 18, 3054–3066, http://dx.doi.org/10.1080/00958972.2014.959507
Synthesis, crystal structure, antioxidation and DNA-binding
properties of a dinuclear copper(II) complex with
bis(N-salicylidene)-3-oxapentane-1,5-diamine
HUILU WU*, YUCHEN BAI, YANHUI ZHANG, ZHEN LI, MINGCHANG WU,
CHENGYONG CHEN and JIAWEN ZHANG
School of Chemical and Biological Engineering, Lanzhou Jiaotong University, Lanzhou, PR China
(Received 18 April 2014; accepted 24 July 2014)
A Schiff base bis(N-salicylidene)-3-oxapentane-1,5-diamine (H2L) and its Cu(II) complex, [Cu2(L)2]
∙CHCl3, have been synthesized and characterized by physicochemical and spectroscopic methods.
Single-crystal X-ray analysis revealed that the complex is a centrosymmetric binuclear neutral entity,
in which Cu(II) is a five-coordinate in a distorted trigonal bipyramidal geometry. The DNA-binding
properties of the free ligand and the complex have been investigated by electronic absorption, fluo-
rescence, and viscosity measurements. The results suggest that the H2L and the complex to DNA
via the intercalation mode and the binding affinity of the complex were higher than that of the H2L.
The intrinsic binding constants Kb of the ligand and the complex are 2.2 × 104 and 2.7 × 104 M−1
,
respectively. Antioxidant assay in vitro shows the Cu(II) complex possesses significant antioxidant
activities and better scavenging activity than the H2L and other antioxidants.
Keywords: Bis(N-salicylidene)-3-oxapentane-1,5-diamine; Copper(II) complex; Crystal structure;
DNA-binding; Antioxidant
1. Introduction
Schiff bases are an important class of compounds in medicinal and pharmaceutical fields.
They show biological applications including antibacterial, antifungal [1–5], and antitumor
activities [6, 7]. Metal–organic complexes offer the possibility of new functional materials,
in which pore size, coordination forms, and functionality could be varied [8]. Diamino
*Corresponding author. Email: wuhl@mail.lzjtu.cn
© 2014 Taylor & Francis

Dinuclear copper(II)
3055
Schiff bases and their complexes have been used as biological models to understand the
structures of biomolecules and biological processes [9, 10].
The syntheses and interactions of transition metal complexes with DNA and protein have
been an active field of research. Interest in this area is aimed at gaining insight into the
mechanism involving site-specific recognition of DNA, the biochemical procedures govern-
ing protein sequencing, foot-printing and folding studies, understanding the reactive models
for protein–nucleic acid interactions, application of probes of DNA structure, and obtaining
information about the rational design and synthesis of new types of pharmaceutical mole-
cules. To search for new restriction enzymes and anticancer therapeutic agents, much effort
has been made in synthesis and property investigation of mimics. These complexes can
bind to DNA in many non-covalent modes, such as ionic bonds, hydrogen bonds, π → π*
interactions, and hydrophobic interactions [11, 12]. The interactions of small inorganic–
organic molecules with DNA have attracted attention because of their interference with
important events in cells of living organisms [13–16].
DNA binding is the critical step for DNA activity. To design effective chemotherapeutic
agents and better anticancer drugs, it is essential to explore the interactions of metal com-
plexes with DNA [17, 18]. An understanding of how these small molecules bind to DNA
will be potentially useful in the design of new drugs, significantly sensitive spectroscopic or
diagnostic probes and reactive agents, which can recognize specific sites or conformations
of DNA [19–22]. In recent years, inorganic–organic metal complexes such as Schiff base
derivatives and their transition metal complexes have been investigated in interactions
between complexes and DNA. Antioxidant activities of the complex were determined by
hydroxyl radical (OH∙) scavenging methods in vitro [23, 24].
In this study, a Cu(II) complex of bis(N-salicylidene)-3-oxapentane-1,5-diamine was syn-
thesized and its interaction with DNA was studied via electronic absorption spectroscopy,
viscosity, and fluorescence spectroscopy. In addition, the antioxidant activities of the com-
plex were determined by hydroxyl radical (HO∙) scavenging methods in vitro.
2. Experimental
2.1. Measurements and methods
C, H, and N elemental analyses were determined using a Carlo Erba 1106 elemental ana-
lyzer. IR spectra were recorded from 4000 to 400 cm−1 with a Nicolet FT-VERTEX 70
spectrometer using KBr pellets. The fluorescence spectra were recorded on a LS-45 spectro-
fluorophotometer. Electronic spectra were taken on a Lab-Tech UV Bluestar spectrophotom-
eter. Antioxidant activities containing the hydroxyl radical (OH∙) were performed in a water
bath with a 722sp spectrophotometer.
All chemicals were of analytical grade. Calf thymus DNA (CT-DNA), ethidium bromide
(EB), nitroblue tetrazolium nitrate, methionine, and riboflavin were obtained from Sigma-
Aldrich Co. (USA) and used without purification. Tris–HCl buffer and EDTA-Fe(II) solu-
tion were prepared using bidistilled water. The stock solution of complex was dissolved in
DMF at 3 × 10−3 M L−1. All chemicals used were of analytical grade. The experiments
involving interaction of the ligand and the complex with CT-DNA were carried out in dou-
ble distilled water buffer containing 5 mM Tris and 50 mM NaCl and adjusted to pH 7.2
with hydrochloric acid.

3056
H. Wu et al.
2.2. Preparation of 3-oxapentane-1,5-diamine
3-Oxapentane-1,5-diamine was synthesized following the procedure [25]. –C4H12N2O
(104.1): (Found (%): C, 45.98; H, 11.50; N, 26.76. Calcd (%): C, 46.13; H, 11.61; N,
26.90). Selected IR data: (KBr ν (cm−1)), 1120 (νC–O–C), 3340 (ν–NH2).
2.3. Preparation of bis(N-salicylidene)-3-oxapentane-1,5-diamine (H2L)
For the synthesis of H2L, salicylaldehyde (10 mM, 1.22 g) in EtOH (5 mL) was added
dropwise to a 5 mL EtOH solution of 3-oxapentane-1,5-diamine (10 mM, 1.04 g). After
completion of addition, the solution was stirred for an additional 4 h at 78 °C. After cooling
to room temperature, the precipitate was filtered. The product was dried in vacuo, giving a
yellow crystalline solid. The synthetic route for H2L is shown in scheme 1. Yield: 1.65 g
(73.0%). –C18H20O3N2 (312.4): (Found (%): C, 69.30; H, 6.51; N, 8.89. Calcd (%): C,
69.21; H, 6.45; N, 8.97). Selected IR data: (KBr ν (cm−1)), 1637(νC=N), 1286(νC–O–C), 3458
(νOH), stretching frequency, respectively. UV–vis (DMF): λ = 275, 311 nm.
2.4. Preparation of [Cu2(L)2]∙CHCl3
To a stirred solution of H2L (0.312 g, 1 mM) in MeOH (5 mL) was added Cu(NO3)2(H2O)3
(0.241 g, 1 mM) in MeOH (5 mL). The blue sediment generated rapidly. The precipitate
was filtered off, washed with MeOH and absolute Et2O, and dried in vacuo. The dried pre-
cipitate was dissolved in CHCl3 to form a blue solution. Blue block crystals of [Cu2(L)2]
∙CHCl3 suitable for X-ray diffraction studies were obtained by evaporation of the CHCl3
for a few weeks at room temperature. Yield: 0.218 g (41.4%). –Cu2C38H38Cl6N4O6
(986.50): (Found (%): C, 57.55; H, 4.88; N, 7.38. Calcd (%): C, 57.82; H, 4.85; N, 7.49).
Selected IR data: (KBr ν (cm−1)), 1147 (νC–O–C), 1627 (νC=N) stretching frequency, respec-
tively. UV–vis (DMF): λ = 294, 370 nm.
2.5. X-ray structure determination of [Cu2(L)2]∙CHCl3
A suitable single crystal was mounted on a glass fiber, and the intensity data were collected
on a Bruker Smart CCD diffractometer with graphite-monochromated Mo Kα radiation
(λ = 0.71073 Å) at 296 K. Data reduction and cell refinement were performed using
Scheme 1. Synthetic route for H2L.

Dinuclear copper(II)
3057
Table 1. Crystallographic data and data collection parameters for [Cu2(L)2]
∙CHCl3.
Formula
Cu2C38H38Cl6N4O6
986.50
Molecular weight (g m−1
)
Crystal system
Space group
Monoclinic
P2(1)/c
Unit cell dimensions
a (Å)
b (Å)
c (Å)
α (°)
13.1746(4)
16.6027(5)
9.4906(3)
90
β (°)
102.3060(10)
90
2028.22(11)
2
γ (°)
Volume (Å3)
Z
T (K)
296(2)
1.615
1.495
1004
D (calculated) (g cm−3
)
Absorption coefficient (mm−1
F (0 0 0)
)
Crystal size (mm)
0.38 × 0.30 × 0.28
2.10–25.00
19,536
4648 [R(int) = 0.0334]
−17 ≤ h ≤ 17,
−21 ≤ k ≤ 20,
−12 ≤ l ≤ 12
Full-matrix least-squares on F2
4648/0/254
1.068
θ Range for data collection (°)
Reflections collected
Independent reflections
Index ranges
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Finial R1 and wR2 [I > 2σ(I)]
R indices (all data)
R1 = 0.0330, wR2 = 0.0824
R1 = 0.0385, wR2 = 0.0941
0.713 and −0.705
Largest difference in peak and hole (e Å−3
)
SMART and SAINT programs. Data reduction and cell refinement were performed using
SAINT programs [26]. The absorption corrections were carried out by the empirical
method. The structure was solved by direct methods and refined by full-matrix least-squares
against F2 using SHELXTL software [27].
All hydrogens were found in different electron maps and were subsequently refined in a
riding-model approximation with C–H distances from 0.93 to 0.97 Å and Uiso(H) = 1.2
Ueq(C) or Uiso(H) = 1.5 Ueq(C). A summary of parameters for the data collections and
refinements are given in table 1.
2.6. DNA binding experiments
Metal complexes can bind to DNA according to three possible patterns: (1) electrostatic
interaction, in which the metal complexes are electrostatically adsorbed on the phosphates
of DNA, (2) intercalative binding, in which the metal complexes intercalate into the base
pairs of the double-stranded DNA, and (3) groove binding, in which the metal complexes
interact with the grooves of DNA [28]. Absorption titration experiments were performed
with fixed concentrations of the compounds, while gradually increasing the concentration of
CT-DNA. The required amount of CT-DNA was added to both the compound solution and
the reference solution to eliminate the absorbance of CT-DNA itself. From the absorption
titration data, the binding constant (Kb) was determined using the equation [29]:
½DNAꢀ=ðea ꢁ ef Þ ¼ ½DNAꢀ=ðeb ꢁ ef Þ þ 1=Kbðeb ꢁ ef Þ

3058
H. Wu et al.
where [DNA] is the concentration of CT-DNA in the base pairs, εa corresponds to the
observed extinction coefficient (Aobsd/[M]), εf corresponds to the extinction coefficient of
the free compound, εb is the extinction coefficient of the compound when fully bound to
CT-DNA, and Kb is the intrinsic binding constant. The ratio of slope to intercept in the plot
of [DNA]/(εa − εf) versus [DNA] gave the value of Kb.
The fluorescence of EB intercalated into DNA can be quenched by the addition of a sec-
ond molecule [30, 31]. The extent of the fluorescence quenching of EB bound to CT-DNA
can be used to determine the extent of binding between the second molecule and CT-DNA.
The competitive binding experiments were carried out in the buffer by keeping [DNA]/
[EB] = 1.13 and varying the concentrations of the compounds. The fluorescence spectra of
EB were measured using an excitation wavelength of 520 nm, and the emission range was
set between 550 and 750 nm. The spectra were analyzed according to the classical Stern–
Volmer equation [32]: I0/I = 1 + Ksv [Q], where I0 and I are the fluorescence intensities at
599 nm in the absence and presence of the quencher, respectively, Ksv is the linear Stern–
Volmer quenching constant, and [Q] is the concentration of the quencher. In these experi-
ments, [CT-DNA] = 2.5 × 10−3 M/L, [EB] = 2.2 × 10−3 M/L.
Viscosity measurements were carried out to further verify the interaction of the com-
plexes with CT-DNA [33]. Viscosity experiments were conducted on an Ubbelodhe viscom-
eter, immersed in a water bath maintained at 25.0 0.1 °C. Titrations were performed for
the complexes (3 μM), and each compound was introduced into the CT-DNA solution
(50 μM) present in the viscometer. Data are presented as (η/η0)1/3 versus the ratio of the
concentration of the compound to CT-DNA, where η is the viscosity of CT-DNA in the
presence of the compound and η0 is the viscosity of CT-DNA alone. Viscosity values were
calculated from the observed flow times of CT-DNA-containing solutions corrected for the
flow time in buffer alone (t0), η = (t − t0) [34].
2.7. Antioxidation study methods
The hydroxyl radicals in aqueous media were generated through the Fenton-type reaction
[35, 36]. The 3 mL reaction mixtures contained 1.0 mL of 40 μg/mL aqueous safranine,
1 mL of 1.0 mM aqueous EDTA-Fe(II), 1 mL of 3% aqueous H2O2, and a series of quanti-
tatively microadding solutions of the tested compound. The sample without the tested com-
pound was used as the control. The reaction mixtures were incubated at 37 °C for 30 min in
a water bath. The absorbance was then measured at 520 nm. All the tests were run in tripli-
cate and are expressed as the mean and standard deviation [37]. The scavenging effect for
OH• was calculated from the following expression:
Scavenging effect % ¼ ðAsample ꢁ ArÞ=ðAo ꢁ ArÞ ꢂ 100%
where Asample is the absorbance of the sample in the presence of the tested compound, Ar is
the absorbance of the blank in the absence of the tested compound, and Ao is the absor-
bance in the absence of the tested compound and EDTA-Fe(II).
3. Results and discussion
The ligand and the complex are very stable in air. They are remarkably soluble in polar
aprotic solvents such as DMF and DMSO; slightly soluble in ethanol, methanol, and chlo-
roform; insoluble in water.

Dinuclear copper(II)
3.1. X-ray structure determination of complexes
3059
The single-crystal X-ray analysis reveals that the complex crystallizes in monoclinic space
group P2(1)/c and is a centrosymmetric binuclear neutral entity. The ORTEP structure of
Cu(II) complex with atom labeling is shown in figure 1. The structure of the complex con-
sists of two divalent Cu(II) ions, two fully deprotonated ligands and one CHCl3. Both Cu
(II) ions are a five-coordinate in a distorted trigonal bipyramidal geometry (figure 1). In the
[Cu2(L)2]∙CHCl3 unit, a 10-membered ring is formed by the two Cu(II) centers and the cor-
responding atoms on the two ligands. Every Cu(II) is N, Ochelated by one ligand and is
further N, O coordinated by another ligand. There are two ligands bridged by two Cu(II)
ions from the macrocyclic complex. The dinuclear coordination sphere results from fusion
of Cu–O and Cu–N bonds. In the ring, the long distance and short distance are 5.937 Å
(Cu–Cu) and 3.829 Å (O–O) in the cycle (figure 2). Selected bond lengths and angles of
the complex are listed in table 2.
The Cu(II) complex exhibits a different hydrogen-bonding pattern built up from non-clas-
sical C–H∙∙∙O hydrogen bonds in the crystal structure (figure 3). C–H∙∙∙O and C–H∙∙∙Cl
hydrogen-bonding interactions play important roles in crystal packing in the complex.
Selected hydrogen bonds are listed in table 3.
3.2. IR and electronic spectra
In free H2L, a strong band is found at 1637 cm−1 together with a weak band at 1286 cm−1.
By analogy with the assigned bands, the former can be attributed to ν(C=N), while the latter
can be attributed to ν(C–O–C). Both bands were shifted to lower frequency by 20–80 cm−1 in
the spectrum of the complex, which implies coordination of oxygen to the metal. The elec-
tronic spectra of the ligands and the metal complex were recorded in DMF solution at room
Figure 1. Molecular structure and atom numberings of [Cu2(L)2]∙CHCl3 with hydrogens omitted for clarity.

3060
H. Wu et al.
Table 2. Selected bond distances (Å) and angles (°) for the complex.
Bond distances
Cu–O(2)
Cu–N(2)
1.9240(15)
1.9601(17)
Cu–O(1)
Cu–N(1)
1.9266(15)
1.9621(18)
Bond angles
O(2)–Cu–O(1)
O(1)–Cu–N(2)
O(1)–Cu–N(1)
O(3)–Cu–O(1)
O(3)–Cu–N(1)
O(3)–Cu–O(2)
153.52(7)
90.05(7)
91.40(7)
121.99(7)
72.66(7)
83.76(7)
O(2)–Cu–N(2)
O(2)–Cu–N(1)
N(2)–Cu–N(1)
O(3)–Cu–O(2)
O(3)–Cu–N(1)
92.94(7)
90.82(7)
168.52(7)
83.79(7)
96.97(7)
Figure 2. A 10-membered ring of complex.
Figure 3. Packing structure of [Cu2(L)2]∙CHCl3.
temperature. The UV bands of H2L (275, 311 nm) are marginally shifted in the complex.
Two absorption bands are assigned to π → π* (benzene) and π → π* (C=N) transitions.
3.3. DNA-binding mode and affinity
Electronic absorption spectroscopy is a useful method to study the binding of DNA with
metal complexes. The absorption spectra of the ligand and the Cu(II) complex in the

Dinuclear copper(II)
3061
Table 3. Selected hydrogen-bonding distances (Å) and angles (°).
D–H···A
d(D–H)
d(H···A)
d(D···A)
\(DHA)
C(8)–H(8A)...O(3)#1
C(11)–H(11B)...O(1)#1
C(10)–H(10B)...Cl(1)#2
C(19)–H(19)...O(3)#3
0.97
0.97
0.97
0.98
2.42
2.44
2.81
2.24
3.010(3)
2.962(3)
3.523(2)
3.190(3)
118.9
113.2
131.0
163.5
Note: Symmetry transformations used to generate equivalent atoms: #1 = −x, −y, −z + 1; #2 = x, y − 1, z; #3 = x,
y + 1, z.
absence and presence of CT-DNA (at a constant concentration of compounds) are given in
figure 4. H2L and the Cu(II) complex exhibit intense absorption bands at 395 and 378 nm
assigned to π–π* transition of the C=N, and addition of increasing amounts of CT-DNA
results in hypochromism and red shift in the UV–vis spectra of the compounds. In the pres-
ent case, with the addition of DNA, H2L, and the Cu(II) complex exhibit hypochromism of
80 and 87% accompanied in the absorption maxima. The spectroscopic changes suggest
Figure 4. Electronic spectra of H2L (a), Cu(II) complex (c) in Tris–HCl buffer upon addition of CT-DNA.
[Complex] = 3.0 × 10−5 M, [DNA] = 2.5 × 10−5 M. The arrows show the emission intensity changes upon increas-
ing DNA concentration. Plots of [DNA]/(εa − εf) vs. [DNA] for the titration of H2L (b), Cu(II) complex (d) with
CT-DNA.

3062
H. Wu et al.
that H2L and the Cu(II) complex have strong interaction with DNA [38–41]. The Kb values
of the H2L and the Cu(II) complex were 2.2 × 104 M−1 (R2 = 0.988 for 15 points) and
2.7 × 104 M−1 (R2 = 0.974 for 15 points), respectively. Hence, the binding strength of Cu
(II) complex is greater than the free ligand. In general, this may be attributed to the Cu(II)
complex with the electric effect resulting in relatively close stacking between the Cu(II)
complex and the DNA base pairs [41].
Measurement of the ability of a complex to affect the EB fluorescence intensity in the
EB-DNA adduct allows determination of the affinity of the complex for DNA, whatever the
binding mode may be. No emission was observed either alone or in the presence of CT-
DNA in the buffer. A competitive binding of the complex to CT-DNA decreases the emis-
sion intensity of EB, and the reduction extent of the emission intensity gives a measure to
verify the DNA binding propensity of the complex.
The fluorescence intensity of the EB-DNA system greatly decreases upon gradual addi-
tion of both compounds (figure 5). The Stern–Volmer constant KSV was obtained as the
slope of the linear plot of I0/I versus compound. From the inset in figure 5, the KSV values
are 2.0 × 102 M−1 (R2 = 0.943 for 16 points) and 4.8 × 102 M−1 (R2 = 0.974 for 16 points)
Figure 5. Emission spectra of EB bound to CT-DNA in the presence of free H2L (a) and Cu(II) complex (c),
λex = 520 nm. The arrows show the intensity changes upon increasing concentrations of the complexes.
Fluorescence quenching curves of EB bound to CT-DNA by free H2L (b) and Cu(II) complex (d). (Plots of I0/I vs.
[complex].)

Dinuclear copper(II)
3063
for H2L and the complex, respectively. These values show that both the free ligand and
complex can compete for DNA-binding sites and so displace EB from the DNA, which is
usually characteristic of intercalative interaction of compounds with DNA; the binding
strength of the complex is greater than that of the free ligand [42].
Optical photophysical techniques are widely used to study the binding of ligands and
their complexes to DNA, but do not give sufficient information to determine a binding
model. Therefore, viscosity measurements were carried out to further clarify the interaction
of metal complexes and DNA. Hydrodynamic measurements that are sensitive to the length
change of DNA (i.e. viscosity and sedimentation) are regarded as the least ambiguous and
the most critical tests of a binding model in solution in the absence of crystallographic
structural data [43, 44]. For the ligand and the complex, as the amount of compound is
increased, the viscosity of DNA increases steadily. The values of (η/η0)1/3 were plotted
against [Complex]/[DNA] (figure 6). In classical intercalation, the DNA helix lengthens as
base pairs are separated to accommodate the ligand, leading to increased DNA viscosity,
whereas a partial, non-classical ligand intercalation causes a bend (or kink) in the DNA
helix, so reducing its effective length and thereby its viscosity [43]. The results shown in
figure 6 provide further evidence that both H2L and the Cu(II) complex intercalate with CT-
DNA. The results from the viscosity experiments confirm that these compounds intercalate
into DNA base pairs, as also indicated by the spectroscopic studies.
Based on the above results, we found that the affinity for DNA is stronger in Cu(II) com-
plex than with the ligand. We attribute this difference to two possible reasons. (i) By com-
parison of the molecular structure of the ligand and Cu(II) complexes, we find the greater
number of coplanar aromatic rings, which facilitate intercalation to the base pairs of double-
helical DNA, may lead to higher affinity for DNA. (ii) The charge transfer of coordinated
ligand caused by the coordination of the central atom lead to the decrease in the charge den-
sity of the plane conjugate system, which is conducive to insertion [45, 46].
3.4. Antioxidant activity
Since H2L and Cu(II) complex exhibit reasonable DNA-binding affinity, it was considered
worthwhile to study other potential aspects, such as antioxidant and antiradical activity. It is
Figure 6. Effect of increasing amounts of the compounds on the relative viscosity at 25.0 0.1 °C.

3064
H. Wu et al.
Figure 7. A plot of scavenging percentage (%) vs. concentration of the Cu(II) complex on hydroxyl radical.
a well-documented fact that some transition metal complexes display significant antioxidant
activity, and therefore, we undertook a systematic investigation on the antioxidant potential
of free H2L and Cu(II) complex against OH∙ radicals with respect to different concentra-
tions of the test compounds.
Figure 7 depicts the inhibitory effect of H2L and the complex on OH∙ radicals. The inhib-
itory effect of the tested complex on OH∙ radicals is concentration related, and the suppres-
sion ratio increases with increasing the sample concentration of the tested concentration.
Mannitol and vitamin C are well-known natural antioxidants, so we also studied the scav-
enging activity of mannitol and vitamin C against hydroxyl radical using the same model
[47]. The 50% inhibitory concentration (IC50) values of mannitol and vitamin C are
9.6 × 10−3 and 8.7 × 10−3 M, respectively. According to the antioxidant experiments, the
IC50 of the H2L and Cu(II) complex are 8.30 × 10−5 and 7.79 × 10−5 M (figure 7). We find
that when arriving at similar suppression ratio, concentration of the complex and H2L are
far less than that of mannitol and vitamin C. The result showed that the Cu(II) complex
exhibits better scavenging activity than H2L, mannitol, and vitamin C. The lower IC50 val-
ues observed in antioxidant assays did demonstrate that the Cu(II) complex has a strong
potential to be applied as a potential drug to eliminate radicals [36–49].
4. Conclusion
A new unsymmetrical dinuclear Cu complex, [Cu2(L)2]∙CHCl3, has been synthesized and
characterized by single-crystal X-ray diffraction based on the Schiff base bis(N-salicylid-
ene)-3-oxapentane-1,5-diamine (H2L). The binding modes of ligand and Cu(II) complex
with CT-DNA have been studied. The photophysical and viscosity measurements indicate
that the compounds interact with CT-DNA through intercalative binding and their affinity to
DNA follows the order complex > ligand, which can be attributed to the difference in steric
hindrance and electron density caused by coordination. In addition, ligand and Cu(II) com-
plex also reveal excellent antioxidant properties, with complex more effective than ligand.
These compounds have potential applications for the development of nucleic acid molecular
probes and new therapeutic reagents for diseases on the molecular level and warrant further
in vivo experiments and pharmacological assays.

Dinuclear copper(II)
3065
Supplementary material
Crystallographic data (excluding structure factors) for the structure in this article have been
deposited with the Cambridge Crystallographic Data Center as supplementary publication
CCDC 835709. Copies of the data can be obtained free of charge on application to the
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK.
Funding
The present research was supported by the National Natural Science Foundation of China [grant number
21367017]; the Fundamental Research Funds for the Gansu Province Universities [grant number 212086]; Natural
Science Foundation of Gansu Province [grant number 1212RJZA037]; “Qing Lan” Talent Engineering Funds for
Lanzhou Jiaotong University.
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