Synthesis, structure, DNA-binding properties, and antioxidant activity of copper(II) and cobalt(II) complexes with bis(N-allylbenzimidazol-2-ylmethyl) benzylamine

Article abstract of DOI:10.1007/s11243-011-9536-5

Bis(N-allylbenzimidazol-2-ylmethyl)benzylamine (babb) and two of its complexes, [Cu(babb)(pic)₂]·H₂O (1) and [Co(babb)₂](pic)₂ (2) (pic = picrate), have been synthesized and characterized by physico-chemical and

Full text of DOI:10.1007/s11243-011-9536-5

Transition Met Chem (2011) 36:819–827
DOI 10.1007/s11243-011-9536-5
Synthesis, structure, DNA-binding properties, and antioxidant
activity of copper(II) and cobalt(II) complexes
with bis(N-allylbenzimidazol-2-ylmethyl)benzylamine
•
Hui-Lu Wu Jing-Kun Yuan Ying Bai
•
•
•
Fei Jia Bin Liu Fan Kou Jin Kong
•
•
Received: 8 June 2011 / Accepted: 6 September 2011 / Published online: 20 September 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Bis(N-allylbenzimidazol-2-ylmethyl)benzylamine
(babb) and two of its complexes, [Cu(babb)(pic)2]ꢀH2O (1)
and [Co(babb)2](pic)2 (2) (pic = picrate), have been synthe-
sized and characterized by physico-chemical and spectro-
scopic methods. Single crystal X-ray diffraction revealed that
the two complexes have similar distorted octahedral struc-
tures, but the degree of distortion and the coordinated atoms
are different. The DNA-binding properties of the free ligand
and its two complexes have been investigated by electronic
absorption, fluorescence, and viscosity measurements. The
results suggest that all three compounds bind to DNA via
an intercalative binding mode, and their binding affinity for
DNA follows the order 2 [1 [ligand. Additionally, both
complexes exhibited potential antioxidant properties in in
vitro studies, and complex 1 was the more effective.
Benzimidazole is a typical heterocyclic ligand with
nitrogen as the donor atom. Interest in exploring benz-
imidazole derivatives and their metal complexes has been
increasing, since the recognition that many of these mate-
rials may serve as models that minic both the structure and
reactivity of metal sites in complex biological systems and
can also possess a broad spectrum of biological activity
[5, 6]. Due to their privileged structure and properties [7],
benzimidazoles and their derivatives exhibit a wide variety
of pharmacological activities such as fungicides or anti-
helminthics, among others [8]. Hence, transition metal
complexes containing benzimidazole ligands are a subject
of intensive research not only owing to their rich coordi-
nation chemistry but also due to a number of established
and potential application areas [9, 10].
In previous studies [11–14], we have investigated that
the coordinating ability of various benzimidazole ligands. In
this study, the synthesis, characterization, and DNA-binding
activities of two transition metal complexes with bis(N-
allylbenzimidazol-2-ylmethyl)benzylamine are presented.
According to relevant reports in the literature [15–18], similar
transition metal complexes can exhibit antioxidant activity.
We therefore also conducted an investigation into the hydro-
xyl radical scavenging properties of these complexes.
Introduction
Transition metal complexes are currently being used to
bind and react at specific sequences of DNA in a search for
novel chemotherapeutics and DNA probes and for the
development of highly sensitive diagnostic agents [1–3].
Therefore, an understanding of how these small molecules
bind to DNA will potentially be useful in the design of such
new compounds, which can recognize specific sites or
conformations of DNA [2–4].
Experimental
C, H, and N elemental analyses were determined using a
Carlo Erba 1106 elemental analyzer. Electrolytic conduc-
tivity measurements were made with a DDS-307 type
conductivity bridge using 3 9 10-3 mol L-1 solutions in
DMF at room temperature. IR spectra were recorded in the
4,000–400 cm-1 region with a Nicolet FT-VERTEX 70
spectrometer using KBr pellets. Electronic spectra were
H.-L. Wu (&) ꢀ J.-K. Yuan ꢀ Y. Bai ꢀ F. Jia ꢀ B. Liu ꢀ F. Kou ꢀ
J. Kong
School of Chemical and Biological Engineering,
Lanzhou Jiaotong University, Lanzhou 730070,
People’s Republic of China
e-mail: wuhuilu@163.com
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Transition Met Chem (2011) 36:819–827
concentration of the quencher. In these experiments [CT-
DNA] = 2.5 9 10-3 mol/L, [EB] = 2.2 9 10-3 mol/L.
Viscosity experiments were conducted on an Ubbelodhe
viscometer, immersed in a water bath maintained at
25.0 0.1 °C. Titrations were performed for the com-
plexes (3–30 lM), and each compound was introduced into
CT-DNA solution (42.5 lM) present in the viscometer.
Data were analyzed as (g/g0)1/3 versus the ratio of the
concentration of the compound to CT-DNA, where g is the
viscosity of CT-DNA in the presence of the compound and
g0 is the viscosity of CT-DNA alone. Viscosity values were
calculated from the observed flow time of CT-DNA-con-
taining solutions corrected from the flow time of buffer
alone (t0), g = (t - t0) [25].
taken on Lab-Tech UV Bluestar and Spectrumlab 722sp
spectrophotometers. Fluorescence spectra were recorded
on a LS-45 spectrofluorophotometer. 1H NMR spectra were
recorded on a Varian VR300-MHz spectrometer with TMS
as an internal standard.
Calf thymus DNA (CT-DNA) and ethidium bromide
(EB) were purchased from Sigma. All chemicals used were
of analytical grade. All the experiments involving inter-
action of the ligand and the complexes with CT-DNA were
carried out in doubly distilled water buffer containing
5 mM Tris and 50 mM NaCl and adjusted to pH 7.2 with
hydrochloric acid. A solution of CT-DNA gave a ratio of
UV absorbance at 260 and 280 nm of about 1.8–1.9,
indicating that the CT-DNA was sufficiently free of protein
[19]. The CT-DNA concentration per nucleotide was deter-
mined spectrophotometrically by employing an extinction
coefficient of 6,600 M-1 cm-1 at 260 nm [20].
Hydroxyl radical scavenger measurements
Hydroxyl radicals were generated in aqueous media through
the Fenton-type reaction [26, 27]. The aliquots of reaction
mixture (3 mL) contained 1.0 mL of 0.10 mmol aqueous
safranin, 1 mL of 1.0 mmol aqueous EDTA–Fe(II), 1 mL of
3% aqueous H2O2, and a series of quantitative microaddi-
tions of solutions of the test compound. A sample without the
tested compound 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 triplicate and are expressed as the mean and
standard deviation (SD) [28]. The scavenging effect for OHꢀ
was calculated from the following expression:
DNA-binding study
Absorption titration experiments were performed with
fixed concentrations of the complexes, while gradually
increasing the concentration of CT-DNA. To obtain the
absorption spectra, the required amount of CT-DNA was
added to both the compound and reference solutions, in
order to eliminate the absorbance of CT-DNA itself. From
the absorption titration data, the binding constant (Kb) was
determined using the equation [21]:
À
Á
À
Á
À
Á
½DNAꢁ= ea ꢂ ef ¼ ½DNAꢁ= eb ꢂ ef þ 1=Kb eb ꢂ ef
Scavenging ratio ð%Þ ¼ ½ðAi ꢂ A0Þ=ðAc ꢂ A0Þꢁ ꢃ 100%
where [DNA] is the concentration of CT-DNA in base pairs,
ea corresponds to the observed extinction coefficient (Aobsd/
[M]), ef corresponds to the extinction coefficient of the free
compound, eb 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]/
(ea – ef) versus [DNA] gave the value of Kb.
where Ai = absorbance in the presence of the test com-
pound; A0 = absorbance of the blank in the absence of the
test compound; Ac = absorbance in the absence of the test
compound, EDTA–Fe(II) and H2O2.
Synthesis of babb
The enhanced fluorescence of EB in the presence of DNA
canbequenchedbythe additionofasecondmolecule [22, 23].
The extent of fluorescence quenching of EB bound to
CT-DNA can be used to determine the extent of binding
between the second molecule and CT-DNA. Competitive
binding experiments were carried out in the buffer by keeping
[DNA]/[EB] = 1 and varying the concentrations of the com-
pounds. 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 [24]:
Bis(N-allylbenzimidazol-2-ylmethyl)benzylamine (babb) was
synthesized following a slight modification of the procedure in
ref [29]. Bis(2-benzimidazol-2-ylmethyl)benzylamine (7.34 g,
20 mmol) was reacted with potassium (1.56 g, 40 mmol) in
tetrahydrofuran (150 mL). Allyl bromide (4.84 g, 40 mmol)
was then added. The resulting solution was concentrated and
recrystallized from ethanol to give pale yellow block crystals
[30]. Yield: 5.46 g (61%); m.p.:113–115 °C. Found (%): C,
77.8; H, 6.5; N, 15.6. Calcd. (%) for C29H29N5: C, 78.0; H, 6.3;
1
N, 15.7. H–NMR (DMSO–d6 400 MHz) d/ppm: 3.45 (m,
4H, –CH2–Ar), 3.85 (s, 4H, –CH2–benzimidazol), 4.87–5.68
(m, 10H, –CH2–CH=CH2), 7.22 (m, 5H, H–benzene ring),
7.27–7.64 (m, 8H, H–benzimidazol ring). UV–vis (k, nm):279,
286. FTIR (KBr m/cm-1): 737, m(o–Ar); 1,265, m(C–N); 1,461,
m(C=N), 1,643, m(C=C).
I0=I ¼ 1 þ KSV½Qꢁ
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
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Transition Met Chem (2011) 36:819–827
821
Preparation of complex 1
compositions are [Cu(babb)(pic)2]ꢀH2O, and [Co(babb)2]-
(pic)2. The electrolytic conductivity of complex 2 shows
that it is 1:2 electrolyte in DMF [33]. In theory, complex 1 is
neutral, but the conductivity shows that it is a 1:1 electrolyte
in DMF, which may be attributed to partial ionization of the
discrete [Cu(babb)(pic)2] in the DMF.
To a stirred solution of babb (223.5 mg, 0.25 mmol) in hot
EtOH (10 mL) was added Cu(pic)2 (129.94 mg, 0.50 mmol)
in EtOH (2 mL). A yellow crystalline product formed
rapidly. The precipitate was filtered off, washed with EtOH
and absolute Et2O, and dried under vacuum. The crude
product was dissolved in MeCN to form a yellow solution
into which Et2O was allowed to diffuse at room tempera-
ture. Yellow crystals of 1 suitable for X-ray measurement
were obtained after several weeks. Yield: 160.5 mg (64%).
Found (%): C, 49.9; H, 3.5; N, 15.6. Calcd. (%) for
C41H35CuN11O15: C, 49.7; H, 3.7; N, 15.5. Km(DMF,
297 K): 77.9 S cm2 mol-1. UV–vis (k, nm): 275, 280, 381.
FTIR (KBr m/cm-1): 746, m(o–Ar); 1,269, m(C–N); 1,363,
m(O–N–O); 1,481, m(C=N), 1,635, m(C=C) [30].
The IR spectra of the two complexes are closely related to
that of the free ligand babb. One of the most diagnostic
changes occurs in the region between 1,650 and 1,250 cm-1
.
The spectrum of free babb shows a strong band at
1,462 cm-1 and weak bands at 1,643 and 1,408 cm-1
,
attributable to the m(C=N) and m(C=C) frequencies of the
benzimidazole group [34–37], respectively. The location of
these two bands was slightly shifted for both complexes; the
band at 1,462 cm-1 is shifted to 1,481 and 1,483 cm-1 for
complexes, respectively, which implies direct coordination
of all four imine nitrogen atoms to the central metal atom
[38]. Information regarding the possible bonding modes of
the picrate and benzimidazole rings may also be obtained
from the IR spectra [39].
Preparation of complex 2
Complex 2 was prepared by a similar procedure as for com-
plex 1, using Co(pic)2 instead of Cu(pic)2. Yield: 250.3 mg
(63%). Found (%): C, 70.1; H, 6.2; N, 14.3 Calcd. (%) for C70
H62 Co N16 O14: C, 70.3; H, 6.3; N, 14.1. Km(DMF, 297 K):
133.04 S cm2 mol-1. UV–vis (k, nm): 281, 381. FTIR (KBr m/
cm-1): 746, m(o–Ar); 1,269, m(C–N); 1,366, m(O–N–O);
1,483 cm-1, m(C=N), 1,633 cm-1, m(C=C) [30].
DMF solutions of the ligand and its complexes show,
as expected, almost identical UV spectra. The UV bands
of the free ligand (286, 279 nm) are only marginally red-
shifted (5–6 nm) for complex 1, which is evidence of C=N
coordination to the metal center. These bands are assigned
to p ? p* (imidazole) transitions [12]. Conversely, the
bands are marginally blue-shifted (5–6 nm) for complex 2,
this phenomenon also shows that C=N is involved in
coordination to the metal center [40]. The picrate bands
(observed at 381 nm both complexes) are assigned to
n ? p* and p ? p* transitions [11].
X-ray crystallography
A suitablesingle crystal was mounted on a glass fiber, and the
intensity data were collected on a Bruker Smart CCD dif-
fractometer with graphite-monochromated Mo–Ka radiation
˚
(k = 0.71073 A) at 296 K. Data reduction and cell refine-
X-ray structures of the complexes
ment were performed using the SMART and SAINT pro-
grams [31]. The structure was solved by direct methods and
refined by full-matrix least-squares against F2 of data using
SHELXTL software [32]. All H atoms were found in dif-
ference electron maps and subsequently refined in a riding-
model approximation with C–H distances ranging from 0.93
The crystal structure of complex 1 consists of discrete
[Cu(babb)(pic)2] and solvent water molecules. The solvent
water molecules are present in the crystal lattice, but have
no direct interactions with the [Cu(babb)(pic)2]. The OR-
TEP structure (30% probability ellipsoids) of [Cu(babb)-
(pic)2] with atom numberings is shown in Fig. 1.
˚
to 0.97 A and Uiso(H) = 1.2 Ueq(C). Basic crystal data,
description of the diffraction experiment, and details of the
structure refinement are given in Table 1. Selected bond
distances and angles are presented in Table 2.
The central copper(II) atom is six-coordinate with a
CuN3O3 environment. The ligand acts as a tridentate
N-donor, with the remaining coordination sites occupied by
three O atoms from two picrates. The coordination geom-
etry of the Cu(II) may be best described as distorted
octahedral with (O1, O2, O8, N1) providing the equatorial
plane. The maximum deviation distance (N1) from the
least-squares plane calculated from the four coordination
Results and discussion
Characterization and structures of the complexes
˚
atom atoms is 0.19 A, indicating that those atoms are
The two complexes are soluble in DMF, DMSO, and ace-
tonitrile, but insoluble in water and other organic solvents,
such as methanol, ethanol, petroleum ether, trichlorome-
thane, etc. The elemental analyses show that their
almost in a plane. The average bond length between the
copper and the apical nitrogen atoms (N3, N5) is 1.975(3)
˚
˚
A, which is about 0.247 A shorter than the bond average
length between the copper and the four coordinated
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Transition Met Chem (2011) 36:819–827
Table 1 Crystal and structure
refinement data for complex
[Cu(babb) (pic)2]ꢀH2O (1)
and [Co(babb)2](pic)2 (2)
Complex
1
2
Molecular formula
Molecular weight
Crystal system
Space group
C41H35CuN11O15
C70H62CoN16O14
1,410.29
985.34
Monoclinic
Triclinic
P21/c
P-1
˚
a (A)
13.960(8)
13.9512(16)
13.9610(17)
19.159(2)
˚
b (A)
14.369(8)
˚
c (A)
24.633(11)
a (8)
b (8)
c (8)
90
89.6910(10)
69.8690(10)
74.0040(10)
3,351.4(7)
112.59(2)
90
3
˚
V (A )
4,562(4)
Z
4
2
qcald (mg m-3
)
1.435
1.398
F (000)
2,028
1,466
Crystal size (mm)
0.33 9 0.31 9 0.28
2.12–25.50
0.40 9 0.38 9 0.30
2.28–25.00
-16,15/-16,16/-22,19
23,116
h range for data collection (8)
h/k/l (max, min)
-11,16/-17,17/-29,28
22,789
Reflections collected
Independent reflections
Refinement method
8,412 [R (int) = 0.0451]
Full-matrix least-squares on F2
11,656 [R (int) = 0.0264]
Full-matrix least-squares on F2
Data/restraints/parameters
Goodness-of-fit on F2
8,412/11/613
11,656/41/910
1.045
1.019
Final R1, wR2 indices [I [ 2r (I)]
R1, wR2 indices (all date)
Largest differences peak and hole
0.0656, 0.1922
0.1137, 0.2325
1.637 and -0.517
0.0561, 0.1485
0.0888, 0.1759
0.809 and -0.562
-3
˚
(e A
)
DNA-binding properties
nitrogen atoms in the equatorial plane. The bond angle
of the two atoms (N3–Cu–N5) in axial positions is
163.52(14)°. Therefore, the geometry around the Cu(II) is a
distorted octahedron [41–44].
Electronic absorption spectroscopy has been widely
employed to determine the binding characteristics of metal
complexes with DNA [45–47]. We have investigated the
binding mode of DNA with these complexes through
absorptiontitrationexperiments. Theabsorptionspectraofthe
free ligand and complexes 1 and 2 in the absence and presence
of CT-DNA (at a constant concentration of complex) are
given in Fig. 3. With increasing DNA concentrations, the
hypochromisms are 18.3% at 275 nm for free babb; 20.2% at
280 nm for complex 1; and 35.4% at 281 nm for complex 2.
The kmax for free babb increased from 275 to 276, while that
for the complex 1 increased from 280 to 282 nm and that for
complex 2 increased from 281 to 282 nm, i.e., slight red shifts
of about 1–2 nm under identical experimental conditions. The
hypochromism suggested that the compounds interact with
CT-DNA [48]. The Kb values of free babb and complexes
1 and 2 were 2.26 9 103 M-1 (R = 0.98 for 15 points),
7.92 9 104 M-1 (R = 0.98 for 6 points in the linear part), and
9.49 9 104 M-1 (R = 0.99 for 16 points), respectively.
Hence, thebindingstrengthofcomplex2 isgreaterthanthatof
complex 1 and the free ligand.
The crystal structure of complex 2 consists of discrete
[Co(babb)2]2? cations and two picrate anions which sur-
round the [Co(babb)2]2?. The ORTEP structure (30%
probability ellipsoids) of the [Co(babb)2]2? with atom
numberings is shown in Fig. 2.
The crystal structure of complex 2 is similar to that of
complex 1. The equatorial plane is provided by atoms N1,
N3, N6, and N7, where the largest deviation from the mean
˚
plane is 0.356 A, and the Co atom is out of this plane by
˚
only 0.014 A. The axial positions are occupied by the
atoms N5 and N9. The distances between the axial atoms
˚
˚
N5, N9, and the equatorial plane are 2.033 A and 2.038 A,
respectively. The bond angle formed the two axial atoms
(N5–Co–N9) is 145.31(11)°. The most obvious difference
between complexes 1 and 2 is the different degree of
deformation from a regular octahedron, which is apparently
caused by the steric effect of the ligand. The coordination
geometry of complex 1 is much closer to regular
octahedral.
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Transition Met Chem (2011) 36:819–827
823
Table 2 Selected bond
˚
Complex
Bond distances
1
2
distances (A) and angles (°)
in complex 1 and 2
Cu–O(1)
1.958(3)
Co(1)–N(3)
2.075(3)
Cu–N(5)
1.968(3)
Co(1)–N(7)
2.109(3)
Cu–N(3)
1.981(3)
Co(1)–N(9)
2.129(3)
Cu–N(1)
2.101(3)
Co(1)–N(5)
2.147(3)
Cu–O(8)
2.385(3)
Co(1)–N(6)
2.410(3)
Cu–O(2)
2.444(4)
Co(1)–N(1)
2.529(3)
Bond angles
O(1)–Cu–N(5)
O(1)–Cu–N(3)
N(5)–Cu–N(3)
O(1)–Cu–N(1)
N(5)–Cu–N(1)
N(3)–Cu–N(1)
O(1)–Cu–O(8)
N(5)–Cu–O(8)
N(3)–Cu–O(8)
N(1)–Cu–O(8)
O(1)–Cu–O(2)
N(5)–Cu–O(2)
N(3)–Cu–O(2)
N(1)–Cu–O(2)
O(8)–Cu–O(2)
97.31(14)
99.16(13)
163.52(14)
176.33(12)
81.51(14)
82.08(13)
98.77(11)
90.05(12)
86.98(12)
84.73(11)
77.24(12)
89.15(16)
94.93(16)
99.24(12)
175.79(11)
N(3)–Co(1)–N(7)
N(3)–Co(1)–N(9)
N(7)–Co(1)–N(9)
N(3)–Co(1)–N(5)
N(7)–Co(1)–N(5)
N(9)–Co(1)–N(5)
N(3)–Co(1)–N(6)
N(7)–Co(1)–N(6)
N(9)–Co(1)–N(6)
N(5)–Co(1)–N(6)
N(3)–Co(1)–N(1)
N(7)–Co(1)–N(1)
N(9)–Co(1)–N(1)
N(5)–Co(1)–N(1)
N(6)–Co(1)–N(1)
103.75(11)
96.04(10)
106.58(11)
102.04(11)
97.60(11)
145.31(11)
168.55(10)
76.04(10)
73.31(9)
89.29(10)
74.29(10)
167.16(10)
86.26(10)
70.81(10)
108.46(9)
Fig. 2 Molecular structure and atom numberings of the
[Co(babb)2]2? with hydrogen atoms omitted for clarity
p ? p* stacking interactions; increasing numbers of ben-
zene rings may lead to higher affinity for DNA, which is
consistent with the experimental results. In addition, elec-
trostatic attraction may be another reason for the different
affinities for DNA. Since [Cu(babb)(pic)2] is neutral, while
[Co(babb)2]2? is a cation, electrostatic interactions will
Fig. 1 Molecular structure and atom numberings of [Cu(babb)(pic)2]
with hydrogen atoms and solvent water molecule omitted for clarity
Considering these experimental results, we speculate
that the planar elements of the structures, for example,
benzene rings, have a direct effect on the affinity for DNA,
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Transition Met Chem (2011) 36:819–827
Fig. 3 Electronic spectra of a free babb, c complex 1, and e complex
2 in Tris–HCl buffer upon addition of CT-DNA. [Compound] =
3 9 10-5 M-1, [DNA] = 2.5 9 10-5 M-1. The arrow shows the
emission intensity changes upon increasing DNA concentration. Plots
of [DNA]/(ea–ef) versus [DNA] for the titration of b ligand, d complex
1 and f complex 2 with CT-DNA
help to strengthen the interactions between the complex
and DNA.
for DNA, whatever the binding mode may be. If a complex
can displace EB from DNA, the fluorescence of the solu-
tion will be reduced due to the fact that free EB molecules
are readily quenched by the solvent water [49]. For all the
compounds, no emission was observed either alone or in
In general, measurement of the ability of a complex to
affect the intensity of EB fluorescence in the EB-DNA
adduct allows determination of the affinity of the complex
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Transition Met Chem (2011) 36:819–827
825
Fig. 4 Emission spectra of EB bound to CT-DNA in the presence of
complexes 1 (a) and 2 (c); [Complex] = 3 9 10-5 M; kex = 520 nm.
The arrows show the intensity changes upon increasing concentrations
of the complexes. Fluorescence quenching curves of EB bound to CT-
DNA by complexes 1 (b) and 2 (d). (Plots of I0/I versus [Complex])
the presence of CT-DNA in the buffer. The addition of the
free ligand does not produce any significant changes of the
intensity or position of the band at 599 nm of the DNA-EB
system, indicating that free babb cannot displace EB from
the DNA-EB complex. The fluorescence quenching of
DNA–bound EB by the complexes 1 and 2 is shown in
Fig. 4. The behavior of both complexes is in good agree-
ment with the Stern–Volmer equation, which provides
further evidence that the complexes bind to DNA. The Ksv
values for complexes 1 and 2 are 6.44 9 104 (R = 0.98 for
8 points) and 6.64 9 104 M-1 (R = 0.98 for 7 points),
respectively. The data suggest that the interaction of
complex 2 with CT-DNA is stronger than that of complex
1, consistent with the UV-V is results discussed above.
Hydrodynamic measurements that are sensitive to DNA
length changes are regarded as the least ambiguous and
most critical tests of a binding model in solution in the
absence of crystallographic structural data [50, 51]. For the
Fig. 5 Effect of increasing amounts of the compounds on the relative
viscosity of CT-DNA at 25.0 0.1 °C
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Transition Met Chem (2011) 36:819–827
Fig. 6 The inhibitory effect of
complexes 1 (a) and 2 (b) on
OHꢀ radicals; the suppression
ratio increases with increasing
concentration of the test
compound
free ligand and the complexes, as increasing amounts of
the compounds are added, the viscosity of DNA increases
steadily. The values of (g/g0)1/3 were plotted against
[compound]/[DNA]. In classical intercalation, the DNA
helix lengthens as base pairs are separated to accommodate
the bound ligand, leading to increased DNA viscosity,
whereas a partial, nonclassical ligand intercalation causes a
bend (or kink) in the DNA helix and so reduces its effective
length and thereby its viscosity [19].
modes of these compounds with CT-DNA have been
studied. The photophysical and viscosity measurements
indicate that the compounds interact with CT-DNA
through intercalative binding. In addition, their affinity
to DNA follows the order 2 [ 1 [ babb, which can be
attributed to a more planar structure upon coordination to
the metal. The antioxidant activities of the compounds
were also investigated, and the results show that complex 1
exhibits effective scavenging of hydroxyl radicals.
The effects of the two complexes on the viscosity of
CT-DNA are shown in Fig. 5. The viscosity of CT-DNA
increased steadily with the increasing amounts of the
complex, providing further evidence that the two com-
plexes intercalate with CT-DNA [52]. The results from the
viscosity experiments also demonstrate that the binding
strength of complex 2 is greater than that of complex 1.
Supplementary data
Crystallographic data (excluding structure factors) for the
structures reported in this study have been deposited with
the Cambridge Crystallographic Data Center with refer-
ence numbers CCDC 827273 and 827272. Copies of the
data can be obtained, free of charge, on application to
the CCDC, 12 Union Road, Cambridge CB2 1EZ, UK.
Tel: ?44-01223-762910; fax: ?44-01223-336033; e-mail:
deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk.
Antioxidant activity
We compared the abilities of the present compounds to
scavenge hydroxyl radicals with those of the well-known
natural antioxidants mannitol and vitamin C, using the
same method as reported in a previous study [53]. The
50% inhibitory concentration (IC50) value of mannitol and
Acknowledgments The authors acknowledge the financial support
and grant from ‘Qing Lan’ Talent Engineering Funds by Lanzhou
Jiaotong University. The grant from ‘Long Yuan Qing Nian’ of Gansu
Province also is acknowledged.
vitamin C are about 9.6 9 10-3 and 8.7 9 10-3 M-1
respectively. According to the antioxidant experiments, the
IC50 values of complexes 1 and 2 are 2.31 9 10-6 M-1
,
,
6.82 9 10-5 M-1, respectively, (Fig. 6), which implies
that complex 1 exhibits better scavenging activity than
complex 2, as well as mannitol and vitamin C. We suggest
that the mechanism of action of complex 1 involves the
redox process of copper (Cu2?/Cu?) [54, 55].
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