Crystal structures, cyclic voltammetry and DNA binding of two mononuclear nickel(II) complexes

Article abstract of DOI:10.1007/s11243-013-9779-4

Two unsymmetrical complexes, [NiL¹]ClO₄ (1) and [NiL²]ClO₄ (2) have been synthesized and characterized by IR, UV, ES-MS and single crystal X-ray diffraction, where HL¹ and HL² are, respectively, the [1+1] condensation products of 2,6-diformyl-4-X-phenol (X = F or CH₃) with N ¹-(2- aminoethyl)-N ²-(4-nitrobenzyl) ethane-1,2-diamine. The coordination geometry of the metal in both complexes can be approximately described as square planar with a mean plane deviation of 0.032 A in complex 1 and 0.027 A in complex 2, respectively. The binding activities of the complexes toward calf-thymus DNA have been analyzed by spectroscopy and viscosity methods. The binding constants of 1 and 2 obtained from UV spectroscopic studies are 5.43 × 10⁵ and 1.83 × 10⁵ M^-1, respectively, while the linear Stern-Volmer quenching constants obtained from fluorescence spectroscopic studies are 0.83 × 10³ and 0.71 × 10³ M^-1, respectively. The cyclic voltammograms of the complexes show a pseudo-reversible electrochemical process.

Full text of DOI:10.1007/s11243-013-9779-4

Transition Met Chem (2014) 39:111–118
DOI 10.1007/s11243-013-9779-4
Crystal structures, cyclic voltammetry and DNA binding of two
mononuclear nickel(II) complexes
•
Yang Wang Jia-Wei Mao Chao Ding
•
•
•
Zhi-Quan Pan Jian-Fen Li Hong Zhou
•
Received: 29 August 2013 / Accepted: 29 October 2013 / Published online: 12 November 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract Two unsymmetrical complexes, [NiL1]ClO4
(1) and [NiL2]ClO4 (2) have been synthesized and char-
acterized by IR, UV, ES-MS and single crystal X-ray dif-
fraction, where HL1 and HL2 are, respectively, the [1?1]
condensation products of 2,6-diformyl-4-X-phenol (X = F
or CH3) with N1-(2-aminoethyl)-N2-(4-nitrobenzyl) ethane-
1,2-diamine. The coordination geometry of the metal in
both complexes can be approximately described as square
cyclic voltammograms of the complexes show a pseudo-
reversible electrochemical process.
Introduction
Schiff base complexes play a significant role in the field of
coordination chemistry and have been extensively studied,
mainly due to their structural varieties and structure-related
bioactivities [1–4]. Schiff base complexes can be used as
DNA structural probes, oxidation catalysts, DNA cleaving
agents, potential anticancer drugs, enzyme models and so
on [5–8]. In general, the bioactivities of such complexes
depend on the structures of the ligands and the metal types.
They can bind to DNA in many non-covalent modes,
involving ionic bonding, hydrogen bonding and hydro-
phobic interactions, and many can cleave the DNA.
Complexes with aromatic rings can bind DNA through p–p
stacking as well as coordination interactions with the metal
center. Our previous studies showed that dinuclear Ni(II)
complexes containing three benzyl groups have interesting
DNA cleavage and binding activities [9]. In continuation of
this approach, two mononuclear Ni(II) complexes [NiL]?,
where HL is the [1?1] condensation product of the same
aldehyde with different amines, have been synthesized and
characterized. The DNA binding activities of these com-
plexes have been also investigated. The synthesis of the
complexes is shown in Scheme 1.
˚
planar with a mean plane deviation of 0.032 A in complex
˚
1 and 0.027 A in complex 2, respectively. The binding
activities of the complexes toward calf-thymus DNA have
been analyzed by spectroscopy and viscosity methods. The
binding constants of 1 and 2 obtained from UV spectro-
scopic studies are 5.43 9 105 and 1.83 9 105 M-1
,
respectively, while the linear Stern–Volmer quenching
constants obtained from fluorescence spectroscopic studies
are 0.83 9 103 and 0.71 9 103 M-1, respectively. The
Electronic supplementary material The online version of this
article (doi:10.1007/s11243-013-9779-4) contains supplementary
material, which is available to authorized users.
Y. Wang ꢀ J.-F. Li
School of Chemical and Environmental Engineering, Wuhan
Polytechnic University, Wuhan, People’s Republic of China
Y. Wang ꢀ J.-W. Mao ꢀ C. Ding ꢀ Z.-Q. Pan ꢀ H. Zhou (&)
Key Laboratory for Green Chemical Process of Ministry of
Education, Wuhan Institute of Technology, Wuhan,
People’s Republic of China
e-mail: hzhouh@126.com
Experimental
J.-W. Mao
National Quality Supervision and Inspection Center For
Petroleum and Natural Gas Products, Sichuan Institute of
Product Quality Supervision and Inspection, Chendu,
People’s Republic of China
All solvents were obtained from commercial sources and
used without purification. The aldehydes 2,6-diformyl-4-X-
phenol (X = F or CH3) were prepared according to the
123

112
Transition Met Chem (2014) 39:111–118
Scheme 1 The synthesis of the
complexes
literature methods [10, 11]. N1-(2-aminoethyl)-N2-(4-
nitrobenzyl)ethane-1,2- diamine was prepared by our pre-
vious method [9]. Tris(hydroxymethyl)amino-methane
(Tris) and ethidium bromide (EB) were purchased from
Toyobo Co. Calf-thymus DNA(CT-DNA) was obtained
from Sigma.
Table 1 Crystal data and structure refinement for complex 1
Empirical formula
Formula weight
Crystal system
Space group
C19H20FN4NiO4ClO4
545.55
Triclinic
P-1
˚
a (A)
9.3562(15)
9.7874(16)
13.700(2)
110.688(2)
96.185(2)
106.833(2)
1,092.0(3)
2
IR spectra were measured using KBr disks on a Vector
22 FT-IR spectrophotometer. Elemental analyses were
performed on a Perkin–Elmer 240 analyzer. Electrospray
mass spectra were determined on a Finnigan LCQ, using
methanol as mobile phase, and a sample concentration of
1.0 mmol/dm3. UV–Vis spectra were recorded on a UV-
2450 spectrophotometer. Fluorescence spectroscopic stud-
ies were carried out on an F-7000 FL Spectrophotometer.
Cyclic voltammograms were run on a CHI model 750 B
electrochemical analyzer in DMF solution containing tet-
ra(n-butyl)ammonium perchlorate (0.1 M) as the support-
ing electrolyte. A three-electrode cell was used, which was
equipped with a glassy carbon-working electrode, a plati-
num wire as the counter electrode and a Ag/AgCl electrode
as the reference electrode. Scanning rates were in the range
of 50–200 mV s-1. Half-wave potentials, E1/2, were the
average of Epa and Epc, the measured error was 2 mV.
Viscosity experiments were conducted on an Ubbelohde
viscometer.
˚
b (A)
˚
c (A)
a (°)
b (°)
c (°)
3
˚
Volume (A )
Z
D (calc) (g/cm3)
Mu (Moka) [/mm]
F(000)
1.659
1.075
560
Crystal size[mm]
Temp., K
0.22 9 0.24 9 0.28
293
˚
Mo Ka radiation(A)
h range (deg)
0.71073
2.3, 26.0
Nref, Npar
4204, 307
6098, 4204, 0.003
3473
Tot., uniq. data R(int)
Observed data [I [ 2.0sigma(I)]
R, wR2, S
0.0549, 0.1438, 1.02
-0.43, 0.45
3
Min. and Max. Resd. Dens.[e/A ]
˚
Preparation of [NiL1]ClO4 (1)
C19H20N4FNiO4ClO4 (%): C, 41.8; H, 3.7; N, 10.3. Found:
C, 41.6; H, 3.4; N, 10.6. IR(KBr, m/cm-1): 3,209 m(N–H),
1,635 m(C=N), 1,688 m(C=O), 1,094, 624 m(ClO4-).
To a solution of 2,6-diformyl-4-fluorophenol (0.084 g,
0.5 mmol) and Ni(OAc)2ꢀ4H2O (0.124 g, 0.5 mmol) in
anhydrous ethanol (15 mL), a mixture of N1-(2-amino-
ethyl)-N2-(4-nitrobenzyl)ethane-1,2-diamine
hydrobro-
Preparation of [NiL2]ClO4 (2)
mide (0.241 g, 0.5 mmol) and NaOH (0.060 g, 1.5 mmol)
in distilled water (10 mL) was added dropwise. The mix-
ture was stirred at ambient temperature for 8 h followed by
addition of NaClO4 (0.070 g, 0.5 mmol). The resulting
solution was stirred for further 8 h and filtered. Orange
block crystals suitable for the X-ray measurement were
obtained by evaporation of the filtrate at room temperature
for 2 weeks. Yield: 0.104 g (38 %). Anal. Calc. for
Complex 2 was prepared by the same procedure as
described above, except that 2,6-diformyl-4-methylphenol
was used instead of 2,6-diformyl-4-fluorophenol. Yield:
0.139 g (51 %). Anal. Calc. for C20H23N4NiO4ClO4 (%):
C, 44.4; H, 4.3; N, 10.3 Found: C, 44.4; H, 4.3; N, 10.4.
IR(KBr, m/cm-1): 3,210 m(N–H), 1,628 m(C=N), 1,681
m(C=O), 1,093, 624 m(ClO4-).
123

Transition Met Chem (2014) 39:111–118
113
Crystal structure determination
fluorescence was recorded [17]. The Stern–Volmer
quenching constant was determined from the equation I0/
I = 1 ? K[Q] [18], where I0 and I are the emission
intensities in the absence and the presence of the complex,
respectively.
The crystallographic data were measured on a Bruker AXS
˚
SMART diffractometer (Mo Ka radiation, 0.71073 A). Data
reduction and cell refinement were performed with the
SMART and SAINT programs. The structures were solved by
direct methods (Bruker SHELXTL) and refined on F2 by full-
matrix least squares (Bruker SHELXTL) using all unique data
[9]. Hydrogen atoms were located geometrically and refined
in riding mode. The non-H atoms were refined with aniso-
tropic displacement parameters. Calculations were performed
using the SHELX-97 crystallographic software package. The
crystallographic data of complex 1 are summarized in
Table 1, and the detailed crystallographic data of complex 2
have been reported in our previous work [12].
Viscosity measurements were carried out using a capil-
lary viscometer at a constant temperature (25.0 0.1 °C).
Each set of data was measured three times, and the averages
are presented as (n/n0)1/3 versus molar ratio of complex to
DNA [19], where n and n0 are the viscosity of DNA in
presence and absence of the complex, respectively. Vis-
cosity values were calculated from the observed flow times
of DNA containing solutions corrected for the flow time in
buffer alone (t0), n = (t - t0) [20]. Flow times were mea-
sured with a digital stopwatch.
DNA binding experiments
Results and discussion
The CT-DNA was dissolved in 100 mL Tris–HCl buffer
(50 mM Tris–HCl, 50 mM NaCl and pH = 7.2). The
concentration of CT-DNA was calculated according to
Beer–Lambert’s Law A = ebc, where e is the molar
extinction coefficient, 6,600 M-1cm-1 (nudeotide)-1 at
260 nm [13]. The absorption ratio A260/A280 was within the
range of 1.8–2.0, indicating that this solution was suffi-
ciently free from protein [14]. The calculated DNA con-
centration was 3.47 9 10–4 M.
Synthesis and characterization
In the IR spectra of the complexes, strong absorption bands
at 1,635 cm-1 for 1 and 1,628 cm-1 for 2 are observed,
which are assigned to the m(C=N) stretching vibrations.
Bands at 1,681 cm-1 for 1 and 1,688 cm-1 for 2 are
attributed to the m(C=O) stretching vibrations, indicating
that there is a CHO group in each complex. The slight
wavelength differences are attributed to the different sub-
stituents (–F in 1, –CH3 in 2). Comparing the IR spectra of
the free ligands and their complexes, the m(N–H) absorp-
tion bands shift from 3,439 cm-1 for the free ligands to
3,209 1 cm-1 for the complexes, consistent with coor-
dination of the nitrogen atoms to the metal. Strong bands at
1,093 1 and 624 cm–1 for the complexes can be attrib-
The UV–vis experiments were carried out at fixed
concentration of the complexes (50 lM) and varying the
concentration of DNA (0–50 lM). Absorption spectra were
recorded using cuvettes of 1 cm path length. Before mea-
surements, the mixtures of DNA and complex were incu-
bated for 30 min at room temperature. The intrinsic
binding constant was determined using the equation [15]:
-
uted to the ClO4 anions [1]. Hence, one spectroscopic
data are in agreement with the crystal structures of the
complexes.
½DNAꢁ=Eap ¼ ½DNAꢁ=E þ 1=ðKbEÞ
where Eap = ea - ef, E = eb - ef, ea, ef and eb correspond
to Aobsd/[Ni], the extinction coefficient for the free com-
plex, and the extinction coefficient for the complex in the
fully bound form, respectively. Plots of [DNA]/(ea - ef)
versus [DNA] gave the binding constant Kb as the ratio of
the slope to the intercept [16].
The UV–Vis spectra (see supporting material S1) of
complexes 1 and 2 both show sharp absorptions at 238 nm,
assigned to p ? p* inter ligand transitions, plus moderate
absorptions at 432 nm for 1 and 429 nm for 2, which may
be due to the d8 configuration of Ni2?, giving rise to a
charge transfer (CT) transition [21].
To further clarify the interactions between these com-
plexes and DNA, the decrease in fluorescence intensity of
the EB–DNA system (EB = ethidium bromide) caused by
intercalation of the complexes has been measured, as
shown in Fig. 7. The experiments were performed at a
fixed EB-DNA solution concentration (2 9 10–5 M EB,
2.87 9 10–5 M DNA), to which increments of the complex
solutions ranging from 0 to 1.2 9 10–4 M were added.
The solutions were equilibrated for 10 min before the
The ES–MS spectra of the complexes in methanol
solution are shown in supporting material S2. The spectra
are dominated by peaks at m/z 445.08 for 1 and 441.08 for
2, corresponding to [NiL1]? (calc. 445.07) and [NiL2]?
(calc. 441.10), respectively, indicating that the two cations
are stable in methanol solution. These assignments are
supported by the good agreement between the theoretical
and experimental isotope distributions, shown in the inset
of the figures.
123

114
Transition Met Chem (2014) 39:111–118
are invariably smaller than those associated with six-
membered rings [22, 23]; the corresponding values in
complex 1 are \N1–Ni1–N2 = 86.08(17)°, \N2–Ni1–
N3 = 87.09(16)°, \O1–Ni1–N1 = 95.78(15)°, which are
in accordance with the general trend. The coordination
polyhedron of the metal center can be approximately
described as square planar, such that the four coordination
atoms are coplanar with a mean plane deviation of
˚
0.032 A, while the deviation of Ni(II) from the base plane
˚
is 0.051 A. The two aromatic rings in the ligand are almost
perpendicular, with a dihedral angle of 83.5°. The coordi-
˚
nation bond distances are in the range of 1.825–1.944 A,
and the Ni–O distances are shorter than the Ni–N distances.
In the crystal, cations and anions are linked by weak C(N)–
HꢀꢀꢀO hydrogen bonding, and the relevant H-bond param-
eters are given in Table 3. The hydrogen bonding inter-
actions of one molecular unit with adjacent ones are
depicted in Fig. 2.
Fig. 1 Perspective views of the complex 1. Hydrogen atoms are
omitted for clarity (ellipsoids are drawn at 30 % probability level)
Complex 2 is almost isostructural with complex 1,
except for the different substituent on the phenyl groups, F
in L1 and CH3 in L2. The angles within the chelate rings
around the nickel atom are \N2–Ni1–N3 = 86.28(13)°,
\N3–Ni1–N4 = 85.82(14)°, \O1–Ni1–N2 = 96.39(14)°,
respectively, similar to those of complex 1. The hydrogen
bonding interactions of one molecular unit with adjacent
ones are depicted in supporting material S3.
Crystal structures of the complexes
Perspective views of the complexes are given in Figs. 1
and 3, together with the atom numbering schemes. Selected
bond lengths and angles are listed in Table 2.
The molecular structure of complex 1 contains a
[NiL1]? cation and a ClO4 anion. The deprotonated
-
ligand coordinates with nickel(II) in a tetradentate manner,
by means of three amine nitrogen atoms and one oxygen
from phenol. This arrangement results in one six-mem-
bered chelate ring and two adjacent five-membered chelate
rings. According to the literature, the angles around the
nickel atom associated with five-membered chelate rings
Cyclic voltammetry
The cyclic voltammograms of the complexes were recor-
ded in DMF solution using tetrabutyl ammonium
Fig. 2 View of the hydrogen
bonding interactions of one
molecular unit with adjacent
ones in complex 1. Hydrogen
atoms except those involved in
hydrogen bonding are omitted
for clarity
123

Transition Met Chem (2014) 39:111–118
115
˚
Table 3 Hydrogen bonding distances (A) and angles (°) for the
complex 1
˚
˚
D–HꢀꢀꢀA
d (D—H) A d(HꢀꢀꢀA) A D(DꢀꢀꢀA)
\DHA°
˚
A
N2–H2ꢀꢀꢀO3
0.9100
0.9100
0.9100
0.9100
0.9300
0.9700
0.9700
0.9700
2.5500
2.3700
2.2100
2.5200
2.4300
2.5200
2.4400
2.4700
3.369(5)
3.136(6)
3.015(6)
3.332(7)
3.324(5)
3.259(6)
3.292(6)
3.185(5)
150.00
142.00
147.00
149.00
161.00
133.00
146.00
130.00
N2–H2ꢀꢀꢀO4
N3–H3AꢀꢀꢀO13
N3–H3AꢀꢀꢀO14
C7–H7ꢀꢀꢀO12
C9–H9BꢀꢀꢀO5
C9–H9BꢀꢀꢀO12
C11–
H11AꢀꢀꢀO11
C12–H12BꢀꢀꢀO1 0.9700
2.5900
2.5600
2.5900
2.4400
3.017(5)
3.271(6)
3.324(5)
2.768(6)
107.00
133.00
136.00
101.00
C15–H15ꢀꢀꢀO5
C18–H18AꢀꢀꢀO1 0.9300
C19–H19ꢀꢀꢀO1 0.9300
0.9300
Fig. 3 Perspective views of the complex 2. Hydrogen atoms are
omitted for clarity (ellipsoids are drawn at 30 % probability level)
UV spectroscopic studies
˚
Table 2 Selected bond lengths (A) and bond angles (°) for the
complex 1
Electronic absorption spectroscopy is a useful technique
for characterization of the binding mode between metal
complexes and DNA [24]. The UV absorption spectra of
the complexes are shown in Figs. 5 and 6. Complex 1
shows a moderate absorption at 431 nm, which can be
assigned to a charge transfer transition. Upon the addition
of CT-DNA solution, the spectrum of complex 1 shows
obvious hyperchromicity, up to about 17 %. The hyper-
chromic effect might be ascribed to electrostatic binding,
or to partial uncoiling of the helix structure of DNA,
making the DNA bases more exposed [3, 25]. The value of
binding constant Kb obtained from the plot of [DNA]/Eap
Bond
Distance
Bond
Distance
Ni1–O1
Ni1–N2
1.825(3)
1.895(4)
Ni1–N1
Ni1–N3
1.853(4)
1.944(3)
Bond
Angles
Bond
Angles
O1–Ni1–N1
O1–Ni1–N2
O1–Ni1–N3
95.78(15)
177.65(15)
90.93(14)
N1–Ni1–N2
N1–Ni1–N3
N2–Ni1–N3
86.08(17)
171.46(17)
87.09(16)
versus [DNA] is 5.43 9 105 M-1
.
perchlorate (TBAP) as supporting electrolyte in the scan
range from -1.0 to -0.5 V. The scan rate was varied in the
range of 50–200 mVs-1, with the results shown in Fig. 4.
The cyclic voltammograms of complex 1, showed a pair
of anodic and cathodic peaks, which can be attributed to
The UV absorption spectrum of the complex 2 also shows
absorption at 431 nm, but addition of DNA now results in
hypochromism, reaching a maximum of about 14 %. The
binding constant Kb = 1.83 9 105 M-1, indicating that
complex 2 interacts with DNA by intercalation, involving
strong stacking interactions of the aromatic chromophore of
the complex with the DNA base pairs. Comparing the DNA
binding constants of the complexes with those of similar
mononuclear Ni(II) complexes [26–29], it is found that all
these complexes have considerable binding capacity.
Although the DNA binding constants vary with their struc-
tural differences, the interaction modes of these complexes
with DNA are all intercalative. There is no obvious rela-
tionship between the structures of the complexes and their
DNA binding properties. Nevertheless, considering that the
interaction mode of DNA with these complexes is interca-
lative, the aromatic groups in the complexes are likely to play
an important role in the binding.
the Ni(II)–Ni(I) redox couple. For the scan at 100 mV s-1
,
the cathodic peak potential (Epc) is -0.887 V and the
anodic peak potential (Epa) is -0.746 V; the half-wave
potential calculated from (Epc ? Epa)/2 is -0.817 V; the
separation (DEp) of the anodic and cathodic peak potentials
is 0.141 V and the ratio of cathodic and anodic peak cur-
rents ipc/ipa is 2.3, indicative of a pseudo-reversible elec-
trochemical process [9]. The values of ipc/m1/2 (m = 50,
100, 200 mVs-1, respectively) are approximately the same,
indicating that this process is mainly diffusion controlled
[1]. In complex 2, at a scan of 100 mV s-1, E1/2 =
-0.816 V, DEp = 0.134 V, ipc/ipa = 2.3, again character-
istic of a pseudo-reversible electrochemical process.
123

116
Transition Met Chem (2014) 39:111–118
0.5
0.4
0.5
0.4
c
c
b
b
0.3
0.3
a
a
0.2
0.2
0.1
0.1
0.0
0.0
-0.1
-0.2
-0.3
-0.1
-0.2
-0.3
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
E / V
E / V
Fig. 4 Cyclic voltammograms in DMF solution in the range from -1.0 to -0.5 V (supporting electrolyte = TBAP, 0.1 M), Scan rate,
ma = 50 mV s-1, mb = 100 mV s-1, mc = 200 mV s-1 (left: complex 1, right: complex 2)
a
b
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
6
↑
1
0.90
0.85
0.80
0.75
0.70
0.65
6
↑
1
2.5
2.0
1.5
1.0
0.5
0.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
[DNA] /10-4
M
420
440
460
480
500
520
300
350
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 5 a Absorption spectra of complex 1 in Tris–HCl buffer
(pH = 7.2) solution upon addition of DNA. [complex 1] =
100 lM, [DNA] = 0 – 0.87 lM. Arrows show the absorption
changes upon increasing the concentration of DNA. b Enlarged plot
of the absorption changes at 431 nm and plot of [DNA]/Eap versus
[DNA] for complex 1
a 0.6
b
1
↓
8
1
7
0.60
0.5
↓
6
6
6
5
4
0.4
0.3
0.2
0.1
0.0
0.55
3
2
1
0
0.50
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
[DNA] / 10-4
M
0.45
420
440
460
480
500
520
300
350
400
450
500
550
600
650
Wavelength (nm)
Wavelength (nm)
Fig. 6 a Absorption spectra of complex 2 in Tris–HCl buffer
(pH = 7.2) solution upon addition of DNA. [complex 2] =
100 lM, [DNA] = 0–0.87 lM. Arrows show the absorption changes
upon the increasing concentration of DNA. b Enlarged plot of the
absorption changes at 431 nm and plot of [DNA]/Eap versus [DNA]
for complex 2
123

Transition Met Chem (2014) 39:111–118
117
100
80
60
40
20
0
1.30
1.20
1.15
1.10
1.05
1.00
0.95
1
1
100
↓
↓
1.25
5
5
1.20
80
1.15
1.10
60
1.05
40
20
0
1.00
0
2
4
6
8
10
12
0
2
4
6
8
10 12 14 16 18 20 22
[complex]/10-5
M
[complex]/10-5
M
550
600
650
700
750
800
550
600
650
700
750
800
Wavelength(nm)
Wavelength(nm)
Fig. 7 Emission spectra of EB bound to DNA in the absence (1) and presence (2–5) of the complexes. Inset Stern–Volmer quenching plots of
EB bound to DNA. [EB] = 20 lM, [DNA] = 28.7 lM, [complex] 1–5 = 0–200 lM; kex = 520 nm (left: complex 1; right: complex 2)
Fluorescence spectroscopic studies
1.10
In order to further investigate the binding of these com-
1
1.08
1.06
1.04
1.02
1.00
plexes to DNA, their ability to displace EB from the
intercalated EB-DNA complex was investigated [2, 25].
Emission spectra of the EB-DNA system in the absence
and presence of the complexes and the associated Stern–
Volmer quenching plots are shown in Fig. 7. The emission
spectra of EB bound to DNA exhibit a single peak at
596 nm. As the concentration of complex 1 is increased,
the emission intensity decreases. The linear Stern–Volmer
quenching constant obtained from the slope of the plot of
I0/I versus [Q] is 0.83 9 103 M-1for complex 1.
2
0.0
0.2
0.4
0.6
0.8
1.0
The same situation occurs for complex 2; the emission
intensity decreases as the concentration of complex 2 is
increased, and the linear Stern–Volmer quenching constant
KSV is 0.71 9 103 M-1, which is very similar to that of a
dinuclear nickel(II) complex with a similar ligand con-
taining three phenzyl groups [9]. However, the binding
constant of 2 is 4.3 times lower than that of the dinuclear
nickel(II) complex, suggesting a synergetic effect of the
metal centers in DNA binding. The binding and quenching
constants of complex 1 are larger than those of 2, which
can be only ascribed to the different substituents. The
fluorine atom in 1 can interact with the adjacent phenyl
group; together with its smaller steric requirement, this
would increase the p–p stacking interaction between the
base pairs of DNA and complex 1, leading to a stronger
interaction.
r
Fig. 8 Effects of increasing amounts of complexes 1 and 2 on the
relative viscosities of CT-DNA solutions at 25.0 0.1 °C;
[DNA] = 200 lM, r = [complex]/[DNA]
will loosen the DNA helix structure, resulting in the
overall increase of DNA length and so increasing the
DNA solution viscosity. In contrast, hydrogen bonding
and van der Waals interactions along the groove produce
no significant change in the viscosity of DNA solutions
[30].
The results of viscosity studies are shown in Fig. 8. As
the concentration of the complexes is increased, the vis-
cosity of the DNA solution also increases, providing fur-
ther evidence that the binding mode of the complexes with
DNA is intercalation. The effect of complex 1 on the vis-
cosity is much higher than that of 2 at the same ratio of
DNA/[complex], revealing that the complex 1 intercalates
more strongly than 2. This result is in agreement with the
observation that the binding constant of 1 is 3 times larger
than that of 2.
Viscosity studies
Viscosity measurements are usually regarded as an
effective physical method for the characterization of DNA
binding mode, since intercalation into the DNA base pairs
123

118
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Conclusions
H (2004) Inorg Chem 43:
Two unsymmetrical mononuclear nickel(II) complexes
with different substituents have been prepared and char-
acterized by spectroscopy, cyclic voltammetry and single
crystal X-ray diffraction. The DNA binding abilities vary
somewhat with the substituent, such that binding of the
F-substituted complex is higher than that of CH3-substi-
tuted complex. Comparison of these results with those
available in the literature suggests that a synergetic effect
from more than one metal center would increase the DNA
binding ability.
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Supplementary material
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Trans Met Chem 36:489–498
Crystallographic data have been deposited with the Cam-
bridge Crystallographic Data Center. The CCDC number
of complex 1 is 950658. Copy of the data can be obtained
free of charge on application to The Director, CCDC, 12
Union Road, Cambridge CB2, 1EZ, UK (fax: ?44-1223-
336033); E-mail: deposit@ccdc.cam.ac.uk or http://www.
ccdc.cam.ac.uk).
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Acknowledgments We acknowledge financial support from the
Natural Science Foundation of China (21171135 and 20971102) and
21. Sun L, Li YG, Dong XW, Guo MX (2012) Trans Met Chem
37:361–366
China Hubei Provincial Science
(2011BFA020).
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