Y. Wang et al. / Journal of Molecular Structure 1036 (2013) 361–371
363
2.3.3. Preparation of [Ni2L2(OAc)2]Brꢀ0.5H2O (2)
Complex 2 was prepared by the same procedure as described
above except that 2,6-diformyl-4-bromophenol was used instead
of 2,6-diformyl-4-methylphenol. Yield: 0.155 g (31.3%). Anal. Calc.
for C34H43N8Br2Ni2O9.5 (%): C, 41.13; H, 4.37; N, 11.29 Found: C,
41.19; H, 4.33; N, 11.28. IR(KBr,
m m(N@H), 1647
/cmꢂ1): 3276
m(C@N), 1399, 1541
m
(OAcꢂ).
Scheme 2. General redox process of the free and DNA bound complexes.
2.4. Cleavage of 4-nitrophenyl phosphate disodium salt hexahydrate
(pNPP) experiments
illustrate the binding affinity of Ni(II) and Ni(I) to DNA, the follow-
ing equation:
The hydrolysis activity of the complexes toward 4-nitrophenyl
phosphate (pNPP) was examined by UV–vis spectroscopy in
DMF/Tris–HCl buffer (1/3, v/v) solution at pH = 7.4. The kinetic
measurements were performed by monitoring the hydrolysis prod-
uct (4-nitrophenol, NP) at 25 °C at a wavelength of 406 nm. Stock
solutions were freshly prepared before performing the kinetic
measurement and the ionic strength was maintained at
I = 0.10 M with potassium chloride. The pseudo-first-order rate
constant (kobs (sꢂ1)) of the cleavage of NPP was obtained by the
method of initial rate.
E0 ꢂ E0 ¼ 0:059 logðKred=Kox
Þ
ð2Þ
b
f
was applied: where E0f and Eb0 are the formal potentials of the Ni(II)/
Ni(I) couple in the free and bound forms, respectively. The binding
constant was calculated according to the equation:
1=½DNAꢃ ¼ Kð1 ꢂ AÞ=½1 ꢂ ði=i0Þꢃ ꢂ K
ð3Þ
where K is the binding constant, i and i0 are the peak currents with
and without DNA, A is the proportionality constant, meanwhile the
binding constant was calculated according to the equation:
2.5. DNA-binding experiments
Cb=Cf ¼ K ½free base pairsꢃ=s
ð4Þ
where s is the binding site size in terms of base pairs (base pairs can
be expressed as [DNA]/2), K is binding constant, Cf and Cb denote
the concentration of the free and DNA-bound species, respectively.
The Cb/Cf ratio was determined by
CT-DNA stock solution was prepared by dissolving CT-DNA in
Tris–HCl buffer (100 mL, 50 mM Tris–HCl, 50 mM NaCl, pH = 7.2).
The concentration, in terms of nucleotide, of CT-DNA stock solution
was determined by employing an extinction coefficient of
6600 Mꢂ1cmꢂ1 (nucleotide)ꢂ1 at 260 nm [15]. The ratio of the UV
absorbance of the CT-DNA solution at 260 and 280 nm is about
1.83:1, indicating that this solution is free from protein [16]. The
DNA concentration is 3.47 ꢁ 10ꢂ4 M calculated from Beer–Lam-
Cb=Cf ¼ ði0 ꢂ iÞ=i
ð5Þ
where i and i0 represent the peak currents of the complexes in the
presence and absence of DNA[22].
Viscosity measurements were performed to further clarify the
interaction between metal complexes and DNA [23,24]. Experi-
bert’s Law A =
The UV–vis experiments were carried out in the condition of
fixing the concentration of complexes (50 M) and varying the
concentrations of DNA (0–50 M). The intrinsic binding constant
e
bc.
l
ments were carried out using a capillary viscometer at
l
25.0 0.1 °C. Each set of data measured in triplicate, averages are
presented as (N/N0)1/3 vs. molar ratio of complex to DNA, where N
is the viscosity of CT-DNA in the presence of the complex and N0
is the viscosity of CT-DNA alone. Viscosity values were calculated
from the observed flow time of CT-DNA containing solutions cor-
rected for the flow time in buffer alone (t0), N = (t ꢂ t0) [25].
(Kb) of the complexes with CT-DNA were determined using the
equation [17]:
½DNAꢃ=Eap ¼ ½DNAꢃ=E þ 1=ðKbEÞ; Eap
¼
ea
ꢂ
ef ; E ¼ eb
ꢂ
ef
ð1Þ
where ea ef and eb corresponding to Aobsd/[Ni], the extinction coef-
,
ficient for the free complex, and the extinction coefficient for the
complexes in the fully bound form, respectively. A plot of [DNA]/
3. Results and discussion
(ea
ꢂ
e
f) vs. [DNA] gives Kb as the ratio of slope to intercept [18].
3.1. Characterization of the complexes
Based on the fact that the strong fluorescence of EB intercalated
into DNA can be quenched by the replacement of EB with other
molecules [19], the binding interactions of the complexes with
DNA were also investigated by fluorescence quenching experi-
ment. The quenching experiments were conducted by adding ali-
quots of 0–1.2 ꢁ 10ꢂ4 M solutions of the Ni(II) complexes to the
solution containing 2 ꢁ 10ꢂ5 M EB and 2.8ꢁ10ꢂ5 M CT-DNA in
Tris–HCl buffer [20]. The Stern–Volmer equation I0/I = 1 + K[Q]
was used to determine the quenching constant [21], where I0 and
I are the emission intensities in the absence and the presence of
the complex, respectively.
Cyclic voltammograms were run on a CHI model 750 B electro-
chemical analyzer in DMF solution containing tetra(n-
butyl)ammonium perchlorate (TBAP) as the supporting electrolyte.
A three-electrode cell was used, which was equipped with a glassy
carbon-working electrode, a platinum wire as the counter elec-
trode and a Ag/AgCl electrode as the reference electrode. Scanning
rate was 100 mV sꢂ1. The solution was deaerated for 15 min before
measurements.
The complexes were obtained by the reaction of 2, 6-diformyl-
4-X-phenol (X = CH3, Br) and N1-(2-aminoethyl)-N2-(4-nitroben-
zyl)ethane-1,2-diamine in the presence of Ni(OAc)2ꢀ4H2O and
NaClO4 in water/ethanol solution. They are stable in air and have
a good solubility in DMF, DMSO, methanol and ethanol solution.
3.1.1. X-ray structure
The perspective view of [Ni2L1(OAc)2]+ (1) and [Ni2L2(OAc)2]+
(2) are given in Fig. 1, together with the atom numbering scheme.
The summary of the crystallographic data is listed in Table 1. Se-
lected bond lengths and angles relevant to the nickel coordination
polyhedrons are listed in Table 2.
The molecular structure of 1 contains a [Ni2L1(OAc)2]+ cation
and a ClOꢂ4 anion. The coordination polyhedron of each metal ion
in 1 can be approximately described as a distorted octahedron,
including three nitrogen atoms from the ligand, one oxygen from
phenol and two oxygen atoms from two acetate groups. The two
Ni ions have the same coordination environment. The basal plane
consists of O1, N1, O4 and N2 with the mean plane deviation of
In the cyclic voltammogram experiment, the redox process can
be considered as Ni(II)Ni(II)/Ni(II)Ni(I), shown in Scheme 2. To