Efficient DNA cleavage mediated by mononuclear mixed ligand copper(II) phenolate complexes: The role of co-ligand planarity on DNA binding and cleavage and anticancer activity

Article abstract of DOI:10.1016/j.jinorgbio.2012.04.018

The new mononuclear copper(II) complexes [Cu(L)(H₂O) ₂]⁺ 1 and [Cu(L)(diimine)]⁺ 2-6, where LH = 2-[(2-dimethylaminoethylimino)methyl]phenol and diimine = 2,2′-bipyridine (bpy) (2), or 1,10-phenanthroline (phen)

Full text of DOI:10.1016/j.jinorgbio.2012.04.018

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Journal of Inorganic Biochemistry 114 (2012) 94–105
Contents lists available at SciVerse ScienceDirect
Journal of Inorganic Biochemistry
journal homepage: www.elsevier.com/locate/jinorgbio
Efficient DNA cleavage mediated by mononuclear mixed ligand copper(II) phenolate
complexes: The role of co-ligand planarity on DNA binding and cleavage and
anticancer activity
Paramasivam Jaividhya a, Rajkumar Dhivya b, Mohamad Abdulkadhar Akbarsha c,d, Mallayan Palaniandavar a,e,
⁎
a
Centre for Bioinorganic Chemistry, School of Chemistry, Bharathidasan University, Tiruchirappalli 620024, Tamil Nadu, India
Department of Animal Science, Bharathidasan University, Tiruchirappalli 620024, Tamil Nadu, India
Mahatma Gandhi-Doerenkamp Center for Alternatives to Use of Animals in Life Science Education, Bharathidasan University, Tiruchirappalli, 620024, Tamil Nadu, India
Department of Food Sciences and Nutrition, College of Food Sciences and Agriculture, King Saud University, Riyadh 11451, Saudi Arabia
b
c
d
e
Central University of Tamil Nadu, Thiruvarur 610001, India
a r t i c l e i n f o
a b s t r a c t
The new mononuclear copper(II) complexes [Cu(L)(H2O)2]+ 1 and [Cu(L)(diimine)]+ 2–6, where LH=2-[(2-
dimethylaminoethylimino)methyl]phenol and diimine=2,2′-bipyridine (bpy) (2), or 1,10-phenanthroline
(phen) (3), or dipyrido[3,2-f:2′,3′-h]quinoxaline (dpq) (4) or dipyrido[3,2-a:2′,3′-c]phenazine (dppz) (5) or
11,12-dimethyldipyrido[3,2-a:2′,3′-c]phenazine (dmdppz) (6), have been isolated and characterized. The X-ray
crystal structures of 2 contains the monomeric complex molecule with a trigonal bipyramidal distorted square py-
ramidal (TBPDSP) coordination geometry, while 4 and 6 with square pyramidal distorted trigonal bipyramidal
(SPDTBP) coordination geometry. The amine nitrogen of −NMe2 group of the tridentate primary ligand is located
at one of the corners of the square plane in 2 and 6 but in the axial position in 4. The interaction of the complexes
with calf thymus DNA has been investigated using UV-visible and fluorescence spectroscopy, and viscosity measure-
ments to understand the effect of diimine co-ligands on the mode and extent of DNA binding. The complexes 4 and 5
interact with calf thymus DNA more strongly than the other complexes through partial intercalation of the extended
planar ring of the dpq (4) and dppz (5) co-ligands with the DNA base stack. All the complexes, except 1, effect the
double strand DNA cleavage of plasmid DNA and 5 cleaves plasmid DNA in the absence of a reductant at a concen-
tration (40 μM) lower than 4. It is remarkable that all the complexes display cytotoxicity against human breast can-
cer cell lines (MCF-7) and human cervical epidermoid carcinoma cell lines (ME 180) with potency higher than the
currently used chemotherapeutic agent cisplatin and that 5 exhibits cytotoxicity higher than the other complexes.
© 2012 Elsevier Inc. All rights reserved.
Article history:
Received 15 March 2012
Received in revised form 26 April 2012
Accepted 26 April 2012
Available online 8 May 2012
Keywords:
Mixed ligand copper(II) complexes
DNA binding
DNA cleavage
Anticancer activity
1. Introduction
ligands, and they are presently undergoing evaluation in clinical trials
[8,9]. The new compounds exploit passive and active targeting strategies
While many drug molecules are “organic” in nature, other elements
in the periodic table, particularly metals, offer a much more diverse
chemistry and have important therapeutic applications [1]. In modern
medicine, the most striking example is cisplatin, which is a metal coordi-
nation compound containing no organic units and is currently one of the
leading drugs used against cancer [2–7]. However, its effectiveness is
limited by its high toxicity and incidence of drug resistance. This has pro-
vided motivation for the search for transition metal-based drugs with a
wider spectrum of activity and lower systemic toxicities. Some of the re-
cent successes are compounds based on the bis-am(m)ineplatinum(IV)
and ruthenium-based coordination compounds with N-heterocyclic
to overcome aspects of drug resistance. So, it is essential to increase the
variety of potential metal-based drugs, which may achieve higher activ-
ity, enabling the administration of a lower dose, attack on different types
of tumor cells, overcome drug resistance problems, and exhibit better se-
lectivity and lower toxicity than cisplatin. So, there is considerable atten-
tion focused on the design of new metal-based anticancer drugs that
exhibit enhanced selectivity and novel modes of DNA interaction like
non-covalent interactions that mimic the mode of interaction of biomol-
ecules [10]. Copper [11–13] and ruthenium [14–23] complexes are
regarded as promising alternatives to platinum complexes and several
copper complexes [11–13,24,25] have been now proposed as potential
anticancer substances, demonstrating remarkable anticancer activity
and showing general toxicity lower than platinum compounds. So many
other novel transition metal complexes as well as small molecule based
antitumor agents have been developed and some of them are under clin-
ical trial [26–30]. Copper(II) complexes are considered the best alterna-
tives to cisplatin as copper is biocompatible, and exhibits a significant
⁎
Corresponding author at: Centre for Bioinorganic Chemistry, School of Chemistry,
Bharathidasan University, Tiruchirappalli 620024, Tamil Nadu, India. Tel.: +91 431
2407053; fax: +91 431 2407043.
E-mail addresses: palanim51@yahoo.com, palaniandavarm@gmail.com
(M. Palaniandavar).
0162-0134/$ – see front matter © 2012 Elsevier Inc. All rights reserved.
doi:10.1016/j.jinorgbio.2012.04.018

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95
role in biological systems. Also, synthetic copper(II) complexes have been
reported to act as pharmacological agents, potential anticancer and cancer
inhibiting agents [31–35]. Very recently, certain mixed ligand copper(II)
complexes, which strongly bind and cleave DNA, exhibit prominent anti-
cancer activities and regulate apoptosis [36–40]. Therefore, designing
suitable copper complexes for DNA binding and cleavage under both ox-
idative and hydrolytic conditions, depending upon the recognition ele-
ments in the ligand, is of remarkable importance in considering the
advantages of processes that produce fragments similar to those formed
by restriction enzymes [41–47].
The complexes, which cleave DNA through an oxidative pathway,
requires a co-reactant such as an oxidizing or reducing agent, light, or
redox-active metal center in addition to the principal cleavage agent
[48,49]. The disadvantage of oxidative cleavage agents is that they
produce diffusible radicals which give rise to multiple cleavage sites
by modifying the deoxyribose moiety, which results in fragments
that cannot be re-ligated [48,49]. On the other hand, agents that pro-
mote the hydrolytic cleavage of the phosphodiester backbone of DNA
do not suffer from these drawbacks [48,49]. Therefore, there has been
a substantial increase in the development of reagents suitable for
cleaving DNA hydrolytically under physiological conditions, which
could be useful not only in molecular biology but also in drug design.
The present work stems from our continued interest in designing
mixed ligand copper(II) complexes that are capable of cleaving DNA
in the absence of a reductant. As part of our ongoing interest in de-
veloping new metal complexes as dsDNA binders, we have decided
to explore the DNA binding properties of simple and mixed ligand
complexes. So, in the present investigation we have isolated mixed
ligand redox-active copper(II) complexes of the general formula
[Cu(L)(H2O)2](ClO4) 1 and [Cu(L)(diimine)](ClO4) 2–6, where LH
is 2-((2-dimethylamino)ethylimino)methyl)phenol and diimine is
2,2′-bipyridine (bpy) 2 or 1,10-phenanthroline (phen) 3 or dipyrido-
[3,2-d:2′,3′-f]-quinoxaline (dpq) 4 or dipyrido[3,2-d:2′,3′-f]phenazine
(dppz) 5 or 11,12-dimethyldioyrido[3,2-a:2′3′-c]phenazine (dmdppz)
6 (Scheme 1). As DNA is the primary pharmacological target of many
antitumor compounds, the interaction of the metal complexes 1–6
with calf thymus DNA (CT DNA) is of paramount importance in under-
standing the mechanism of tumor inhibition for the treatment of cancer.
We have chosen phenolate containing ligand as primary ligand as they
are considered privileged ligands in medicinal chemistry with great po-
tential for chemotherapeutic application [50]. Also, copper(II) complexes
containing phenolate ligands can show antiproliferative activities be-
cause of the properties of the coordinate ligands alone, or the structural
and electronic properties which are ascribed to their coordination
with metal center [50]. Very recently, we have reported mixed ligand
copper(II) complexes of the type [Cu(tdp)(tmp)](ClO4), where H(tdp)
is 2-[(2-(2-hydroxyethylamino)ethylimino)methyl]phenol as efficient
DNA- and protein-binding and -cleaving agents, which show prominent
cytotoxicity towards human cervical epidermoid carcinoma cell line (ME
180) [39]. Also, we have reported the mixed ligand copper(II) complexes
of the type [Cu(L-tyr)(diimine)](ClO4), where tyr is L-tyrosine with a
phenolate moiety, which act as efficient DNA binding and cleaving
agents and induce apoptotic mode of cancer cell death [40]. Very recent-
ly, Reedijk and coworkers have reported that the complex [Cu(pyrimol)
Scheme 1. Possible coordination geometries of simple and mixed ligand copper(II) complexes of primary (L) and diimine (N-N) co-ligands.

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P. Jaividhya et al. / Journal of Inorganic Biochemistry 114 (2012) 94–105
Cl], where HPyrimol is 4-methyl-2-[(pyrid-2-ylmethylene)amino]-
phenol, shows efficient self-activated DNA cleavage and cytotoxic effects
on L1210 murine leukemia and A2780 human ovarian carcinoma cell
lines [51]. Sadler and his co-workers have reported mixed ligand bis(-
salicylato)copper(II) complexes with diimines as co-ligands to
exhibit cytotoxic and antiviral activities [52]. Neves et al. reported
mononuclear copper(II) complexes of [Cu(HL1)Cl2], where H(L1) is
2-[(bis(pyridylmethyl)amino)methyl]-4-methyl-6-formylphenol effect
double-strand DNA cleavage and amide bond cleavage [53]. The choice
on the aromatic diimine co-ligand is made to find out which part of the
molecule is determinant in endowing strong DNA binding affinity on
the complex and also in the light of the considerations reported already
by our group. The propensity and mode of DNA binding of the complexes
has been studied by using absorption spectral titration, competitive DNA
binding studies and viscosity measurements. The DNA cleavage proper-
ties and anticancer activities of the complexes have been also investigat-
ed. Interestingly, the complex 5, which shows the strongest DNA binding
affinity and efficient cleavage of plasmid DNA due to the partially inter-
calating dppz coligand, exhibits the highest anticancer activity against
human breast cancer cell lines (MCF-7) and human cervical epidermoid
carcinoma cell lines (ME 180) with its potency being higher than that of
cisplatin.
corresponding buffer to required concentrations for all experiments.
The ability of 1–6 to cleave DNA was examined by following the conver-
sion of supercoiled plasmid DNA to open circular DNA or linear DNA
using agarose gel electrophoresis to separate the cleavage products.
2.3. Synthesis of ligands
2.3.1. Synthesis of 2-((2-dimethylamino)ethyliminomethyl)phenol
Salicylaldehyde (1.02 g, 10 mmol) in methanol (20 mL) was added
dropwise to N,N′-di-methylethylenediamine (0.88 g, 10 mmol) in meth-
anol (10 mL). The mixture was stirred for 24 h to get a bright yellow so-
lution. The resulting solution was evaporated, the yellow oily residue
dried in vacuum and used as such for preparing the copper(II) complexes
(yield, 1.83 g, 95%). 1H NMR (CDCl3, 400 MHz): (OH, J=5.0, 1.0, 1H,
HPy); 8.57 (s, 1H, HC=N); 8.33 (dt, J=7.8, 1.0, 1H, HPy); 7.81 (tq,
J=7.8, 1.8, 1H, HPy); 7.38–7.35 (m, 1H, HPy); 7.28–7.16 (m, 3H, HPh);
7.08 (dd, J=7.8, 1.3, 1H, HPh); 2.47 (3H, H3C).
2.3.2. Synthesis of diimine ligands
The diimine ligands dpq [56], dppz [57], and 11,12-dmdppz [57] were
synthesized according to literature protocols.
Caution! During handling of perchlorate salts of metal complexes
with organic ligands care should be taken because of the possibility
of explosion.
2. Experimental
2.1. Materials
2.4. Preparation of copper(II) complexes
The reagents and chemicals were obtained from commercial
sources (Sigma-Aldrich, USA; Himedia, India, Merck, India, Genei,
Bangalore, India). Copper(II) perchlorate hexahydrate, ethidium bro-
mide (EthBr), calf thymus (CT) DNA (highly polymerized and stored
at −20 °C) (Aldrich), N,N-dimethylethylenediamine (Aldrich), 2,2′-
bipyridine and salicylaldehyde (Loba), 1,10-phenanthrloine and 1,2-
phenylenediamine (Merck) and pUC19 supercoiled DNA, agarose
(Genei, Bangalore) were used as received. Ultrapure Milli Q water
(18.2 mΩ) was used in all experiments. Tris–HCl was prepared by the
reported procedure [54]. The commercial solvents were distilled and
then used for preparation of complexes.
2.4.1. Preparation of [Cu(L)(H2O)2](ClO4) (1)
The copper(II) complex was isolated by adding 2-((2-dimethylamino)
ethyl-iminomethyl)phenol (0.10 g, 0.5 mmol), which was deprotonated
by treating with triethylamine (0.5 mmol) in methanol solution, to
copper(II) perchlorate (0.19 g, 0.5 mmol) in methanol (10 mL) and then
stirring the solution at 40 °C for 1 h. The resulting precipitate was collect-
ed by suction filtration, washed with cold methanol and finally dried in
vacuum over P4O10. Anal. Calcd. for [Cu(L)(H2O)](ClO4): C, 45.23; H,
6.52; N, 9.52. Found: C, 45.43 45.23; H, 6.58; N, 9.63%. Yield: 0.15 g (60%).
2.4.2. Preparation of [Cu(L)(bpy)](ClO4) (2)
The complex 2 was prepared by addition of a methanolic solution
(10 mL) of bpy (0.078 g, 0.5 mmol) and 2-((2-dimethylamino)
ethyliminomethyl)phenol (0.10 g, 0.5 mmol), which was deprotonated
by using triethylamine (0.5 mmol), to a solution of copper(II) perchlo-
rate hexahydrate (0.185 g, 0.5 mmol) in methanol (10 mL) and then
stirring at 40 °C for 2 h. The green crystalline solid obtained was collect-
ed by suction filtration, washed with small amounts of cold methanol
and diethyl ether and then dried in vacuum. Green colored crystals of
2 suitable for X-ray diffraction studies were obtained by dissolving the
complex in aqueous methanol and allowing it to crystallize. Anal.
Calcd. for [Cu(L)(bpy)](ClO4): C, 49.23; H, 4.22; N, 10.52. Found: C,
49.41; H, 4.54; N, 10.98%. Yield: 0.21 g (80%).
2.2. Experimental methods
Microanalyses (C, H and N) were carried out with a Vario EL ele-
mental analyzer. UV–VIS spectroscopy was recorded on a Shimadzu
2450 UV-VIS spectrophotometer using cuvettes of 1 cm path length.
1H NMR spectra were recorded on a Bruker 400 MHz NMR spectrome-
ter. Mass spectrometry was performed on QTOF ESI-MS spectrometer.
Emission intensity measurements were carried out by using a Jasco F
6500 spectrofluorometer. The viscosity measurements were carried out
on a Schott Gerate AVS 310 automated viscometer thermostat at 25 °C
in a constant temperature bath.
Solutions of DNA in the buffer 50 mM NaCl/5 mM Tris–HCl buffer
(pH=7.1) in water gave the ratio of UV absorbance at 260 and
280 nm, A260/A280, of 1.9, indicating that the DNA was sufficiently free
of protein [55]. Concentrated stock solutions of DNA were prepared in
a 50 mM NaCl/5 mM Tris–HCl buffer and sonicated for 25 cycles, where
each cycle consisted of 30 s with 1 min intervals using Branson Ultra
Probe sonicator. The concentration of CT DNA in nucleotide phosphate
(NP) was determined by UV absorbance at 260 nm after 1:100 dilutions
by taking the extinction coefficient, ε260 as 6600 M−1 cm−1. Stock solu-
tions of DNA were stored at 4 °C and used after no more than 4 days.
Supercoiled plasmid pUC19 DNA was stored at: −20 °C and the con-
centration of pUC19 DNA in base pairs were determined by UV ab-
sorbance at 260 nm after appropriate dilutions taking ε260 as
13100 M−1 cm−1 [39]. Concentrated stock solutions of metal com-
plexes were prepared by dissolving calculated amounts of copper com-
plexes in respective amounts of solvent and diluted suitably with the
2.4.3. Preparation of [Cu(L)(phen)](ClO4) (3)
This complex was prepared by adopting the procedure used for the
isolation of 2 but using phen instead of bpy. The dark green crystalline
solid obtained was collected by suction filtration, washed with small
amounts of cold methanol and diethyl ether and then dried in vacuum.
Anal. Calcd for [Cu(L)(phen)](ClO4): C, 51.34; H, 4.34; N, 10.48. Found:
C, 51.69 (51.34); H, 4.56; N, 10.52%. Yield: 0.24 g (90%).
2.4.4. Preparation of [Cu(L)(dpq)](ClO4) (4)
This complex was prepared by adopting the procedure used for the
isolation of 2 but using dpq instead of bpy. Dark green colored crystals
of 4 suitable for X-ray diffraction studies were obtained by dissolving
the complex in aqueous methanol and allowing it to crystallize. Anal.
Calcd for [Cu(L)(dpq)](ClO4): C, 51.20; H. 3.62; N, 14.33. Found:
51.49; H, 3.95 N, 14.35%. Yield: 0.22 g (76%).

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97
Table 1
2.4.5. Preparation of [Cu(L)(dppz)](ClO4) (5)
Crystallographic data for 2, 4 and 6.
This complex was prepared by adopting the procedure used for
the isolation of 2 but using dppz instead of bpy. The green crystalline
solid obtained was collected by suction filtration, washed with small
amounts of cold methanol and diethyl ether and then dried in vacu-
um. Anal. Calcd. for [Cu(L)(dppz)](ClO4): C, 54.10; H, 3.60; N, 13.20.
Found: C, 54.72; H, 3.96; N, 13.33%. Electrospray ionization mass
spectrometry (ESI-MS): [Cu(L)(dppz)]+ displays a peak at m/z
536.18 (calcd. 536.14). Yield: 0.25 g (80%).
[Cu(L)(bpy)]ClO4 [Cu(L)(dpq)]ClO4 [Cu(L)(dmdppz)]ClO4
(2) (4) (6)
Empirical formula
C21 H23 Cl Cu N4 C25 H23 Cl Cu N6 C62 H57 Cl2 Cu2 N12
O5
O7
O10
Formula weight
Crystal system
Space group
510.42
Triclinic
P-1
618.48
Monoclinic
P2(1)/c
1328.18
Triclinic
P-1
Crystal size
0.20×0.20×0.22 0.12×0.10×0.08 0.20×0.20×0.22
Temperature (K)
293
293
293
λ, Ǻ (Mo Kα
)
1.158
0.977
0.812
2.4.6. Preparation of [Cu(L)(dmdppz)](ClO4) (6)
a, Ǻ
b, Ǻ
c, Ǻ
α °
β°
γ°
Z
Density
7.430 (2)
12.212 (3)
12.693 (3)
88.72 (2)
73.13 (3)
84.52 (2)
2
8.24400(6)
17.3817(5)
18.7260(6)
90.00
97.197 (3)
90.00
13.4467(7)
15.0111(7)
16.2537(8)
88.429(4)
82.144(3)
80.612(3)
2
This complex was prepared by adopting the procedure used for the
isolation of 2 but using 11,12-dmdppz instead of bpy. The green crystal-
line solid obtained was collected by suction filtration, washed with small
amounts of cold methanol and diethyl ether and then dried in vacuum.
Green colored crystals of 6 suitable for X-ray diffraction studies were
obtained by dissolving the complex in aqueous methanol and allowing
it for crystallization. Anal. Calcd. for [Cu(L)(dmdppz)](ClO4): C, 56.02,
H, 4.60, N, 12.33. Found: C, 56.10; H, 4.60, N, 12.65%. Yield: 0.24 g (72%).
4
1.545
1.543
1.376
(calculated),
Mg/m3
θ for data
collection
Refinement
method
1.68, 34.12
1.60, 29.61
1.37, 22.33
Full-matrix least- Full-matrix least- Full-matrix least-
2.5. X-ray crystallography
squares on F2
squares on F2
31029/7462
squares on F2
43435/8113
Unique reflections 26976/7588
The single crystals of 2, 4 and 6 of suitable size selected from the
mother liquor were mounted on the tip of a glass fiber and cemented
using epoxy resin. Intensity data for the crystals were collected using
MoKα (λ=0.71073 Å) radiation on a Bruker SMART APEX diffractom-
eter equipped with a CCD area detector at 100 and 293 K. The SMART
[58] program was used for collecting frames of data, indexing reflec-
tions, and determination of lattice parameters; SAINT program for inte-
gration of the intensity of reflections and scaling; SADABS [59] program
for absorption correction, and the SHELXTL [60] program for space
group and structure determination, and least-squares refinements on
F2. The structure was solved by heavy atom method and other non-
hydrogen atoms were located in successive difference Fourier synthe-
ses. Crystal data and additional details of the data collection and refine-
ment of the structure are presented in Table 1. The selected bond
lengths and angles are listed in Table 2.
[R(int)
]
[R (int)=0.028] [R (int)=0.0306] [R (int)=0.1146]
F (000)
526
1268
1370
aR indices
(all data)
S
R=0.0518,
wR2=0.1537
1.051
R=0.0520,
wR2=0.1527
1.077
R=0.0806,
wR2=0.2126
0.997
Largest difference
in peak and hole,
e Ǻ−3
1.052, −0.756
0.548, –0.772
1.105, –0.384
R=Σ | | Fo | - | Fc| |/Σ| Fo |, bwR2 ={Σ w[(Fo2 –Fc2)2/Σ w [(Fo2)2]}1/2
.
a
Table 2
Selected bond distances (Ǻ) and angles (degree) for complexes 2, 4 and 6.
Complex 2
Cu1\O5
Cu1\N1
Cu1\N2
Cu1\N3
Cu1\N4
1.917(18)
1.932 (2)
2.085 (2)
2.238 (2)
2.024 (2)
O5\Cu1\N1
O5\Cu1\N2
O5\Cu1\N3
O5\Cu1\N4
N1\Cu1\N2
N1\Cu1\N3
N1\Cu1\N4
N2\Cu1\N3
N2\Cu1\N4
N3\Cu1\N4
93.29(9)
165.27(9)
99.07(9)
88.08(8)
83.95(9)
108.78(9)
173.52(8)
95.52(9)
93.12(9)
77.21(9)
2.6. DNA binding experiments
2.6.1. Electronic absorption and fluorescence spectra
Concentrated stock solutions of metal complexes were prepared
by dissolving them in aqueous methanol (1:5, methanol/water) and
diluting them with 50 mM NaCl/5 mM Tris–HCl buffer solution at
pH 7.1 buffer to required concentrations for all the experiments. For
absorption and emission spectral experiments the DNA solutions
were pretreated with solutions of metal complexes to ensure no
change in the metal complex concentrations. Absorption spectral titra-
tion experiments were performed by maintaining a constant concentra-
tion of the complex and varying the nucleic acid concentration. This was
achieved by dissolving an appropriate amount of the metal complex
and DNA stock solutions while maintaining the total volume constant
(1 mL). This results in a series of solutions with varying concentrations
of DNA but with a constant concentration of the complex. The absor-
bance (A) was recorded after successive additions of CT DNA.
Complex 4
Cu1\N1
Cu1\N2
Cu1\N3
Cu1\N4
Cu1\O1
1.937(3)
2.092(3)
2.247(3)
2.026(2)
1.928(1)
O1\Cu1\N1
O1\Cu1\N4
N1\Cu1\N4
O1\Cu1\N2
N1\Cu1\N2
N4\Cu1\N2
O1\Cu1\N3
N1\Cu1\N3
N4\Cu1\N3
N2\Cu1\N3
91.77(10)
89.90 (9)
178.09 (10)
155.82 (11)
83.87 (12)
94.98 (11)
107.21 (10)
100.92 (11)
77.69 (9)
96.96 (10)
The EthBr displacement assay was used to determine the apparent
DNA binding constants (Kapp) of the complexes. The emission intensity
of EthBr was used as spectral probe, as it is known to show reduced
emission intensity in buffer solution because of solvent quenching,
and an enhancement in emission intensity is observed when EthBr
intercalatively binds to CT DNA [61]. The competitive binding of com-
plexes 1–6 to CT DNA could lead to the displacement of the EthBr,
exposing it for solvent quenching of the emission. The binding ability
of the complexes to DNA was determined from the extent of reduction
of the EthBr emission intensity.
Complex 6
Cu1-O1
Cu1-N1
Cu1-N2
Cu1-N3
Cu1-N4
1.918 (6)
1.925 (8)
2.081(8)
2.231 (8)
2.025 (7)
O1\Cu1\N1
O1\Cu1\N2
O1\Cu1\N3
O1\Cu1\N4
N1\Cu1\N3
N1\Cu1\N4
N1\Cu1\N2
N2\Cu1\N4
N3\Cu1\N4
N2\Cu1\N3
93.7(3)
153.6(3)
104.4(3)
89.2(3)
100.2(3)
177.0(3)
83.3(4)
94.6(3)
78.2(3)
101.9(3)