1-s2.0-S1011134414003510-main

Polyethyleneimine anchored copper(II) complexes: Synthesis,
characterization, in vitro DNA binding studies and cytotoxicity studies
Jagadeesan Lakshmipraba a
, Sankaralingam Arunachalam a,⇑
, Anvarbatcha Riyasdeen b
,
Rajakumar Dhivya c
, Mohammad Abdulkader Akbarsha b
a
School of Chemistry, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India
b
Mahatma Gandi-Doerenkamp Centre, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India
c
Department of Biomedical Science, Bharathidasan University, Tiruchirappalli 620 024, Tamil Nadu, India
a r t i c l e i n f o
Article history:
Received 18 August 2014
Received in revised form 15 November 2014
Accepted 19 November 2014
Available online 27 November 2014
a b s t r a c t
The water soluble polyethyleneimine–copper(II) complexes, [Cu(phen)(L-tyr)BPEI]ClO4 (where
phen = 1,10-phenanthroline, L-tyr = L-tyrosine and BPEI = branched polyethyleneimine) with various
degree of copper(II) complex units in the polymer chain were synthesized and characterized by elemental
analysis and electronic, FT-IR, EPR spectroscopic techniques. The binding of these complexes with CT-
DNA was studied using UV–visible absorption titration, thermal denaturation, emission, circular dichro-
ism spectroscopy and cyclic voltammetric methods. The changes observed in the physicochemcial prop-
erties indicated that the binding between the polymer–copper complexes and DNA was mostly through
electrostatic mode of binding. Among these complexes, the polymer–copper(II) complex with the highest
degrees of copper(II) complex units (higher degrees of coordination) showed higher binding constant
than those with lower copper(II) complex units (lower degrees of coordination) complexes. The complex
with the highest number of metal centre bound strongly due to the cooperative binding effect. Therefore,
anticancer study was carried out using this complex. The cytotoxic activity for this complex on MCF-7
breast cancer cell line was determined adopting MTT assay, acridine orange/ethidium bromide (AO/EB)
staining and comet assay techniques, which revealed that the cells were committed to specific mode
of cell death either apoptosis or necrosis.
Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction
During the past decade, there has been tremendous interest in
the synthesis and studies pertaining to the interaction of various
metal complexes with DNA and also screening of these complexes
for their anticancer activities so as to replace cisplatin [1–5].
Hence, much attention has been targeted on the design of metal-
based complexes, which can bind to DNA. Interaction of metal
complexes with DNA should be useful in the development of
molecular probes and new therapeutic agents.
Cancer research mainly targets the DNA molecule which is the
origin of uncontrolled cell division. To block this cell division, there
is a need for developing DNA targeted chemotherapy drugs. Sev-
eral small molecules/metal complexes have been interacted with
DNA to correlate their effects of binding/cleavage behavior on cyto-
toxicity [6,7]. In these aspects of drug designing, some of the issues
like solubility, transfection efficiency, targeted delivery, etc. may
enter as practical problems. To overcome these problems, drug
carriers like cationic polymers, surfactants, liposomes and dentri-
mers were employed for efficient drug delivery [8].
Copper is a physiologically important metal element that plays
an important role in the endogenous oxidative DNA damage asso-
ciated with aging and cancer [9]. Copper(II) complexes bearing
1,10-phenanthroline ligand have been widely used due to their
high nucleolytic efficiency [10] and numerous biological activities
such as antitumor, anti-candida and antimicrobial activities
[11,12]. It has been reported that a binuclear copper(II) complex
containing 1,10-phenanthroline and a trinuclear copper(II) com-
plex containing di-(2-picolyl)amine bind strongly with DNA and
cleave more effectively than their corresponding monomeric com-
plexes [13–15]. The reports in the literature that advocate design
studies on metal complexes that cooperative effect arising from
such non-covalent interactions would be a valuable principle in
the development of new metal based probes which recognize bio-
molecular targets with high specificity [16].
Recent literature indicate that mixed ligand copper(II) com-
plexes have been receiving considerable attention for various
http://dx.doi.org/10.1016/j.jphotobiol.2014.11.005
1011-1344/Ó 2014 Elsevier B.V. All rights reserved.
⇑ Corresponding author. Fax: +91 431 2407043.
E-mail address: arunasurf@yahoo.com (S. Arunachalam).
Journal of Photochemistry and Photobiology B: Biology 142 (2015) 59–67
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology B: Biology
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reasons. Copper(II) complexes having amino acid ligand are of
interest due to their biological relevance, good DNA binding ability,
antimicrobial and anticancer activities [17]. Also, there are reports
on drug polymer conjugates as potential candidates for the selec-
tive delivery of anticancer agents to tumor tissues. Particularly,
polyethyleneimine (PEI), possesses quite a number of advantages
as polymer chelating agent, such as good water solubility, high
content of functional groups, suitable molecular weights as well
as good physical and chemical stabilities [18]. Stable polyethylene-
imine–copper(II) complexes are reported where copper ions are
hard to elute from the polymer domain to the bulk solution and
binding constant values are 1–5 order magnitude greater in the
polymer–chelate systems than in the monomeric Cu-complex
systems [19].
Our research group has been involved in the synthesis and stud-
ies on mixed polymer–copper(II) complexes and their DNA binding
properties for the past several years [20,21]. We have reported syn-
thesis and nucleic acid binding of polyethyleneimine–copper(II)
complexes with L-valine and L-arginine as a co-ligand [22,23]. In
the present work we report the synthesis, DNA binding and antitu-
mor properties of polyethyleneimine–copper(II) complexes con-
taining L-tyrosine as co-ligand. L-tyrosine contains benzene ring
which can influence the binding of these complexes with DNA
through p–p interactions.
2. Experimental section
2.1. Materials
Calf thymus DNA and branched polyethyleneimine (BPEI) (Mw
ca. 25,000) were obtained from Sigma–Aldrich, Germany and were
used as obtained. Copper(II) chloride dihydrate, 1,10-phenanthro-
line (Merck, India) and tyrosine (Loba Chemie, India) and were used
as received. The precursor complex, [Cu(phen)(L-tyr)(H2O)]ClO4,
was prepared as reported earlier [24]. A solution of calf thymus
DNA in the buffer gave a UV absorbance ratio of $1.8–1.9: 1 at 260
and 280 nm, indicating that the DNA was sufficiently free of protein.
The concentration of CT DNA in base pairs was determined by UV
absorbance at 260 nm by taking the molar extinction coefficient
value 13,200 MÀ1
cmÀ1
for DNA at 260 nm [25,26]. All the experi-
ments involving the interaction of the polymer–copper(II) complex
with DNA were carried out using buffer containing 5 mM Tris–HCl/
50 mM NaCl at pH 7.0 in twice distilled water. MCF-7 cell line was
obtained from National Centre For Cell Science (NCCS), Pune.
2.2. [Cu(phen)(L-tyr)BPEI]ClO4
To a solution of branched polyethyleneimine (BPEI) (0.15 g,
3.4 mmol) dissolved in ethanol (15 ml), [Cu(phen)(L-tyr)
(H2O)]ClO4 (0.8 g, 1.4 mmol) in water was added slowly with
stirring. The mixture was heated between 50 and 60 °C for 15 h
in a water bath with stirring. The resulting dark blue solution
was dialyzed at 15 °C against distilled water for 4–5 days. The
solvent was then evaporated in a rotary evaporator under reduced
pressure at room temperature. The dark-bluish filmy substance
obtained was pulverized and dried. Yield = 0.23 g for x = 0.182.
(Anal. Calc.: C, 33.77, H, 4.40, N, 14.50, O, 5.81, Cu 18.35, Found:
H, 5.02, N, 14.25, O, 5.77, Cu 18.37% (x = 0.182 obtained from
carbon content). IR (KBr, cmÀ1
): m(NAH) 3446, m(CAC) 2924, m(COOA)
1099, m(C@C) 1471, m(C@N) 1390, 1099, m(CAH) 852, m(CAH) 730; UV
(kmax, nm,(e, MÀ1
cmÀ1
)): 227 (28,120) 272 (54,240), 294
(41,830), 645 (11,090). EPR (77 K, g|| 2.211 and g 2.017; RT
giso = 2.0753.
Polymer–copper(II) complex samples with various numbers of
copper(II) complex units bound to the polymer chain were synthe-
sized by changing the amount of reactants in the reaction solution,
the duration of the reaction time and the reaction temperature.
2.3. Physical measurements
Elemental analysis was determined at Sophisticated Analytical
Instrument Facility (SAIF), Lucknow, India. Absorption spectra
and thermal denaturation studies were recorded on a UV–Vis–
NIR Cary300 Spectrophotometer using cuvettes of 1 cm path
length in tris buffer solution, and emission spectra were recorded
on a JASCO FP 770 spectrofluorimeter. FT-IR spectra were recorded
on a FT-IR JASCO 460 PLUS spectrophotometer with samples pre-
pared as KBr pellets. EPR spectra were recorded on a JEOL-FA200
EPR spectrometer at room temperature and at LNT in methanol
solution. Absorption titration experiments of polymer–copper(II)
complexes in buffer (50 mM NaCl–5 mM Tris–HCl, pH 7.0) were
performed by using a fixed complex concentration to which incre-
ments of the DNA stock solutions were added. Polymer–copper(II)
complex-DNA solutions were incubated for 10 min before the
absorption spectra were recorded. Equal amount of DNA was
added to both the complex and reference solutions to eliminate
the absorbance of DNA itself.
For fluorescence quenching experiments DNA was pretreated
with ethidium bromide (EB) for 30 min. The polymer–copper(II)
complexes were then added to this mixture and their effect on
the emission intensity was measured. Samples were excited at
450 nm and emission was observed between 500 and 700 nm. Cir-
cular dichroism spectra were recorded at room temperature using
the same tris buffer. For the cyclic voltammetry experiments, the
electrode surfaces were freshly polished with alumina powder
and then sonicated in ethanol and distilled water for 1 min prior
to each experiment. Then the electrodes were rinsed throughly
with distilled water. Cyclic voltammetric experiments were per-
formed at 25.0 ± 0.2 °C in a single compartment cell with a three-
electrode configuration (glassy carbon working electrode,
platinum wire auxiliary electrode and saturated calomel reference
electrode). The solution was deoxygenated with nitrogen gas for
20 min prior to experiments.
2.4. Cell culture
The MCF-7 cancer cells were cultured in RPMI 1640 medium
(Sigma–Aldrich, St. Louis, MO, USA), supplemented with 10% fetal
bovine serum (Sigma, USA) and 10,000 IU of penicillin and
100 lg mlÀ1
of streptomycin as antibiotics (Himedia, Mumbai,
India), in 96 well culture plates, at 37 °C, in a humidified atmo-
sphere of 5% CO2, in a CO2 incubator (Forma, Thermo Scientific,
USA). All the experiments were performed using cells from passage
15 or less.
2.4.1. Cytotoxicity assay (MTT assay)
The polymer–copper(II) complex of the highest degree of coor-
dination was dissolved in DMSO, diluted in culture medium and
used to treat the model cell line over a complex concentration
range of 3–30 lg mlÀ1
for a period of 24 h and 48 h. DMSO at
0.5% concentration in the culture medium was used as negative
control. We have used cisplatin as positive control. A miniaturized
viability assay using 3-(4,5-di-methylthiazol-2-yl)-2,5-diphenyl-
2H-tetrazolium bromide (MTT) (Sigma, USA) (5 mg/ml in Phos-
phate-Buffered Saline (PBS)) was added to each well and the plates
were wrapped with aluminum foil and incubated at 37 °C for 4 h.
By this treatment a purple formazone product was formed due to
the reduction of MTT by the mitochondrial enzyme succinate
dehydrogenase of the cells [27]. The purple formazan product
was dissolved by addition of 100 ll of 100% DMSO to each well.
The absorbance was monitored at 570 nm (measurement) and
60 J. Lakshmipraba et al. / Journal of Photochemistry and Photobiology B: Biology 142 (2015) 59–67

630 nm (reference) using a 96 well plate reader (Bio-Rad, Hercules,
CA, USA). Data were collected for four replicates each and used to
calculate the respective means. The percentage of inhibition was
calculated, from this data, using the formula:
The IC50 value was determined as the complex drug concentra-
tion that is required to reduce the absorbance to half that of the
control.
2.4.2. Acridine orange (AO) and ethidium bromide (EB) staining
Acridine orange/ethidium bromide staining was performed as
follows: the cell suspension of each sample containing 5 Â 105
cells, was treated with 25 ll of AO and EB solution (1 part of
100 lg mlÀ1
AO and 1 part of 100 lg mlÀ1
EB in PBS) and examined
in a fluorescent microscope (Carl Zeiss, Germany) using an UV filter
(450–490 nm). Three hundred cells per sample were counted in
tetraplicate for each dose point. The cells were scored as viable,
apoptotic or necrotic as judged by the staining, nuclear morphol-
ogy and membrane integrity [28], and the percentages of apoptotic
and necrotic cells were then calculated. Morphological changes
were also observed and photographed.
2.4.3. Comet assay
DNA damage was detected by adopting comet assay as reported
earlier [29]. Cells were suspended in low-melting-point agarose in
PBS and pipetted out to microscope slides pre-coated with a layer
of normal-melting-point agarose. Slides were chilled on ice for
10 min and then immersed in lysis solution (2.5 M NaCl, 100 mM
Na2EDTA, 10 mM Tris, 0.2 mM NaOH, pH 10.01 and Triton X-100)
and the solution was kept for 4 h at 4 °C in order to lyse the cells
and to permit DNA unfolding. Thereafter, the slides were exposed
to alkaline buffer (300 mM NaOH, 1 mM Na2EDTA, pH > 13) for
20 min to allow DNA unwinding. The slides were washed with buf-
fer (0.4 M Tris, pH 7.5) to neutralize excess alkali and to remove
detergents before staining with ethidium bromide (5 ll in
10 mg mlÀ1
). Photographs were obtained using the fluorescent
microscope. One hundred and fifty cells from each treatment group
were digitalized and analyzed using CASP software. The images
were used to estimate the DNA content of individual nuclei and
to evaluate the degree of DNA damage representing the fraction
of total DNA in the tail.
3. Results and discussion
3.1. Degree of coordination
The structure of the water soluble polymer–copper(II) complex
is shown in Fig. 1. In this figure ‘x’ represents the degree of coordi-
nation, which is the number of moles of copper(II) chelate per mole
of the repeating unit (amine group) of polymeric ligand. If the
entire repeating units (amine group) in the polymer are coordi-
nated to copper, then the value of x is 1. The degree of coordination
(x) was calculated from carbon content value [19,30,31]. The
degree of coordination thus obtained for the polymer–copper(II)
complex samples of the present work are 0.059, 0.149, 0.182.
The stability of the polymer–copper(II) complexes in solution
was verified occasionally by keeping the solution of the polymer–
copper(II) complexes in dialysis bags, small amount of the solution
from the dialysis bags was taken and the stability was confirmed
through absorption spectroscopy. Any free copper complex ion or
copper ion in the solution outside the dialysis bag was not observed
(which was verified using spectrophotometric method), indicating
that polymer–complexes were very stable during the handling of
our experiments. Also viscosity, is a significant experiment which
gives information about interaction in aqueous solution. In our
case, even after dialysis we do not have change in viscosity. This
proves that the complexes are very stable in solution.
3.2. Characterization of polymer–copper(II) complexes
The FT-IR spectra for the polymer–copper(II) complexes dis-
played stretching frequencies around 1471 cmÀ1
and 1390 cmÀ1
which can be attributed to the ring stretching frequencies viz.,
m(C@C) and m(C@N), respectively, of the coordinated 1,10-phenan-
throline and these are at slightly lower frequencies than that of
uncoordinated 1,10-phenanthroline. The m(CAH) out-of-plane
bending values, around 852 cmÀ1
and 730 cmÀ1
, for the phenan-
throline ligand were shifted to 838 cmÀ1
and 693 cmÀ1
, respec-
tively, in the complexes. These shifts can be explained by the fact
that each of the two nitrogen atoms of phenanthroline ligands
donates a pair of electrons to the central copper atom forming
coordinate bond [32]. The band obtained around 2924 cmÀ1
can
be assigned to CAC stretching vibration of aliphatic CH2 of BPEI
whereas the broad band observed around 3446 cmÀ1
can be
assigned to the NAH stretching of BPEI [33]. The uncoordinated
amino acid exhibited a stretch in the region 1750–1700 cmÀ1
cor-
responding to m(COOH). In the complexes, this band was shifted to
1630 cmÀ1
indicating the coordination of carboxylate group to the
copper(II) ion. The very strong band around 1099 cmÀ1
can be
assigned to the presence of perchlorate anion. The stretching fre-
quencies around 508 cmÀ1
and 491 cmÀ1
can be attributed, respec-
tively, to copper–nitrogen and copper–oxygen stretching.
The UV–visible absorption spectra of all the complexes were
recorded in the region 200–800 nm. All the complexes displayed
four bands in the regions 230–645 nm. In the UV region, the
absorption bands below 300 nm are attributed to intra-ligand tran-
sitions whereas in the visible region, the band around 645 nm is
assigned as d–d transition.
The solid state EPR spectra of the polymer–copper(II) complex
(x = 0.203) was recorded in X-band frequencies at room tempera-
ture as well as in frozen solution (77 K) in methanol as solvent
(Fig. 2). The room temperature EPR spectra of the polymer–cop-
per(II) complexes showed single isotropic feature at giso = 2.070–
2.075, and this broadening of isotropic peak is due to intermolecu-
lar spin exchange. This intermolecular type of spin exchange is
caused by the strong spin coupling which occurs during a coupling
Cu
N
H
N
H
(1-x)
x
N
N
.ClO4
NH2
OH
O O
Fig. 1. Schematic representation of [Cu(phen)(L-tyr)BPEI]ClO4.
½Mean absorbance of untreated cellsðcontrolÞ À Mean absorbance of treated cellsðtestÞŠ Â 100:
Mean absorbance of untreated cellsðcontrolÞ
J. Lakshmipraba et al. / Journal of Photochemistry and Photobiology B: Biology 142 (2015) 59–67 61

of two paramagnetic species. At liquid nitrogen temperature we
observed three peaks with the third being broad. This is because
the copper complex units have been mounted on a polymer chain
resulting in some spin–spin coupling between the copper complex
units which leads to a small amount of broadening. In liquid nitro-
gen temperature the complexes showed the values of g|| = 2.211–
2.216 and g = 2.016–2.020. The existence of g|| > g > 2.00 sug-
gests that dx
2
À dy
2
is the ground state with the d9
(Cu2+
) configu-
ration and square pyramidal geometry.
3.3. Absorption studies
Electronic absorption spectroscopy is an effective method to
examine the binding mode of DNA with polymer–copper(II) metal
complexes. Thus, in order to provide evidence for the binding of
polymer–copper(II) complexes to DNA, the binding process was
monitored by absorption spectroscopy by following the changes
in absorption band intensity and its position. On addition of
DNA, the absorption spectra of polymer–copper(II) complex
showed hyperchromism and slight red shift (Fig. 3). The experi-
mental results derived from the UV–visible titration experiments
suggest that positively charged polymer-complexes can bind to
DNA, probably to the phosphate groups, by electrostatic interac-
tion resulting in the stabilization of DNA duplex. Nevertheless,
the metal complex units present in the polymer chain contain aro-
matic moieties so the binding of the complexes involving partial
intercalation of an aromatic ring between the base pairs of DNA
cannot be ruled out. From the above studies the intrinsic binding
constants (Kb) were determined from the increase of absorption
at 294 nm calculated by absorption spectral titration. In order to
compare quantitatively the binding affinity with nucleic acids
between polymer–copper(II) complexes having different degrees
of coordination, the intrinsic binding constants Kbs of the com-
plexes were determined using Eq. (4) by assuming a simple model,
in which the reaction between the nucleic acid site, P and the cop-
per complex unit of the polymer complex, D to form the nucleic
acid bound complex, PD as:
P þ D

Kb
PD ð1Þ
Kb ¼ ½PDŠ=½PŠ½DŠ ð2Þ
where [PD], [P] and [D] represent the respective equilibrium con-
centrations of nucleic acid bound copper complex units, nucleic acid
sites in base pairs and the copper complex units of the polymer
complex.
A ¼ eD½DŠ þ ePD½PDŠ ð3Þ
CD=A À eDCD ¼ ð1=ePD À eDÞ þ 1=ðePD À eDÞKb1=½PŠ ð4Þ
where eD and ePD are the molar extinction coefficient of the free cop-
per complex units and apparent molar extinction coefficient of the
nucleic acid bound copper complex units respectively, CD total con-
centration of copper complex units and A is the experimental absor-
bance. An iterative procedure was employed as per the method
provided in Ref. [34] to arrive at the Kb values (first [D] set equal
to CD, then, once a first estimate of Kb and (ePD À eD) are obtained,
a new value of [D] was calculated and so on until convergence is
achieved). This procedure yields a binding constant value (Kb) for
each complex.
As seen from Table 1 the binding constants observed for poly-
mer–copper(II) complexes are higher those that of similar type of
simple metal complexes like [Cu(phen)(L-tyr) H2O]ClO4 (Kb = 3.75 -
Â 103
MÀ1
) as well as the polymer alone PEI (Kb = 1.2 MÀ1
) [35,36].
However, they are very much lower than the potential intercala-
tors like ethidium bromide (Kb, 7.0 Â 107
MÀ1
in 40 mM Tris/HCl,
pH 7.9) [37] and the partially intercalating complexes like
[Co(phen)2(dppz)]3+
(Kb = 9.09 Â 105
MÀ1
) and [Ru(imp)2(dppz)]2+
(Kb = 2.19 Â 107
MÀ1
) [38], which implies that these complexes
bind to DNA relatively less strongly than classical intercalators
and partial intercalators. Also, as seen from the Table, it was
observed that the binding constant changes with degree of coordi-
nation of copper(II) units in the polymer chain; greater the ratio of
copper(II) centres in the polymer chain, higher was the binding
constant because when one copper(II) complex unit binds with
DNA it will cooperatively act to increase the overall binding ability
of the other copper(II) complex units to DNA.
Fig. 2. EPR spectrum of [Cu(phen)(L-tyr)(BPEI)]ClO4 (x = 0.203) in methanol at
liquid nitrogen temperature.
200 300 400 500 600
0.0
0.5
1.0
0.00001 0.00002 0.00003
0.0000005
0.0000010
0.0000015
0.0000020
0.0000025
0.0000030
[DNA]/εa−εf
[DNA]
Absorbance
Wavelength, nm
Fig. 3. Absorption spectra of [Cu(phen)(L-tyr)BPEI]ClO4 (x = 0.182) in the absence of
DNA and in the presence of DNA, [complex] = 3 Â 10À5
M, [DNA] = 0–3.2 Â 10À5
M
(inset: plot of [DNA]/(ea À ef) vs. [DNA]).
Table 1
The intrinsic binding constant (Kb) of [Cu(phen)(L-tyr)BPEI]ClO4, with DNA and RNA
and thermal melting temperature in the presence of [Cu(phen)(L-tyr)BPEI]ClO4 with
different degree of coordination.
Complex Degree of
coordination (x)
Kb (MÀ1
)
±0.04
Ksv (MÀ1
)
±0.03
[Cu(phen)(L-tyr)BPEI]ClO4 0.059 2.10 Â 104
2.13 Â 104
0.149 2.03 Â 105
2.78 Â 104
0.182 7.80 Â 105
3.37 Â 104

3.4. Ethidium bromide displacement assay
All polymer–copper(II) complexes were non-emissive upon
excitation of the MLCT band, either in aqueous solution or in the
presence of DNA. The competitive binding experiments with a
well-established quenching assay based on the displacement of
the intercalating EB from ct-DNA was carried out in order to get
further information regarding the DNA binding properties of poly-
mer–metal complexes. The quenching of emission intensity of DNA
bound EB (Fig. 4) was analyzed through Stern–Volmer equation, I0/
I = 1 + Ksv[Q], where I0 and I are the fluorescence intensities in the
absence and presence of the complex, respectively, Ksv is the linear
Stern–Volmer constant and Q is the concentration of polymer–
copper(II) complex [39,40]. A plot of I0/I vs. [Q] was drawn and
Ksv was obtained from the ratio of slope to intercept (Table 1). As
seen from the Table, the Ksv value increases with increase in degree
of coordination of polymer–copper(II) complex. This is attributed
to the cooperative binding between copper(II) units on the same
polymer chain with DNA. This cooperative effect increases with
degree of coordination.
3.5. Effect of ionic strength
The change in fluorescence intensity of cationic copper(II)–poly-
mer complexes to DNA in the presence of NaCl can be used to verify
whether the binding mode is electrostatic or intercalative; a linear
relation between fluorescence intensity and concentration of NaCl
is highly indicative of an electrostatic mode of interaction whereas
a non-dependence of fluorescence intensity on ionic strength indi-
cates intercalation [41,42]. It is observed that as the concentration
of NaCl increases, the relative fluorescence intensity due to ethi-
dium bromide increases (Fig. 5). This is due to the competitive bind-
ing of Na+
ions to DNA which decreases the binding affinity of the
copper(II)–polymer complex to DNA. As the concentration of NaCl
increases, a linear increase of fluorescence is noticed, indicating
that the cationic copper(II)–polymer complex-DNA interactions
for the polymers studied are electrostatic. [43,44]
3.6. Circular dichroism spectral studies
CD spectral technique is useful method to monitor the
conformational variations of DNA during complex-DNA interac-
tions and achieve information on changing DNA conformation by
the binding of the metal complex to DNA. DNA has a major
longwave positive peak centred at 275 nm and the intensity of this
positive peak is similar in magnitude to that of the negative peak
centred at 245 nm (Fig. 6) corresponding to the p–p stacking of
the base pairs and right handed helicity of B-form DNA in buffer
solution [45]. Addition of polymer–copper(II) complex (x = 0.182)
to B-form DNA has been shown to induce a B to A transition, result-
ing in a CD spectrum with characteristics totally different from
those of B-form DNA; the long wave positive peak is larger with
a maximum at $270 nm and a very large shortwave peak results
below $230 nm [46]. Thus, the increased ellipticity observed at
275 nm when polymer–copper(II) complex binds to DNA can be
interpreted as unwinding of B form of DNA due to a decrease in
twist angle. This can be tentatively interpreted as B form of DNA
becoming more ‘A-like’ upon binding polymer–copper(II) complex.
3.7. Cyclic voltammetry studies
The binding of polymer–copper(II) complexes with DNA was
further confirmed by cyclic voltammetric studies. The cyclic
500 600 700
0
100
200
300
400
Intensity
Wavelength, nm
Fig. 4. Emission spectra of EB bound to DNA, [EB] = 2 Â 10À4
M in the absence of
complex and in the presence of complex (x = 0.182), [DNA] = 2 Â 10À3
M, [com-
plex] = 0–1 Â 10À4
M.
0.0 0.2 0.4 0.6 0.8
0.0
0.2
0.4
0.6
0.8
1.0
RelativeFluoresence
[NaCl] mM
1
2
3
Fig. 5. Titration of DNA[DNA] = 2 Â 10À4
M in the presence of ethidium bro-
mide[EB] = 2 Â 10À4
M in the presence of polymer–copper(II) complexes[com-
plex] = 1 Â 10À4
M as a function of NaCl concentration.
240 280 320
-80
-40
0
40
80
DNA
DNA+ complex
CD,mdeg
Wavelength, nm
Fig. 6. Circular dichroism spectra in the absence (black) and in the presence
[Cu(phen)(L-tyr)BPEI]ClO4 (x = 0.182), [complex] = 12 Â 10À5
M (red) with DNA,
[DNA] = 9 Â 10À5
M. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)

voltammogram of polymer–copper(II) complex (x = 0.182) in the
absence and presence of DNA is shown in Fig. 7. In the absence
of DNA, the cathodic peak potential (Epc) and the anodic peak
potential (Epa) of our complex are 339 mV and 574 mV, respec-
tively, with a large peak-to-peak separation, DEp, of 235 mV and
the ratio of cathodic to anodic peak current (ipc/ipa) is 1.05 indicat-
ing a quasi-reversible redox process [47]. The formal potential (E1/
2) which is taken as the average of Epc and Epa is 0.457 V in the
absence of DNA, whereas in the presence of DNA a negative shift
in E1/2 by 0.055 V along with increase in DEp of 30 mV has been
observed. The ipc/ipa value also increased with the increase of the
DNA concentration. Literature report [48] have pointed out that
the shift direction of electrochemical potential of metal complex,
after reacting with DNA, is related to its binding mode with DNA.
A positive shift of the peak potential and the negative shift indicat-
ing that electrostatic mode of interaction.
The dependence of cathodic current on scan rate was also inves-
tigated. In the case of our polymer–copper(II) complex the plot of
cathodic current vs. the square root of the scan rate (m1/2
), was lin-
ear for the complex alone, and the complex in the presence of DNA,
which indicates that the electrochemical process are diffusion
controlled process [48].
4. Cytotoxic assay
4.1. MTT assay
In vitro cytotoxicity of polymer–copper(II) complexes was eval-
uated by MTT assay on MCF-7 cells. The cytotoxic effects of the
polymer–copper(II) complex of the highest degree of coordination
was examined on cultured MCF-7 human breast cancer cells by
exposing cells for 24 h and 48 h to medium containing the complex
at 3–30 mg mlÀ1
concentration (Fig. 8). The polymer–copper(II)
complex inhibited the growth of the cancer cells significantly, in
a dose- and duration-dependent manner. The cytotoxic activity
was determined according to the dose values of the exposure of
the complex required to reduce survival to 50% (IC50), compared
to untreated cells. The IC50 values of the complexes are 20.4 ± 2.5
and 14.3 ± 1.7 lg mlÀ1
after 24 h and 48 h respectively. The poly-
mer–copper (II) complex showed highly effective cytotoxic activity
against MCF-7 cancer cells and the IC50 value of the complex was
lesser for 48 h treatment group than for 24 h treatment group.
Inspite of its high cytotoxic activity against MCF-7 cells, the cyto-
toxic effectiveness was relatively lower when compared to cis-
platin, the IC50 values of which were 13.71 ± 0.5 and
12.56 ± 0.8 lg mlÀ1
for 24 h and 48 h treatment periods, respec-
tively. However, cytotoxic potential apart, cisplatin has been estab-
lished to produce toxic side effects [49] which is not expected with
the polymer–copper(II) complex in present study [50]. The cyto-
toxic effect of the polymer–copper(II) complex may be interpret-
able as due to its amphiphilic nature [51] and, hence, would
penetrate the cell membrane easily, reduce the energy status in
tumors and also to alter hypoxia status in the cancer cell microen-
vironment, which are factors that would influence the antitumor
acidity. It is known that phenanthroline-containing metal com-
plexes have a wide range of biological activities such as antitumor,
antifungal, apoptosis [52–54], interaction with DNA thereby inhib-
iting replication, transcription, and other nuclear functions and
arresting cancer cell proliferation so as to arrest tumor growth.
4.2. Assessment of cell death based on morphological features
Apoptosis is a gene-controlled cell death process, which is char-
acterized by DNA fragmentation, chromatin condensation and
marginalization, membrane blebbing, cell shrinkage, and fragmen-
tation of cells into membrane-enclosed vesicles or apoptotic bodies
to be phagocytosed by macrophages [55]. To further confirm the
mode of cell death induced by the complex on cancer cells AO/EB
(acridine orange/ethidium bromide) staining (Apoptosis Assays)
was adopted, which would reveal the changes in the gross cytology
of the cell with special reference to cytoplasm and nucleus. After
treatment of MCF-7 cancer cells, polymer–copper(II) complex of
the highest degree of coordination, at the respective IC50 concen-
trations for 24 h and 48 h, the cells were observed for the gross
cytological changes. The treated cells revealed all the above cyto-
logical changes (Fig. 9). These cytological changes indicated that
the cells were committed to cell death, mostly, apoptosis and to
a certain extent necrosis.
4.3. Single-cell gel electrophoresis (Comet assay)
Among the different techniques used for measuring and analyz-
ing DNA strand breaks in mammalian cells, the single cell gel elec-
trophoresis assay (Comet assay) is considered as a rapid, simple,
visual and sensitive technique to asses DNA fragmentation typical
of toxic DNA damage and of an early stage of apoptosis [56]. As
Fig. 7. Cyclic voltammograms of [Cu(phen)(L-tyr)BPEI]ClO4 (x = 0.182), [com-
plex] = 1 Â 10À3
M (black) in the presence of DNA (red) [DNA] = 0–8.0 Â 10À4
M,
scan rate: 50 mV sÀ1
. (For interpretation of the references to color in this figure
legend, the reader is referred to the web version of this article.)
Fig. 8. Inhibition of in vitro cancer cells growth by [Cu(phen)(L-tyr) BPEI]ClO4
(x = 0.182).

Control 24h 48h
0
20
40
60
80
100%ofcells
Normal
Apoptosis
Necrosis
Fig. 9. Photomicrographs of control (the cells were viable as inferred from the green – fluorescence) and AO/EB stained MCF-7 cancer cells treated with the [Cu(phen)(L-
tyr)BPEI]ClO4 (x = 0.182) at 20.4 and 14.3 lg mlÀ1
concentration for 24 and 48 h. Scale bar: 35 lm. The graph shows data on percentage of cells that are normal afflicted with
apoptosis and necrosis in the control and 24 h and 48 h treatment groups. (For interpretation of the references to color in this figure legend, the reader is referred to the web
version of this article.)
Control 24 h 48 h
0
20
40
60
80
100
120
140
%ofcells
Dead
Highly Damaged
Damaged
Slightly Damaged
Intact
Fig. 10. Comet images of DNA double strand breaks at 12 and 24 h treatment of [Cu(phen)(L-tyr)BPEI]ClO4 (x = 0.182) at 20.4 and 14.3 lg mlÀ1
concentration. Cells were
grown in RPMI1640 medium containing FBS at 10% final concentration, and streptomycin (10 mg mlÀ1
) and penicillin (10,000 IU mlÀ1
) as antibiotics. The duration-
dependence of the DNA damage is revealed. Scale bar: 35 lm. DNA damage in MCF-7 cell populations as defined according to the percentage of DNA in the tail.

shown in Fig. 10, the images were used to estimate the DNA con-
tent of individual nuclei and to evaluate the degree of DNA damage
representing the fraction of total DNA in the tail. Cells were
assigned to five groups: 0–20% (intact), 20–40% (slightly damaged),
40–60% (damaged), 60–80% (highly damaged) and >80% (dead).
The results revealed that DNA damage was induced in MCF-7 can-
cer cells by the polymer–copper(II) complex, and the incidence was
greater at 48 h than at 24 h, as shown in Fig. 10.
5. Conclusions
Water soluble polyethyleneimine coordinated–copper(II) com-
plexes containing phenanthroline and L-tyrosine as co-ligands with
various degrees of coordination were synthesised. The complexes
were characterized adopting various spectroscopic techniques
and elemental analysis. The binding between the polymer–cop-
per(II) complexes and DNA was assessed in relation to the polymer
complex with different degrees of copper complex content in the
polymer chain. The electronic absorption spectral studies, emission
studies and ionic strength effect showed that these complexes bind
to DNA via electrostatic modes of binding. These studies indicates
that the binding affinity toward DNA increases with the increase
in the number of copper centres in the polymer. The changes in cir-
cular dichroism and cyclic voltammetry studies of the binding
between one of our complexes in the presence of DNA confirm
the above mentioned modes of binding. Thermal denaturation
studies of the binding between our complexes and DNA reveal that
the complex with higher degree of coordination binds with DNA
and stability enhanced. The polymer–copper(II) complex of the
highest degree of coordination showed good cytotoxic activity
against MCF-7 cancer cell with mostly through apoptosis although
a few cells succumbed to necrosis.
Acknowledgments
We are grateful to the UGC-SAP and DST-FIST programmes of
the Department of Chemistry, Bharathidasan University. Council
of Scientific and Industrial Research (CSIR), New Delhi is acknowl-
edged for financial support [Scheme. No. 09/475(0154)/2010-EMR-
I dated. 09/02/2011] for Senior Research Fellowship to JLP. One of
the authors, SA., thanks for sanction of research schemes, Grant No.
SR/S1/IC-13/2009 of DST, Grant No. 01(2461)/11/EMR-II of CSIR
and also Grant No. 41-223/2012(SR) of UGC. Grants from Doerenk-
amp-Zbinden Foundation, Switzerland, and King Saud University,
Riyadh, Kingdom of Saudi Arabia to MAA are gratefully
acknowledged.
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