Picolinic acid based Cu(II) complexes with heterocyclic bases - Crystal structure, DNA binding and cleavage studies

Article abstract of DOI:10.1016/j.ejmech.2014.03.084

In view of the importance of picolinic acid (PA) in preventing cell growth and arresting cell cycle, new PA based metallonucleases were designed with a view to study their DNA binding and cleavage abilities. Three new Cu(II) complexes [Cu(II)(DPPA)].4H₂O (1),[Cu(II)(DPPA)(bpy)].5H₂O (2) and [Cu(II)(DPPA)(phen)].5H₂O (3), were synthesized using a picolinic acid based bifunctional ligand (DPPA) and heterocyclic bases (where DPPA: Pyridine-2-carboxylic acid {2-phenyl-1-[(pyridin-2-ylmethyl)-carbonyl]- ethyl}-amide; bpy: 2, 2′-bipyridine and phen: 1, 10-phenanthroline). DPPA was obtained by coupling 2-picolinic acid and 2-picolyl amine with l-phenylalanine through amide bond. Complexes were structurally characterized by a single crystal X-ray crystallography. The molecular structure of 1 shows Cu(II) center essentially in a square planar coordination geometry, while complex 2 shows an approximate five coordinated square-pyramidal geometry. Eventhough we could not isolate single crystal for complex (3), its structure was established based on other techniques. The complex (3) also exhibits five coordinate square pyramidal geometry. The complexes show good binding affinity towards CT-DNA. The binding constants (K_b) decrease in the order 1.35 ± 0.01 × 10⁵ (3) > 1.23 ± 0.01 × 10⁵ (2) > 8.3 ± 0.01 × 10⁴ (1) M ^-1. They also exhibit efficient nuclease activity towards supercoiled pUC19 DNA both in the absence and presence of external agent (H ₂O_2). The kinetic studies reveal that the hydrolytic cleavage reactions follow the pseudo first-order rate constant and the hydrolysis rates are in the range of (5.8-8.0) × 10⁷ fold rate enhancement compared to non-catalyzed double stranded DNA (3.6 × 10 ^-8 h^-1).

Full text of DOI:10.1016/j.ejmech.2014.03.084

Accepted Manuscript
Picolinic acid based Cu(II) complexes with heterocyclic bases – crystal structure, DNA
binding and cleavage studies
Pulimamidi Rabindra Reddy, Nomula Raju, Pallepogu Raghavaiah, Shaik Hussain
PII:
S0223-5234(14)00306-7
10.1016/j.ejmech.2014.03.084
EJMECH 6868
DOI:
Reference:
To appear in: European Journal of Medicinal Chemistry
Received Date: 11 November 2013
Revised Date: 27 January 2014
Accepted Date: 31 March 2014
Please cite this article as: P.R. Reddy, N. Raju, P. Raghavaiah, S. Hussain, Picolinic acid based Cu(II)
complexes with heterocyclic bases – crystal structure, DNA binding and cleavage studies, European
Journal of Medicinal Chemistry (2014), doi: 10.1016/j.ejmech.2014.03.084.
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ACCEPTED MANUSCRIPT
Graphical Abstract
Three new Cu(II) complexes were synthesized, structurally characterized and their DNA binding
and cleavage properties were studied. Gel electrophoresis pattern revealed that these complexes
cleave supercoiled DNA into nicked circular form.

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Highlights
ꢀ
Three new picolinic acid based Cu(II) complexes were synthesized and structurally
characterized.
ꢀ
ꢀ
ꢀ
ꢀ
The DNA binding and cleavage properties of these complexes were studied.
The copper complexes bind to DNA in an intercalative mode of interaction.
The complexes successfully converted supercoiled DNA into nicked circular form.
The kinetic data show that the cleavage reactions follow pseudo first order rate constants

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Picolinic acid based Cu(II) complexes with heterocyclic
bases – crystal structure, DNA binding and cleavage
studies.
a
a
b
Pulimamidi Rabindra Reddy * , Nomula Raju , Pallepogu Raghavaiah and Shaik
a
Hussain
a
Department of Chemistry, Osmania University, Hyderabad, India, Pin: 500007
Phone: +91-40-27171664; Fax: +91-40-27090020,
b
School of Chemistry, University of Hyderabad, Hyderabad, INDIA, Pin: 500046.
*
Corresponding Author: Department of Chemistry, Osmania University, Hyderabad,
INDIA, PIN: 500007; Email: profprreddy@gmail.com; Phone: +91-40-27171664; Fax:
91-40-27090020.
+
1

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Abstract
In view of the importance of picolinic acid (PA) in preventing cell growth and arresting
cell cycle, new PA based metallonucleases were designed with a view to study their DNA
binding and cleavage abilities. Three new Cu(II) complexes [Cu(II)(DPPA)].4H O
2
(
1),[Cu(II)(DPPA)(bpy)].5H O (2) and [Cu(II)(DPPA)(phen)].5H O (3), were
2
2
synthesized using a picolinic acid based bifunctional ligand (DPPA) and heterocyclic
bases (where DPPA: Pyridine-2-carboxylic acid {2-phenyl-1-[(pyridin-2-ylmethyl)-
carbonyl]-ethyl}-amide; bpy: 2, 2'-bipyridine and phen: 1, 10-phenanthroline). DPPA
was obtained by coupling 2-picolinic acid and 2-picolyl amine with L-phenylalanine
through amide bond . Complexes were structurally characterized by a single crystal x-ray
crystallography. The molecular structure of 1 shows Cu(II) center essentially in a square
planar coordination geometry, while complex 2 shows an approximate five coordinated
square-pyramidal geometry. Eventhough we could not isolate single crystal for complex
(3), its structure was established based on other techniques. The complex (3) also exhibits
five coordinate square pyramidal geometry. The complexes show good binding affinity
5
b
towards CT-DNA. The binding constants (K ) decrease in the order 1.35±0.01x10
5
4
-1
(
3)>1.23±0.01 x10 (2)>8.3±0.01x10 (1) M . They also exhibit efficient nuclease
activity towards supercoiled pUC19 DNA both in the absence and presence of external
agent (H O The kinetic studies reveal that the hydrolytic cleavage reactions follow the
2
2).
7
pseudo first-order rate constant and the hydrolysis rates are in the range of (5.8 – 8.0)x10
-8
-1
fold rate enhancement compared to non-catalyzed double stranded DNA (3.6x10 h ).
Keywords: Picolinic acid, Copper(II) complexes, Crystal structure, DNA binding, DNA
cleavage.
2

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1
. Introduction
The interaction of metal complexes with DNA is of great interest for the development of
artificial endonucleases followed by anticancer drug therapies. DNA offers several
potential binding sites for transition metals, including the anionic phosphate backbone,
electron-rich bases, and the major or minor grooves [1]. Transition metal complexes can
associate with DNA mainly in two types of binding motifs: Covalent and non-covalent
binding (intercalation, electrostatic interaction and major/minor groove binding). The
3 2 2
well-studied first generation anticancer complex, cisplatin [cis-Pt(NH ) Cl ], binds
7
covalently to the N -guanine of DNA, causing a distortion to the structure of DNA
double helix leading to serious side effects and cell death [2-5]. Because of this, the usage
of cisplatin is limited. Hence, other metals like copper, [6−8] and ruthenium [9-11] are
now regarded as promising alternatives to platinum in cancer therapy. Copper is most
abundant bio-essential element with two oxidation states (+1 and +2) which are important
in most aerobic organisms, is employed as a structural and catalytic cofactor, and
consequently it is involved in many biological pathways [12]. The coordination flexibility
and distortion ability of Cu(II) complexes contribute significantly to the structural
diversity, which eventually play a vital role in DNA binding and cleavage.
Nitrogen ligands have been extensively used in coordination chemistry, [13, 14]
especially to obtain derivatives that are able to mimic structural, spectroscopic and
catalytic features of active sites of metallo-enzymes [15-18]. As a typical heterocyclic
planar ligands, 1, 10-phenanthroline and 2, 2-bipyridine have attracted attention due to
their diverse and wide-ranging antiviral, photochemical and photophysical properties,
versatile coordination modes and the potential to form supramolecular aggregates through
3

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π…π stacking interactions. Thus, the design of their metal complexes have continuously
increased since many of these materials may serve as models which mimic both the
structure and reactivity of metal ion sites in complex biological systems and possess a
broad spectrum of biological activity [19-21].
Many useful applications of the complexes require that the complexes bind to DNA
through intercalative mode since it is one of the important binding modes that invariably
lead to cellular degradation [22]. Considering the prominent role of intercalators in
enhancing DNA binding, cleavage and biological applications, we have been focusing
our attention in the development of new metallonucleases with increased ligand
aromaticities [23-30]. Recently, it was shown that the treatment with picolinic acid
disordered the cell growth and arrested cell cycle [31]. Picolinic acid also stimulates
programmed cell-death (PCD) in cancer cells and efficiently interrupts the progress of
HIV in vitro [32]. In view of this, we have synthesized a picolinic acid based ligand
(DPPA), with phenylalanine and picolyl amine [33] and isolated its binary and ternary
copper complexes. We reported (Fig.1) herein the synthesis, characterization and x-ray
structure (1&2). In the absence of crystal structure for (3), the geometry was established
based on other techniques. Their interaction with CT-DNA was studied by employing
various biophysical techniques such as thermal denaturation, electronic absorption,
viscosity and fluorescence spectroscopy. The binding constants (K ) for these complexes
b
5
5
4
-1
were determined (1.35±0.01x10 (3), 1.23±0.01 x10 (2) and 8.3±0.01x10 (1) M ). The
DNA cleavage activity of the complexes was evaluated using gel electrophoresis
technique. They bind and cleave DNA efficiently. The DNA hydrolysis rate constants
4

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were also determined. The ternary complexes (2, 3) bind and cleave DNA more
efficiently compared to binary complex (1).
2
. Experimental part
2
.1. Materials
Picolinic acid, picolylamine, phenylalanine-methyl ester, DCC (N,N’-dicyclohexyl
carbodiimide), 2, 2'-bipyridine (bpy), 1, 10-phenanthroline (phen), Cu(OAc) .H O,
2
2
LiOH.H
2
O and ethidium bromide (EB) were obtained from Sigma (99.99% purity) USA
Cl ) were purchased from Merck,
and were of analar grade. Solvents (MeOH, EtOH, CH
2
2
India. The CT-DNA was obtained from Fluka (Switzerland), supercoiled pUC19 DNA,
agarose, tris-base and tris-HCl were obtained from Bangalore Genei (India). The
chemicals were used as supplied.
2
.2. Methods
Elemental analyses were obtained from the microanalytical Heraeus Carlo Elba 1108
elemental analyzer. The molar conductivity was measured on a Digisun digital
conductivity bridge (model: DI-909) with a dip type cell. NMR spectra obtained from
Bruker biospin Avance-III 400 MHz spectrometer. Infrared spectra were recorded on a
-1
Perkin - Elmer FT-IR spectrometer, in KBr pellets in the 4000-400 cm range. Magnetic
susceptibilities of the complexes were recorded at room temperature on a Faraday
balance (CAHN-7600) using Hg[Co(NCS) ] as the internal standard. Diamagnetic
4
corrections were made by using Pascal’s constants [34]. ESI mass spectra of the
complexes were recorded using a Quattro Lc (Micro mass, Manchester, UK) triple
quadruple mass spectrometer with Mass Lynx software. UV-vis spectra of the complexes
were recorded on a Shimadzu 160A spectrophotometer (800-200nm) and Jasco V-530
5

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UV-vis spectrophotometer using 1-cm quartz micro-cuvettes. Gel pattern after the
electrophoresis was photographed by Alpha-Innotec gel documentation system (USA).
2
.3. X-ray crystallographic procedures.
Reddish brown single crystals of 1 and 2 were grown by slow evaporation of reaction
solution (aq. MeOH) for a week at room temperature. A crystal of the complex was
mounted on a glass fiber and used for data collection. Crystal data were collected at 293
K, using Bruker-Nanious SMART APEX CCD single crystal diffractrometer equipped
o
with a graphite monochromator and a Mo Kα fine-focus sealed tube (λ = 0.71073 A )
operated at 2.0 kW. The detector was placed at a distance of 6.0 cm from the crystal. Data
o
were collected with a scan width of 0.3 in ω and an exposure time of 15s/frame. The
SMART software was used for data acquisition and the SAINT-Plus software was used
for data extraction [35]. The absorption corrections were performed with the help of
SADABS program [36] The SHELX-97 programs [37] available in the WinGX package
[38] was used for structure solution and refinement. The ORTEX6a [39] and Platon [40]
packages were used for molecular graphics. All the non-hydrogen atoms were refined
with anisotropic displacement parameters. All the hydrogen atoms could be located in
Difference Fourier map. However, they were relocated at chemically meaning full
positions and were given riding model refinement. The refinement of water hydrogen
atoms were restrained such that they remain in the vicinity of the respective difference
peak. The selected crystallographic data of 1 and 2 was given in Table 1.
2
.4. Syntheses of ligand (DPPA) and complexes (1-3).
6

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The ligand DPPA was synthesized as per the literature [33]. This is described
schematically in scheme 1. The PA-I, PA-II and the final ligand (DPPA) were isolated
†
and characterized. The details are provided in supplementary information .
The ligand DPPA (0.36 g; 1 mmol) was dissolved in methanol (10 mL) and added drop
wise to an aq. methanolic solution (10 mL) of Cu(OAC) . H O (0.198 g; 1 mmol). The
2
2
solution was stirred for 6 h. at room temperature to obtain complex 1. Complex 1 (0.421
g; 1 mmol) was dissolved in aq. methanol (10 mL) and to this a methanolic solution (15
mL) of bpy (0.156 g; 1 mmol) / phen (0.198 g; 1 mmol) was added and stirring was
continued for 3 h. to afford 2 and 3 respectively (Scheme 1). The reaction solutions were
kept for slow evaporation for few days to isolate single crystals. Reddish brown crystals
were obtained for 1 and 2. A brown solid was isolated for complex 3.
[
1]: Yield: (0.357g, 85%). Elemental analyses: (Found: C, 50.70; H, 5.27; N, 11.20. Calc.
+
for C H CuN O : C, 51.06; H, 5.30; N, 11.34%). ESI-MS: m/z 422 (M + H, 100%),
2
1
26
4
6
+
-1
4
3
6
4
44 (M + Na, 34%). UV-vis: λ (H O: MeOH, 9:1)/nm: 257, 393, 530. IR: ν / cm :
max 2 max
438br (OH), 2927w (CH), 1643vs (C=O), 1599s (C=C), 1404 (C=N), 1051, 824, 765,
-1
2
-1
-3
o
90, 520w (Cu-N). µ = 1.69 BM. Λ [Ω cm mol , 10 M in aq. methanol, 25 C] =
eff
M
.60.
[2]: Yield: (0.404g, 70%). Elemental analyses: (Found: C, 55.47; H, 5.39; N, 12.33. Calc.
+
for C31
H36CuN
6
O
7
: C, 55.72; H, 5.43; N, 12.58%). ESI-MS: m/z 578 (M + H, 85%). UV-
-1
vis: λmax (H
2
O:MeOH, 9:1) /nm: 207, 270, 392, 690. IR: νmax / cm : 3425br (OH),
2
1
926w, 2862w (CH), 1615vs (C=O), 1596s (C=C), 1404s (C=N), 1339, 1292, 1157,
-
1
2
-1
-3
084, 1023, 823, 764, 695, 580m (Cu-N). µ = 1.73 BM. [Ω cm mol , 10 M in aq.
eff
o
methanol, 25 C] =12.40.
7

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[
3]: Yield: (0.420g, 70%). Elemental analyses: (Found: C, 56.92; H, 5.19; N, 12.03. Calc.
+
for C33H36CuN6O : C, 57.26; H, 5.24; N, 12.14%). ESI-MS: m/z 602 (M +H, 100%),
7
+
6
24 (M +Na, 28%). UV-vis: λ (H O:MeOH, 9:1)/nm: 222, 264, 395, 710. IR: ν
/
max
max
2
-1
cm : 3427br (OH), 3079, 2925, 2861w (CH), 1618vs (C=O), 1594s (C=C), 1428s (C=N),
-1
2
-1
-3
1
343, 1082, 850, 770, 730, 583m (Cu-N). µ = 1.80 BM. [Ω cm mol , 10 M in aq.
eff
o
methanol, 25 C] =15.70.
2
2
.5. DNA binding
.5.1. Preparation of stock solutions
Concentrated CT-DNA stock solution was prepared in 5 mM tris-HCl/50 mM NaCl in
double distilled water at pH=7.5 and the concentration of DNA solution was determined
-1
-1
by UV absorbance at 260 nm (ε = 6600 M cm ) [41] Solution of CT-DNA in 5 mM tris-
.
HCl/50 mM NaCl (pH =7.5) gave a ratio of UV absorption at 260 nm and 280 nm
A260/A280 of ca. 1.8-1.9, indicating that the DNA was sufficiently free of protein
contamination [42]. The concentration of EB solution was determined
-1
-1
spectrophotometrically at 480 nm (ε = 5680 M cm ) [43] All stock solutions were
.
o
stored at 4 C and were used within couple of days. All DNA binding and cleavage
experiments were carried out in tris buffer. The stock solutions of metal complexes were
prepared in water methanol (9:1) mixture.
Based on competitive binding experiments, it was concluded that the saturation of
binding reaches when the concentration of reacting species (DNA & complexes) are in an
equimolar (1:1) ratio. Accordingly all the measurements were performed only up to that
ratio. The major species present at the medium of interaction were either 1:1 or 1:1:1 for
binary and ternary complexes respectively.
8

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2
.5.2. Thermal denaturation studies
The thermal denaturation studies were performed on Shimadzu 160A spectrophotometer
equipped with a thermostatic cell holder. DNA (30 µM) was treated with complexes 1-3
(30 µM) in a 1:1 ratio in tris buffer at pH 7.5. The samples were continuously heated at
o
-1
the rate of 1 C min while monitoring the absorption changes at 260 nm. The melting
temperature (T ) and the melting interval (ꢀT ) data were obtained from the experiment
as per the reported procedure [44].
.5.3. Interaction of CT DNA with 1-3 by UV-vis titration
m
m
2
Absorption spectra were recorded on Jasco V-530 UV-visible spectrophotometer using 1-
cm quartz micro-cuvettes. Absorption titrations were performed by keeping the
concentration of the complexes (1-3) constant (10 µM) and monitored the charge transfer
bands absorbance changes upon increasing the concentration of CT-DNA from 0-10 µM.
In the reference cell, a blank DNA was placed so as to nullify the DNA absorbance at
measured wavelength. For the complexes 1-3, the binding constants (K ) were
b
determined from the spectroscopic titration data using the following equation [45, 46].
[
DNA]/ (ε -ε ) = [DNA]/ (ε -ε ) + 1/ K (ε -ε ) -------(1)
a f b f b b f
The ‘apparent’ extinction coefficient (ε
a
) was obtained by calculating Aobsd / [Cu]. The
terms ε and ε correspond to the extinction coefficients of free (unbound) and the fully
f
b
bound complexes, respectively. From a plot of [DNA]/(ε
a f
-ε ) Versus [DNA] will give a
slope 1/(ε -ε ) and an intercept 1/K (ε -ε ). K is the ratio of the slope to the intercept.
b
f
b
b
f
b
2
.5.4. Viscosity
Viscometric titrations were performed with an Ostwald Viscometer at room temperature.
The concentration of DNA was 200 µM, complex concentration was varied from 0-200
9

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µM and the flow times were measured with a digital timer. Each sample was measured
three times for accuracy, and an average flow time was determined. Data was presented
1
/3
as (η/ηo) vs. [complex]/[DNA], where η is the viscosity of DNA in the presence of
complex and η that of DNA alone. Viscosity values were calculated from the observed
o
flow time of DNA containing solutions (t) corrected for that buffer alone (5 mM tris
HCl/50 mM NaCl) (t ), η = (t-t ).
o
o
2
.5.5. Competitive binding
These experiments were also performed on Jasco V-530 UV-visible spectrophotometer
using 1-cm quartz micro-cuvettes. Absorption titrations were performed for 1 - 3
complexes by keeping the concentration of the EB (40 µM), DNA (40 µM) constant, and
by varying complex concentration from 0 to 40 µM.
2
.5.6. Fluorescence spectra
Fluorescence spectra were recorded with SPEX- Fluorolog 0.22m fluorimeter equipped
with a 450 W Xenon lamp. The slit widths were 2x2x2x2 and the emission spectral range
was 550-650 nm. Solution containing DNA and EB was titrated with varying
concentrations of 1-3. The concentration of DNA and EB was maintained at 41 µM and
the concentration of complexes was in the range of 0-196 µM. The solutions containing
DNA and EB were mixed in an equimolar ratio and the complexes 1-3 were added and
allowed to equilibrate till a clear solution was obtained. They were excited at 540 nm
(λmax for EB) and the fluorescence emission at 598 nm (λmax) was recorded.
Fluorescence spectra were also utilized to obtain Scatchard plots. For this, titration of
DNA against EB in the absence and presence of copper complexes were performed.
Initial concentration of DNA in tris buffer is 20 µM and the concentrations of 1-3 were
1
0

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kept at 50 µM. After each addition of EB to the solutions containing DNA and copper
complexes, the emission spectra were recorded in the range of 550 to 650 nm with
o
excitation at 540 nm at 25 C. Corrections were made to the data for the volume changes
during the course of titrations. The data were analyzed by the method of Lepecq and
Paoletti [47] to obtain bound (c ) and free (c ) concentration of EB and Scatchard plots
b
f
were obtained by plotting r /c Vs r (where r = c / conc. of DNA).
EB
f
EB
b
2
.6. DNA Cleavage
Electrophoresis experiments were performed using supercoiled pUC19 plasmid DNA (SC
DNA) in accordance with the established procedures. The cleavage of SC DNA (38 ꢁM
base pair concentration) was accomplished by the addition of Cu(II) complexes in the
absence (0-150 ꢁM, 0-80 ꢁM and 0-60 ꢁM of 1-3, respectively) and presence (0-15 ꢁM
of 1 and 0-5 for 2 &3) of 1 mM H O .The total volume of the solution was maintained
2
2
o
constant at 16 µL by adding 5 mM tris buffer. The mixtures were incubated at 37 C for a
period of 2h. The reactions were quenched and the resulting solutions were subjected to
electrophoresis. The analysis involved loading of the solutions onto 1% agarose gels
containing 2.5 ꢁM EB (2 ꢁL), and the DNA fragments separated by gel electrophoresis
(60 V for 2 h in standard tris-acetate-EDTA (TAE) buffer, pH 8). Ethidium-stained
agarose gels were imaged, and densitometric analysis of the visualized bands was used to
determine the extent of DNA cleavage.
Control experiments were carried out using Cu(OAc) .H O, free ligands at a
2
2
concentration of 1 mM. In order to gain an insight into the mechanism of hydrolytic
cleavage: the cleavage of SC DNA (38 ꢁM base-pair concentration of SC DNA) was
carried out at a concentration of 150, 80, 60 ꢁM of complexes 1-3 respectively in the
1
1

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presence of 1 mM DMSO, a known hydroxyl radical scavenger. For oxidative cleavage,
the concentration of complexes was 15 ꢁM for 1 and 5 ꢁM for 2 & 3 in the presence of 1
o
mM DMSO and 1 mM H O . Each solution was incubated at 37 C for 2h and analyzed
2
2
by the procedure described above. For evaluating kinetic data, the complex concentration
was fixed at 100 µM with different incubation times (0-60 min) under identical
experimental conditions.
3
. Results and Discussion
The complexes were soluble in water methanol mixture (9:1). They were non-
hygroscopic and stable in both solid and solution phases. The analytical data for the
complexes are in good agreement with their formulations. The molar conductance values
are too low to account for any dissociation of the complexes in aq.MeOH, suggesting the
non-electrolytic nature of the complexes. The observed magnetic moment values for 1-3
in the range of 1.69-1.80 BM are suggestive of one unpaired electron system with spin
(1/2), which represents the paramagnetic nature of Cu(II) complexes [48]. Important
physico-chemical data for 1-3 were obtained and tabulated in Table 2.
Reddish brown crystals were isolated for complexes 1 and 2 by slow evaporation of
reaction solutions for few days at room temperature. In spite of trying in different
conditions and solvents, we could not succeed in isolating single crystals for 3.
However, the complexes were comprehensively characterized by various established
physico-chemical techniques.
3
.1. Crystal structure description of 1
The ortep diagram of the complex 1 is shown in Fig. 2a and selected bond distances and
angles were given in Table 3. The complex 1 is in monoclinic system with space group
1
2

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C12/c1 and the copper center lies in a N4 coordination environment with four lattice
water molecules for 1. The ligand DPPA coordinates with Cu(II) ion in a tetra-dentate
manner utilizing N atoms of two different functions, namely amidate (deprotonated
amide) (N1 and N2) and pyridine (N3 and N4) as potential donor sites in 1 resulting in
the formation of three five membered chelating rings (CuNCCN). The bond distances
from metal center to donor sites are Cu(1)-N(1) 1.893(3), Cu(1)-N(2) 1.916(3), Cu(1)-
N(3) 2.013(3) and Cu(1)-N(4) 2.027(3) respectively. The pyridyl N-atoms are slightly
elongated from the ideal M-N bond distances. The bond angles around the Cu(II) are not
orthogonal at 82.60(13), 82.68(12), 82.45(11), and 112.24(12) for each N(1)-Cu(1)-N(2),
N(1)-Cu(1)-N(3), N(2)-Cu(1)-N(4) and N(3)-Cu(1)-N(4)1 respectively are slightly
o
deviated from 90 . However the sum of the angles around Cu(II) is very close to 360˚
indicating a planar geometry. Therefore, the coordination geometry around Cu(II) is
slightly distorted from ideal square-planar as elucidated by bond angles and inter planar
angles around the metal atom, which are composed of three five membered chelating
rings (CuNCCN) in complex 1. Dihedral angle [°] between the least-square planes (N1-
Cu-N2) and (N3-Cu-N4) = 2.090(1) ° is a measure of deviation from square-planarity.
3
.2. Crystal structure description of 2
Similarly, the complex 2 is also a monoclinic system with P2(1)/c space group. The ortep
diagram (Fig. 2b) of 2 shows N5 coordination with approximate square pyramidal
geometry around the copper center. In this complex DPPA acts as a tri-dentate ligand in
contrast to its tetra-dentate nature with Cu(1)-N(1) 2.043(3), Cu(1)-N(2) 1.928(3) and
Cu(1)-N(3) 1.956(3) bond distances (Table 3), and occupying three equatorial positions
of basal plane of the square. The secondary ligand bpy occupies one equatorial and one
1
3

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axial position with Cu(1)-N(5) 2.032(3) and Cu(1)-N(6) 2.255(3) bond distances
respectively. The Cu-N axial distance is elongated with respect to the Cu-N basal
9
distances by ~0.20 is typical for d Cu(II) centers with a square pyramidal geometry. The
bond angles between two coordinates in square are 81.89(13), 95.29(13), 82.54(12) and
9
6.95(12) for each N(1)-Cu(1)-N(2), N(1)-Cu(1)-N(5), N(2)-Cu(1)-N(3) and N(3)-Cu(1)-
N(5) respectively. The sum of the square angles is 356.67º which is slightly deviated
from ideal square (360º). The axial nitrogen (N6) also show small angle towards N(1)-
Cu(1)-N(5) plane compared to other planes due to the rigidity of the bpy molecule. This
also accounts for the distortion in the geometry.
3
.3. IR spectra.
The IR spectral data of the free ligand (DPPA) and the Cu(II) complexes with their
relative assignments were studied to characterize their structures. The assignments of the
infrared spectra were made on the basis of literature and Nakamoto [49]. The IR spectra
of the complexes were analyzed in comparison with their free ligand spectra (†Fig.S6a-
c). In the IR spectra of 1-3 (†Fig.S6d-f) a broad peak appeared at 3438, 3425 and 3427
-1
cm respectively for 1-3 were assigned to the υ(O-H) of water molecules present in the
-1
crystal lattice. The IR spectra of ligand exhibited a sharp peak at 3293 cm which was
attributed to the υ(N-H) vibrations, disappeared in the complexes due to the deprotanation
of amide group upon complexation with copper. This indicates that the nitrogen atoms of
the amide group coordinated to Cu(II) ion. An intense stretching absorption detected at
-
1
1
1
655 cm was due to the amide carbonyl group υ(C=O) of DPPA, which was shifted to
-
1
643, 1615 and 1618 cm in complexes 1-3 respectively. The peaks corresponding to the
-1
ring stretching frequencies (ν(C=C) and ν(C=N)) at 1579 and 1454 cm for free bpy and
1
4

ACCEPTED MANUSCRIPT
-1
-1
at 1506 and 1419 cm for phen were shifted to 1596 and 1441cm for 2 (1553 and 1428
-1
cm for 3) upon complexation. And the characteristic low energy pyridine ring in and
-1
out-of-plane hydrogen bending modes of free bpy observed at 758 and 619 cm (850 and
-1
-1
-1
7
34 cm for phen) were shifted to 764 and 703 cm for 2 (848 and 730 cm for 3),
which is an indication of the coordination of the heterocyclic nitrogen to copper. The new
-1
non-ligand peaks at 527, 580 and 583 cm for 1-3 respectively were assigned to υ(Cu-N)
vibration, [50] also indicates that the ligands are coordinated to copper through nitrogen
atoms.
3
.4. UV-vis spectra
The electronic spectra of the Cu(II) complexes (1-3) recorded in aq. methanol at room
temperature show absorption bands with varied intensity. The electronic spectra of 3 was
shown in Fig. 3 (†Fig. S7 for 1and 2). The intense UV bands at 257 (1), 207, 270 (2) and
2
22, 264 nm (3) were assigned to intra-ligand (π-π*) transition of DPPA and bpy/phen
ligands, while the less intense bands in the range 392-395 nm of 1 - 3 were assigned to
the LMCT transitions. Besides, in the visible region a broad band (insets of res. Figures)
4
4
observed at 530 nm for 1 was assigned to A
2 1
→ T transition, which suggests that the
complex was arranged in a distorted square planar geometry. On the other hand for the
2
2
complexes 2 and 3 the bands at 690 and 710 nm were due to B g→ A g transition
1
1
supporting an approximate square pyramidal geometry around copper [51].
3
.5. ESI-MS studies
Formation of the ligand (DPPA) and its Cu(II) complexes was confirmed by electrospray
ionization mass spectrometry (ESI-MS). A peak observed in the DPPA spectrum at m/z
+
3
61 (†Fig. S5 c) assigned to the [M + H] is of molecular ion species. The ESI-MS
1
5

ACCEPTED MANUSCRIPT
+
+
spectra of complex 1 show peaks at m/z 422 [M + H] and 444 [M + Na] , complex 2 at
+
+
m/z 578 [M + H] and complex 3 show peaks at m/z 602 [M + H] and m/z 624 [M +
+
Na] . These m/z values indicate different stiochiometric ratios for the complexes: 1:1
(Cu:DPPA) for 1 and 1:1:1 (Cu:DPPA:bpy and Cu:DPPA:phen) for 2 and 3 respectively.
3
.6. DNA binding
DNA is the primary intracellular target of many anticancer drugs [52]. DNA binding is
the critical step for DNA cleavage in most cases and has importance in understanding the
mechanism of tumor inhibition for the treatment of cancer. Therefore the binding mode
and affinity involved between the complexes (1-3) and CT-DNA was investigated by
thermal denaturation, UV-vis absorption spectra, competitive binding, viscosity and
fluorescence quenching studies in tris-HCl buffer.
3
.6.1. Thermal denaturation
Thermal denaturation experiments were carried out to study the stability of the duplex
CT-DNA in the presence of complexes. The interaction of small molecules with double-
helical DNA may increase or decrease T
m
, the temperature at which the double helix is
broken up into a single-stranded DNA. While increase in T
m
values suggest an
intercalative or phosphate binding, decrease is typical for base binding. The thermal
denaturation profile of DNA in the absence and presence of complexes 1-3 is provided in
o
Fig. 4. As can be seen, an increase of 4-9 C for 1-3 (Table 4) was observed in the T
m
profile of complexes as compared to free DNA. The DNA melting data suggest an
intercalative/phosphate mode of binding with copper complexes. The ꢀT values follow
m
the order of 3>2>1 which is in accordance with their aromaticity.
3
.6.2. Interaction of CT DNA with 1-3 by UV-vis titration
1
6

ACCEPTED MANUSCRIPT
UV-visible absorption spectral measurements were performed for electronic perturbations
for the interaction of complexes with CT-DNA to determine the intrinsic binding
constants (K ) [53]. The absorption spectra of the complex 3 in the absence and presence
b
of increasing amounts of DNA is illustrated in Fig. 5. On the addition of increasing
amounts of DNA to the complex 3, both hypochromic (23%) (14% for 1 19% for 2) and
bathochromic shifts from 264-270 nm (257-261 nm for 1 and 270-274 for 2) were
observed. The hypochromism and bathochromism were suggested to arise due to the
interaction between the electronic state of an intercalating chromophore (complex) and
those of the DNA bases [54, 55]. These results demonstrate that the complex may bind
to DNA through an intercalation of the complex into the double helix structure of DNA.
Similar results were obtained for 1 and 2 (†Fig. S8). The intrinsic binding constant (Kb)
for the association of the complexes with CT-DNA (insets of res. figures) were
4
-1
5
-1
5
-1
determined as 8.3±0.01 x10 M , 1.23±0.01 x10 M and 1.35±0.01 x 10 M for
complexes 1, 2 and 3 respectively (Table 4) by using equation-(1). The lower binding
constant of binary complex (1) as compared to ternary complexes (2 and 3) is due to the
absence of planar heterocyclic bases in the former. The higher DNA binding ability of the
phen complex compared to its bpy analogue is due to the presence of an extended
aromatic phenyl ring (in phen) which might facilitate partial intercalation of the base
moiety through noncovalent π-π interaction with the DNA bases.
3
.6.3. Viscosity
The viscosity measurements were carried out on CT-DNA by increasing the
concentration of the added complexes and EB, a known DNA intercalator for
comparison. The effects of the Cu(II) complexes (1-3) and EB on the viscosity of CT-
1
7

ACCEPTED MANUSCRIPT
DNA was shown in Fig. 6. In the presence of complexes, the viscosity of DNA has been
found to increase similar to that of EB, a proven DNA intercalator, suggesting that the
complexes bind to DNA through an intercalation. This result is consistent with the
classical intercalators [56, 57]. The increased degree of viscosity, which may depend on
their affinity to DNA follow the order 1<2<3<EB. This is attributed to an increase of
aromatic planar rings from complexes 1-3.
3
.6.4. Competitive binding
Intercalative binding was also demonstrated through competitive binding experiments
with EB as an intercalative probe [58] using absorption spectroscopy. The study
involves the addition of complexes to DNA pre-treated with EB and recording of the
intensity of absorption. On interaction with DNA, free EB which has an absorption
maxima ~480 nm was shifted to a higher wavelength followed by a decrease in
absorption. This provides a reasonable evidence for the intercalation of EB into the DNA
base stacks [59]. Fig.7 reflects the above conclusion where the absorption maxima of
free EB at 478 nm was shifted to 481 nm in the presence of DNA. Addition of 3 to the
EB-DNA solution caused an enhancement in the absorption intensity, which is an
indication of competitive binding of 3 with EB to bind to DNA. Similar trends were
observed for 1 and 2 (†Fig. S9). The results clearly suggest that 1-3 bind to DNA through
an intercalative mode by the release of intercalatively bound EB. These results are
consistent with those obtained from the spectroscopic studies and viscosity
measurements.
3
.6.5. Fluorescence spectroscopy
1
8

ACCEPTED MANUSCRIPT
The quenching experiments based on the displacement of an intercalating drug EB from
CT-DNA will provide further information about the relative binding affinity of the
complexes to CT-DNA with respect to EB. The fluorescence quenching curves of EB
bound to DNA in the absence and presence of 3 was given in Fig.8.
It may be seen from the Figure, the EB-DNA system shows characteristic strong emission
at about 598 nm when excited at 540 nm, indicating that the intercalated EB molecules
have been successfully protected by the hydrophobic environment inside the DNA helix
2
from being quenched by H O; as a result the accessibility of solvent molecules to EB is
reduced. A remarkable reduction in emission intensity was observed when 3 was added to
EB-DNA system, characteristic for the intercalative binding of the complex with DNA.
Similar trends were observed for 1 and 2 (†Fig. S10). Some mechanisms have been
proposed to account for this reduction in the emission intensity: the replacement of
molecular fluorophores, [60, 61] electron transfer, [62] energy transfer [63] and proton
transfer [64].
The quenching efficiency for each complex was evaluated by the Stern-Volmer constant
o o
Ksq, which varies with the experimental conditions, I /I = 1+Ksq.r; where I and I are the
fluorescence intensities in the absence and presence of the complex, respectively, and r is
the concentration ratio of the complex to DNA. K is a linear Stern-Volmer quenching
sq
constant. The quenching plots illustrate that the quenching emission of EB-DNA system
by the complexes are in good agreement with the linear Stern Volmer equation [65],
which also suggest that the complexes are bound to DNA. The K value was obtained as
sq
the ratio of the slope to intercept. The Ksq values for the complexes 1-3 are 0.09, 0.17 and
0
.25 respectively (insets of respective figures and Table 4).
1
9

ACCEPTED MANUSCRIPT
Fluorescence Scatchard plots for the binding of EB to CT-DNA in the presence of the
complexes were obtained as described by Lepecq and Paoletti [47]. From the Scatchard
plots, complexes can be classified as Type A or Type B. In Type A complexes, Scatchard
plot show decrease in the slope in the presence of increasing amounts of metal complex,
with no change in the intercept on the abscissa suggesting a competitive inhibition of
ethidium bromide binding (intercalation) [66, 67] whereas Type B complexes show a
change in slope and intercept, suggesting intercalative and covalent interactions with
DNA [68, 69]. The binding isotherms of EB and DNA in the absence and presence of 1-3
were determined experimentally and presented in Fig.9. As can be seen from the plots a
decrease in slope with no change in the intercept was observed upon the addition of 1-3
indicating an intercalative mode of binding of the complexes with DNA. These results
also concur with those obtained from other techniques viz. absorption, viscosity and
emission spectral studies.
In summary, the DNA binding studies revealed that the binding of 1-3 to CT-DNA was
through an intercalation.
3
.7. DNA cleavage.
Natural plasmid DNA mainly has a closed circle supercoiled form (SC), and nicked
circular (NC) and linear forms (LC) as small fractions. Relaxation of supercoiled pUC19
DNA into nicked circular and linear conformation was used to quantify the relative
cleavage efficiency of the complexes (1-3) by agarose gel electrophoresis technique. At
neutral pH, DNA is negatively charged so it moves through the gel from cathode towards
anode. It is also a useful technique to investigate various binding modes of small
molecules to SC DNA. Intercalation of small molecules to plasmid DNA can loosen or
2
0

ACCEPTED MANUSCRIPT
cleave the SC DNA form, which results in the decrease of its (SC DNA) mobility rate and
can be separately visualized by agarose gel electrophoresis method. The gel
electrophoretic mobility assay (Fig.10) reveal the conversion of supercoiled (SC) plasmid
pUC19 DNA into nicked circular (NC) form by complexes 1-3. For DNA cleavage
experiments its concentration was 38 ꢁM base pairs.
3
.7.1. Oxidative cleavage
The ability of the complexes 1-3 in inducing oxidative DNA cleavage was studied. On
reaction with SC DNA in the presence of hydrogen peroxide (H ), all complexes show
2
O
2
efficient nuclease activity (Fig.10a; lanes 7-10 for 1; Fig.10b lanes 8-10 for 2 and Fig.10c
lanes 8-11 for 3). Control experiment with only H O (lane 7) does not show any
2
2
appreciable DNA cleavage (<10%), but in the presence of complexes 1-3 the SC DNA
was totally converted to NC form. Total conversion of SC to NC form was achieved by
complex 1 at a concentration of 10 ꢁM where as only 5 ꢁM was required for complexes 2
and 3. Total inhibition of cleavage was observed in the presence of DMSO, a hydroxyl
radical scavenger. This indicates the possible involvement of hydroxyl radical and/or the
reactive copper-oxo species in the cleavage pathway. On the basis of this observation, the
mechanism of DNA cleavage mediated by 1-3 may be proposed as follows: DNA
cleavage is redox-mediated – The complexes would first interact with DNA through
intercalation to form a Cu(II)---DNA adduct species followed by its reduction by the
external agent (H O ) to a Cu(I)---DNA system, which then generates hydroxyl radicals
2
2
on reaction with O . These hydroxyl radicals would then attack DNA, causing strand
2
scission (Scheme 2). This pathway was proposed by Sigman and co-workers for the
2
1

ACCEPTED MANUSCRIPT
oxidative cleavage reaction of the bis(phen)copper complex [70] which is the most
accepted one.
3
.7.2. Hydrolytic cleavage
Attempts were also made to cleave DNA through hydrolysis of phosphodiester bond.
Since this process does not require any external agents and light it has biological
relevance. When DNA was incubated with increasing concentrations of complexes SC
DNA was degraded to NC form. The catalytic activities of 1-3 are depicted in Fig.10. All
complexes start their activity at a conc. as low as 10 µM. At 100 µM a complete
conversion of supercoiled plasmid DNA into the nicked circular form was observed for 1
(
Fig.10a; lane 6). In contrast complete conversion was achieved at 80 µM (Fig. 10b; lane
) for 2 and at 60 µM for 3 (Fig. 10c; lane 6); on increasing the concentration of 3 to 100
µM further conversion of the NC form to linear form (LF) was observed (Fig. 10c; lane
7
7
). As can be seen from the above results the hydrolysis of DNA follows the order
1
<2<3, this is due to the extended planar aromatic moieties in 1-3 which can insert deeply
in to the bases of DNA.
To ensure that the copper complexes are solely responsible for the cleavage, several
control experiments were performed under identical conditions (Fig.10d). No cleavage of
DNA was observed with free copper and free ligands (1 mM). In a control experiment
with DMSO (1 mM), a known radical scavenger, only slight inhibition (< 2% >) of DNA
•
cleavage was observed. This rule-out the possibility of DNA cleavage via OH –based
depurination pathway and also a possible oxidative cleavage [71] (dioxygen + copper
complex + traces of DNA). This suggests that the DNA cleavage was the result of
hydrolysis of a phosphodiester bond. Thus, the present study reveals that no diffusable
2
2

ACCEPTED MANUSCRIPT
2 2
radical is involved in the DNA cleavage as no external agents like H O , MPA, ascorbic
acid or light were employed in the cleavage experiments.
3
.7.3. Kinetics
The cleavage of pUC19 DNA by 1-3 was kinetically monitored by quantification of SC
and NC forms of DNA. The observed distribution of SC and NC DNA in agarose gel
provides a measure of the extent of hydrolysis of the phosphodiester bond in each
plasmid DNA, and the data were used to perform simple kinetics. The kinetic plots
(
Fig.11 for 3 and †Fig.S11 for 1 and 2) which show a decrease of SC and an increase of
NC forms of DNA versus time, follow the pseudo-first-order kinetics and both the forms
fitted well to a single exponential curve. From these curve fits, the DNA hydrolysis rates
-1
-1
-1
were determined as 2.09 h (R=0.985), 2.45h (R=0.963) and 2.89 h (R= 0.987) for 1-3
respectively. The enhancement of DNA hydrolysis rate constant by metal complexes in
-1
the range of 0.09-0.25 h was considered impressive.[83] With the rate constants of 2.09
-1
7
(
1), 2.45 (2) and 2.89 h (3) it amounts to an order of (5.8-8.0)x10 fold rate
-8
-1
enhancement compared to non-catalyzed double stranded DNA (3.6x10 h ) [72].
4
.
Conclusions
In view of the importance of picolinic acid in preventing cell growth and arresting cell
cycle, we have successfully isolated three new picolinic acid based copper(II)
complexes
[Cu(II)(DPPA)].4H
2
O
(1),
[Cu(II)(DPPA)(bpy)].5H
2
O
(2)
and
[
Cu(II)(DPPA)(phen)].5H
2
O (3) and structurally characterized. Single crystal x-ray
structures were determined for complexes 1 and 2. The complexes arranged in square
planar geometry for 1 and square pyramidal geometry for 2 & 3. Interestingly, the ligand
behaves as a tetradentate in square planar complex and tridentate in square pyramidal
2
3

ACCEPTED MANUSCRIPT
complexes. The DNA binding studies revealed that they bind to CT DNA in an
intercalative mode of interaction. The binding constants decrease in the order 3>2>1
which was attributed to the increasing aromaticities of the complexes. The complexes
exhibit efficient nuclease activity towards supercoiled pUC19 DNA. The cleavage
activity of the complexes increase in the order 1<2<3. The studies suggest the DNA
cleavage was carried out by involvement of hydroxyl radical in the oxidative pathway,
whereas in the hydrolytic cleavage hydroxyl radical was not generated. The present
Cu(II) complexes are solely responsible for hydrolytic cleavage. The kinetic data indicate
that the cleavage reactions follow pseudo first order rate constants and the rates for the
cleavage of DNA phosphodiester bond are impressive. Thus, the picolinic acid based
ligands with increased aromaticities may be developed as DNA recognition elements in
designing efficient metal based anti cancer agents.
Abbreviations.
DPPA
Pyridine-2-carboxylic acid {2-phenyl-1-[(pyridin-2-ylmethyl)-carbonyl]-
ethyl}-amide
PA
Picolinic acid
bpy
2, 2'-Bipyridine
phen
CT-DNA
1, 10-Phenanthroline
Calf-thymus deoxyribonucleic acid
FT-IR spectra Fourier transform Infrared spectra
UV-vis
ESI-MS
EB
Ultraviolet visible
Electro spray ionization mass spectra
Ethidium bromide
2
4

ACCEPTED MANUSCRIPT
MeOH
DMSO
SC DNA
NC
Methanol
Dimethyl sulfoxide
Supercoiled deoxyribonucleic acid
Nicked circular
Acknowledgements. The financial support to PRR from the Council of Scientific and
Industrial Research and University Grants Commission, Govt. of India is gratefully
acknowledged. NR thanks CSIR for Senior Research Fellowship.
Appendix A. †Supplementary Information
Crystallographic data found at Cambridge crystallographic data centre, CCDC: 788099
1
for 1 and 811860 for 2. Synthetic procedure of DPPA ligand, H-NMR spectra of PA-I
1
1
13
(
Fig. S1), H-NMR spectra of PA-II (Fig. S2), H-NMR spectra of DPPA (Fig. S3), C-
NMR spectra of DPPA (Fig. S4), ESI-ms Spectra of PA-I, PA-II and DPPA (Fig. S5),
IR spectra of free ligands and complexes 1-3 (Fig. S6a-f), Electronic absorption spectra
(
Fig. S7). DNA binding study by absorption spectroscopy (Fig. S8), competitive binding
by UV (Fig. S9), fluorescence quenching (Fig. S10) and kinetic study (Fig. S11) of
complexes 1(a) and 2(b).
References
[1] L. J. Boerner, J. M. Zaleski, Curr. Opin. Chem. Biol. 9 (2005) 135–144.
[2] Y. W. Jung, S. J. Lippard, Chem. Rev. 107 (2007) 1387–1407.
[3] S. E. Sherman, S. J. Lippard, Chem. Rev. 87 (1987) 1153–1181.
[4] A. S. Abu-Surrah, M. Kettunen, Curr. Med. Chem. 13 (2006) 1337–1357.
[5] E. R. Jamieson, S. J. Lippard, Chem. Rev. 99 (1999) 2467–2498.
2
5

ACCEPTED MANUSCRIPT
[6] S. Dhar, D. Senapati, P. K. Das, P. Chatopadhyay, M. Nethaji, A. R. Chakravarty, J.
Am. Chem. Soc. 125 (2003) 12118-12124.
7] H. Prakash, A. Shodal, H. Yasul, H. Sakural, S. Hirota, Inorg. Chem. 47 (2008)
045-5047.
8] D. Li, J. Tian, Y. Kou, F. Huang, G. Chen, W. Gu, X. Liu, D. Liao, P. Cheng, S. Yan,
Dalton Trans. (2009) 3574-3583.
[
5
[
[
[
(
[
9] Y. Sun, L. E. Joyce, N. M. Dickson, C. Turro, Chem. Comm. 46 (2010) 2426-2428.
10] A. Jain, J. Wang, E. R. Mashack, B. S. J. Winkel, K. J. Brewer, Inorg. Chem. 48
2009) 2711-2718.
11] L. A. Mullice, R. H. Laye, L. P. Harding, N. J. Buurma, S. J. A. Pope, New J.
Chem. 32 (2008) 2140-2149.
12] L. Z. Li, C. Zhao, T. Xu, H. W. Ji, Y. H. Yu, G. Q. Guo, H. Chao, J. Inorg.
Biochem. 99 (2005) 1076-1082.
13] J. Reedijk, in Heterocyclic Nitrogen-Donor Ligands Comprehensive Coordination
[
[
Chemistry, ed. G. Wilkinson, R. D. Gillard and J. A. McCleverty, Vol. 2, Pergamon
Press, Oxford, 1987, pp. 73-. (and references therein).
[14] D. A. House, in Ammonia and Amines. doi:10.1039/C2DT30512G Comprehensive
Coordination Chemistry, ed. G. Wilkinson, R. D. Gillard and J. A. McCleverty, Vol . 2,
Pergamon Press, Oxford, 1987, pp. 23-.(and references therein).
[
15] F. Grob, A. Müller-Hartmann, H. Vahrenkamp, Eur. J. Inorg Chem. 11 (2000) 2363-
370.
16] W. L. Driessen, R. A. G. Graaf, F. J. Parevliet, J. Reedijk, R. M. de Vos, Inorg.
Chim. Acta 216 (1994) 43-49.
2
[
2
6

ACCEPTED MANUSCRIPT
[17] S. Wang, Q. Luo, X. Wang, L. Wang, K. Yu, J. Chem. Soc., Dalton Trans. (1995)
2
045-2055.
18] H. Adams, N. A. Bailey, J. D. Crane, D. E. Fenton, J. M. Latour, J. M. Williams, J.
Chem. Soc., Dalton Trans. (1990) 1727-1735.
[
[
(
[
19] T. Bortolotto, P. P. Silva, A. Neves, E. C. Pereira-Maia, H. Terenzi, Inorg. Chem. 50
2011) 10519-10521.
20] T. Hirohama, Y. Kuranuki, E. Ebina, T. Sugizaki, H. Arii, M. Chikira, P. Tamil
Selvi, M. Palaniandavar, J. Inorg. Biochem. 99 (2000) 1205- 1219.
[
[
[
[
[
21] S. Anbu, S. Shanmugaraju, M. Kandaswamy, RSC Adv. 2 (2012) 5349-5357.
22] F. He, L. Tao, X. Li, Y. Li, Z. Wu, C. Yan, New J. Chem. 36 (2012) 2078-2087.
23] P. R. Reddy, K. S. Rao, B. Satyanarayana, Tetrahedron Lett. 47 (2006) 7311-7315.
24] P. R. Reddy, P. Manjula, Chem. Biodiv. 4 (2007) 468-480.
25] P. R. Reddy, P. Manjula, T. K. Chakraborty, R. Samanta, Chem. Biodiv. 6 (2009)
7
64-773.
[
[
[
(
[
[
[
26] P. R. Reddy, A. Shilpa, Ind. J. Chem. 49A (2010) 1003-1015.
27] P. R. Reddy, A. Shilpa, Polyhedron, 30 (2011) 565-572.
28] P. R. Reddy, A. Shilpa, N. Raju, P. Raghavaiah, J. Inorg. Biochem. 105
2011)1603-1612.
29] P. R. Reddy, N. Raju, B. Satyanarayana, Chem. Biodiv. 8 (2011) 131-144.
30] P. R. Reddy, N. Raju, Polyhedron, 44 (2012) 1-10.
31] G. Frumento, R. Rotondo, M. Tenetti, G. Domonte, U. Benatti, G. B. Ferrara, J.
Exp. Med. 196 (2002) 459-468.
32] J. A. Fernadez-pol, D. J. Klos, P. D. Hamilton, Anticancer Res. 21 (2001) 3773-.
[
2
7