Novel bio-essential metal based complexes linked by heterocyclic ligand: Synthesis, structural elucidation, biological investigation and docking analysis

Article abstract of DOI:10.1016/j.jphotobiol.2015.11.011

New series of bio-essential metal based complexes linked by Schiff base ligand (L) and 2,2′-bipyridine (bpy) have been synthesized and characterized by diverse spectral techniques such as elemental analysis, magnetic susceptibility, molar conductivity mea

Full text of DOI:10.1016/j.jphotobiol.2015.11.011

Journal of Photochemistry & Photobiology, B: Biology 154 (2016) 67–76
Contents lists available at ScienceDirect
Journal of Photochemistry & Photobiology, B: Biology
journal homepage: www.elsevier.com/locate/jphotobiol
Novel bio-essential metal based complexes linked by heterocyclic ligand:
Synthesis, structural elucidation, biological investigation and
docking analysis
T. Arun a, R. Subramanian b, N. Raman a,
⁎
a
Research Department of Chemistry, VHNSN College, Virudhunagar 626 001, Tamilnadu, India
Centre for Scientific and Applied Research, PSN College of Engineering and Technology, Tirunelveli 627152, Tamilnadu, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 7 October 2015
Received in revised form 6 November 2015
Accepted 10 November 2015
Available online 3 December 2015
New series of bio-essential metal based complexes linked by Schiff base ligand (L) and 2,2′-bipyridine (bpy) have
been synthesized and characterized by diverse spectral techniques such as elemental analysis, magnetic
susceptibility, molar conductivity measurements, FT-IR, UV–Vis., 1H NMR, 13C NMR, EPR and Mass. The spectral
data suggest that the metal complexes espouse octahedral geometry around the metal ions. Interactions of the
complexes with CT DNA have been explored by electronic absorption, ethidium bromide displacement assay,
viscosity measurements, cyclic voltammetry and differential pulse voltammetry in order to evaluate the possible
DNA-binding mode and to calculate the corresponding DNA-binding constants. The DNA interaction studies pro-
pose that the intercalative mode of interaction and the complexes exhibit oxidative cleavage of pUC19 DNA in the
presence of hydrogen peroxide as activator. The synthesized Schiff base ligand and its metal complexes have
been screened for anti-microbial activity by micro dilution method against two Gram-positive bacteria
(Staphylococcus aureus and Bacillus subtilis), two Gram-negative bacteria (Escherichia coli and Salmonella typhi)
and three fungi strains (Fusarium solani, Aspergillus niger and Candida albicans) revealing that the complexes
are good anti-pathogenic agents than the ligand. Moreover, molecular docking analysis has been performed to
confirm the nature of binding of the complexes with DNA.
Keywords:
Heterocyclic ligand
DNA binding
DNA cleavage
Anti-microbial activity
Docking analysis
© 2015 Elsevier B.V. All rights reserved.
1. Introduction
The important function of DNA in living process is that it acts as
storage and transporter of genetic information in a cell and also the
Schiff bases are vital class of ligands due to their synthetic flexibility,
selectivity and sensitivity towards the central metal atom. These ligands
are considered as ‘privileged’ ligands in coordination chemistry due to
simple preparation by the condensation process of primary amines
and an aldehyde [1,2]. Schiff base ligands containing azomethine
(\\HCN) group is particularly suited for coordination of metal ions via
N-atom of the lone pair. Recently, these ligands have received much
consideration mainly because of their wide application in the field of
optical materials [3], catalysis [4], drug design, biological probes [5]
and used as chemical sensor [6]. The Schiff base ligands with nitrogen
and oxygen donor sites in their structures act as good chelating agents
for the transition metal ions. Coordination of Schiff bases with metal
ions such as copper, cobalt, nickel and zinc, often enhances their activi-
ties [7]. Moreover, the complexes of Schiff bases showed promising bio-
logical activity including antitumor, antibacterial, antifungal, herbicidal,
medical imaging, and biological modeling applications [8–10].
most important intercellular target of anticancer drugs [11,12]. The
different sequences present in the DNA are concerned in various regula-
tory progression such as gene expression, gene transcription, mutagen-
esis and carcinogenesis etc. [13]. These progressions can be modified by
the interaction of metal complexes with DNA leading to DNA damage.
More recently, Shahabadi et al., have reported that the interaction be-
tween the small molecules and DNA can cause DNA damage. Further,
it blocks the division of cancer cells resulting in cell death [14,15]. For
these reasons, exploring and designing of novel metal based complexes
capable of interacting with DNA are necessary nowadays. Metal
complexes interact with DNA duplex generally by variety of binding
modes such as non-covalent and covalent interactions [16]. In the
non-covalent interactions, intercalation is one of the most predominant
binding sites owing to its various application in anti-tumor and
molecular biology [17].
On the basis of the above stated facts, the synthesis of new Schiff
base from 3,4-dimethoxybenzaldehyde and o-phenylenediamine has
been reported. The Schiff base ligand has been coordinated to bio-
essential transition metal ions Cu(II), Co(II), Ni(II) and Zn(II) to form
the mixed ligand complexes using 2,2′-bipyridine as ancillary ligand.
⁎
Corresponding author.
E-mail address: ramanphotochem@gmail.com (N. Raman).
http://dx.doi.org/10.1016/j.jphotobiol.2015.11.011
1011-1344/© 2015 Elsevier B.V. All rights reserved.

68
T. Arun et al. / Journal of Photochemistry & Photobiology, B: Biology 154 (2016) 67–76
Further, the ligand and metal complexes have been characterized
successfully by physicochemical and various spectroscopic techniques.
The bio-relevancy of these complexes have been efficiently examined
and explored by DNA binding, DNA cleavage and anti-microbial studies.
Finally, the docking analysis has been executed to confirm the mode of
binding of complexes with DNA.
(BM) 1.81; UV–Vis. in DMSO, nm (transition): 410 (LMCT) and 620
(d–d).
[CoL(bpy)Cl2]: Yield: 71%; brown color; Anal. Calc.(%): C (59.1), H
(4.6), N (8.1) and Co (8.5); Found (%): C (58.2), H (4.2), N (7.6)
and Cu (7.8); FT-IR (KBr) (cm−1): 1595 (−CH = N), 1592
(−C
=
N) (bpy), 354 (M-Cl) and 420 (M-N);
∧
m
(Ω−1 mol−1 cm2) 8.3; μeff (BM) 4.63; UV–Vis. in DMSO, nm
(transition): 388 (LMCT) and 560 (d–d).
2. Experimental
2.1. Materials
[NiL(bpy)Cl2]: Yield: 64%; yellow color; Anal. Calc (%): C (59.1), H
(4.6), N (8.1) and Ni (8.5); Found (%): C (57.3), H (4.1), N (7.5)
and Ni (8.2); FT-IR (KBr) (cm−1) 1602 (−C = N), 1588 (−C = N)
(bpy), 367 (M-Cl) and 434 (M-N); ∧m (Ω−1 mol−1 cm2) 7.2; μeff
(BM) 3.28; UV–Vis. in DMSO, nm (transition): 407 (LMCT) and 710
(d–d).
[ZnL(bpy)Cl2]: Yield: 68%; greenish yellow color; Anal. Calc (%) C
(58.5), H (4.6); N (8.0) and Zn (9.3); Found (%): C (57.6), H (4.2),
N (7.8) and Zn (8.7); FT-IR (KBr) (cm−1): 1598 (\\CHN), 15,854
(CN) (bpy), 362 (M–Cl) and 438 (M–N); 1H NMR (DMSO-d6)
δppm: (Ar–H) 6.4–7.5 (m), (\\CHN) 8.5 (s) and (\\O-CH3) 3.4
(s) ppm; 13C NMR (DMSO-d6): δppm 119.0–146.2 (Ar–C), 158.4
The chemicals involved in this work were of AnalaR grade and
were used without further purification. 3,4-dimethoxybenzal-
dehyde, o-phenylenediamine and 2,2′-bipyridine (bpy) were ob-
tained from Sigma Aldrich. Calf thymus DNA (CT DNA) (Himedia,
India) and pUC19 plasmid DNA (Bangalore Genei, India) were used
for DNA binding and cleavage studies. All other chemicals, solvents
and metal salts were procured from E-Merck, India.
2.2. Physical Measurements
Elemental analysis (C, H and N) data were obtained using a Perkin-
Elmer 240 elemental analyzer. Vibration spectra were performed on
FTIR–Shimadzu model IR-Affinity-1 spectrophotometer using KBr
discs. The NMR spectra of the ligand and Zn(II) complex were recorded
on a Bruker Advance DRX 300 spectrometer operating at room temper-
ature (RT). RT magnetic susceptibility measurements were carried out
on a modified Gouy-type magnetic balance, Hertz SG8-5HJ. The molar
conductivity of the complexes in DMSO solution (10−3 M) was
measured in a deep vision 601 model digital conductometer at RT. The
EPR spectrum was accomplished at liquid N2 temperature (77 K)
using tetracyanoethylene (TCNE) as the g-marker. Shimadzu Model
1601 UV–Visible spectrophotometer and Hitachi F–2500 fluorescence
spectrophotometer were employed to attain the electronic and fluores-
cence spectra, respectively. Cyclic voltammetric (CV) experiments were
achieved on a CHI 620C electrochemical analyzer in freshly distilled
DMSO solution.
(\\CHN), 153.3 (\\CN) and 53.4 (\\O-CH3) ppm;
(Ω−1 mol−1 cm2) 16.4; μeff (BM) diamagnetic; UV–Vis. in DMSO,
nm (transition): 380 (LMCT).
∧
m
2.5. Biological Evaluation
The methods used in the biological evaluation (DNA binding, DNA
cleavage and antimicrobial screening) are detailed in supplementary file.
2.6. Docking Analysis
Docking analysis of L and its metal complexes have been performed
using HEX 6.3 software which is an interactive molecular graphics
program for the interaction, docking calculations and to identify
possible docked poses of the biomolecules. The crystal structure of
DNA duplex of sequence d(CGCGAATTCGCG)2 dodecamer (PDB ID:
1BNA) is downloaded from the protein data bank.
2.3. Synthesis of Schiff Base (L)
For the synthesis of Schiff base ligand (L), an ethanolic solution of
3,4-dimethoxybenzaldehyde (0.2 mM) was added to an ethanolic
solution of o-phenylenediamine (0.1 mM) and the resultant mixture
was refluxed for 4 h. The solid product formed was filtered, washed
and recrystallized from ethanol, dried in vacuo.
[L] Yield: 76%; yellow color; Anal. Calc.(%): C (71.2), H (5.9) and N
(6.9); Found (%): C (70.1), H(5.4) and N (6.5); FT-IR (KBr) (cm−1):
1608 (\\HCN), 2880–2920 (C\\H) and 1400–1600 (CC); 1H NMR
(DMSO-d6) δppm: (Ar–H) 6.4–7.5(m), (\\CHN) 8.6(s), and (\\O-
CH3) 3.4(s) ppm; 13C NMR (DMSO-d6): δppm 119.0–146.2 (Ar–C),
159.7 (\\CHN) and 53.4 (\\O-CH3) ppm; UV–Vis. in DMSO, nm
(transition): 254 (π–π*) and 370 (n-π*)
3. Results and Discussion
New bio-essential metal based complexes were synthesized from
Schiff base ligand (L) and 2,2′-bipyridine. L and derived complexes are
found to be stable over the extended periods. These complexes have
been characterized by spectral techniques such as FT-IR, UV–visible,
1H NMR, 13C NMR, EPR, Mass, elemental analysis, magnetic susceptibil-
ity and molar conductivity measurements. L is soluble in common
organic solvents whereas metal complexes are only soluble in DMF
and DMSO.
3.1. Elemental Analysis and Molar Conductivity Measurements
2.4. Synthesis of Metal Complexes
The general synthetic pathway for the heterocyclic Schiff base ligand
(L) and its mixed ligand metal complexes is given in Scheme 1. The very
similarity between the obtained elemental analysis data of L and the
metal complexes suggests the stoichiometry, [ML(bpy)Cl2]. The DMSO
solution of the metal complexes shows lower molar conductance values
(7.2–16.4 Ω−1 cm2 mol−1) which might be due to the non-electrolytic
nature of the complexes.
To synthesize metal complexes, an equimolar mixture of Schiff base
ligand (L) (0.1 mM), metal chloride (0.1 mM) and 2,2′-bipyridine
(0.1 mM) was mixed in a portion of ethanol (40 mL) and the resultant
mixture was refluxed for 8 h. The resultant product was washed with
ethanol and then recrystallized. The obtained solid product was filtered,
dried in vacuo at 60 °C and kept in a desiccator.
[CuL(bpy)Cl2]: Yield: 69%; brown color; Anal. Calc.(%): C (58.7),
H (4.6), N (8.1) and Cu (9.1); Found (%): C (57.2), H (4.3), N (7.6)
and Cu (8.3); FT-IR (KBr) (cm−1): 1598 (\\CHN), 1586 (\\CN)
(bpy), 376 (M–Cl) and 454 (M–N); ∧m (Ω−1 mol−1 cm2) 12.4; μeff
3.2. FT-IR Spectra
The FT-IR spectra of ligand (L) and metal complexes were compared
and shown in Fig. S1. The IR spectrum of ligand (L) [Fig. S1(a)], shows

T. Arun et al. / Journal of Photochemistry & Photobiology, B: Biology 154 (2016) 67–76
69
due to the coordination of ligand to the metal ions. The charge transfer
(Metal to ligand) transition can also be identified around ~430 nm in UV
spectra of all the metal complexes [20]. However, metal complexes
reveal an extra and much important characteristic band that appears
as the result of d–d transition. This d–d transition is very helpful to
forecast the geometry of the metal complexes. The electronic spectrum
of [CuL(bpy)Cl2] displays the d–d transition band in the region 620 nm,
which is due to 2Eg → 2T2g transition. This d–d transition band powerful-
ly favors a distorted octahedral geometry around the metal ions [21].
The electronic spectrum of [CoL(bpy)Cl2] shows this d–d transition
4T1g(F) → 4T2g(F) at 560 nm as the result of octahedral geometry [22].
The absorption spectrum of [NiL(bpy)Cl2], displays two d–d transition
bands in the region 570 and 690 nm, which are assigned to
3
3
3
3A2g(F) → T2g(F), and A2g(F) → T1g(F), transitions respectively,
which confirms the octahedral geometry of the complexes [23].
3.4. 1H NMR and 13C NMR Spectra
The 1H NMR spectra of the Schiff base ligand (L) and its
[ZnL(bpy)Cl2] complex have been recorded in CDCl3. The obtained 1
H
NMR spectra of Schiff base ligand (L) and [ZnL(bpy)Cl2] complex are
illustrated in supplementary file (Fig. S3). The 1H NMR spectrum of
ligand (L) exhibits singlet at 8.6 (s) ppm due to azomethine protons
(\\CHN) of Schiff base ligand. Moreover, the ligand exhibits the
following signals: phenyl multiplet at 6.4–7.5 (m) ppm and methoxy
protons (\\O-CH3) at 3.4 (s) ppm. In the case of [ZnL(bpy)Cl2], such
distinctive peak of azomethine group (\\CHN) gets shifted to upfield re-
gion at 8.5 (s) ppm, which confirms the coordination of azomethine
group(\\CHN) with Zn(II) metal ion [24]. There is no appreciable
change in all other signals of the complexes.
Scheme 1. Schematic route for the synthesis of Schiff base (\\HCN) ligand (L) and its metal
complexes.
The 13C NMR spectrum of L exhibits aromatic carbon signals
between 119.0 and 146.2 ppm. It also exhibits the other important
peaks at 159.7 and 53.4 ppm, characteristic of \\HCN and methoxy
(−O-CH3) carbon. In the case of [ZnL(bpy)Cl2], such characteristic
peak of\\HCN bond gets shifted to 158.4 ppm and a new peak is also
seen at 154.3 due to the 2,2′-bipyridine (\\CN) carbon. This evidence
of 2,2′-bipyridine (\\CN) carbon greatly supports the coordination
with metal center.
the strong band acquired at ~1608 cm−1, due to azomethine stretching
vibration (\\CHN) acquired in the ligand, indicating that condensation
between 3,4-dimethoxybenzaldehyde and o-phenylenediamine has
taken place, resulting in the formation of desired Schiff base ligand. It
was also confirmed by the significant aromatic\\CC-stretch, which ac-
quired at ~1487 cm−1 as a medium band. On complexation [Fig. S1(b–
d)] a major shift in azomethine (\\CHN) linkage to lower frequency by
~10–15 cm−1, and a new band acquired in the range ~1595–1602 cm−1
suggesting the involvement of the azomethine\\N to the metal ions
[18]. This observation is further supported by M–N characteristic peak
noted at ~420–454 cm−1 in all the complexes. The coordination of
2,2′-bipyridine to the metal ions can be confirmed by the existence of
new characteristic (\\CN-) peaks in all the complexes. As expected,
such (\\CN) band acquired in the region ~1584–1592 cm−1, and it indi-
cates the coordination of 2,2′-bipyridine to the metal ions [19]. In addi-
tion, the IR spectra of all the synthesized complexes show another band
at ~354–376 cm−1, which shows the successful coordination of chlorine
to the metal ions. Thus FT-IR spectral studies assure the formation of L
and the complexes which have 2,2′-bipyridine as the ancillary ligand.
3.5. EPR Spectra
EPR spectroscopy has been recognized as the most excellent
technique to investigate the metal ion environment in the complexes,
i.e., the geometry and nature of the bonding between the metal ion
and its ligands, in particular Cu(II) complexes. In the present study,
EPR spectrum of [CuL(bpy)Cl2] was performed. The recorded EPR spec-
trum of [CuL(bpy)Cl2] is given in supplementary file (Fig. S4). It exposed
two g factors g|| and g┴. The calculated spin Hamiltonian spectral data of
[CuL(bpy)Cl2] are listed in supplementary file (Table S1). From this
spectral data, it is found that g|| (2.11) N g┴ (2.02) N ge (2.0023) which
indicates the unpaired electron is present in the d2x-y2 orbital and it is
likely to be octahedral geometry as conferred in UV–Vis. spectroscopy
[25]. The g|| value of Cu(II) complex can be used as a measure of the co-
valent character of the metal–ligand interactions. If the value is more
than 2.3, the metal–ligand bond is essentially ionic and the value less
than 2.3 is indicative of covalent character. In the present EPR results,
lesser value of g|| (2.11) is compared to 2.3 which is a good evidence
for the covalent character of Cu–Nitrogen interactions, as suggested by
Kivelson and Neiman [26]. From the values of g factors, the exchange
coupling factor (G) can be expressed by the following equation:
3.3. UV–Vis. Spectroscopy
The UV–Vis. spectra of the Schiff base ligand (L) and its metal
complexes recorded in DMSO solution are given in Fig. S2. In the UV
spectrum of ligand (L) [Fig. S2(a)], notably two characteristic absorption
bands are observed. The emergence of higher energy absorbtion band at
254 nm can be attributed to the π–π* transition of π electrons present in
aromatic phenyl ring (CC) and azomethine chromophores. The
observed lower energy absorption band at 370 nm, is due to the n–π*
transition of non-bonded electrons available in (\\CHN) groups.
Furthermore, the UV–Vis. spectra of the complexes displayed the
characteristic of π–π* and n–π* transition bands below 370 nm, but
they slightly differ in their wavelength or absorption, which may be
ꢀ
ꢁ
À
Á
G ¼ gjj−2 = g┴−2
It has been already reported that, if G N 4.0, the local tetragonal axes
are aligned parallel or slightly misaligned. If G b 4.0, significant

70
T. Arun et al. / Journal of Photochemistry & Photobiology, B: Biology 154 (2016) 67–76
exchange coupling might be present and considerable misalignment
will be there. In this study, the observed G value of [CuL(bpy)Cl2] is
5.5 which means that the local tetragonal axes are aligned parallel or
slightly misaligned and exchange coupling could be negligible [27].
The observed empirical ratio of g||/A|| value for the [CuL(His)2] is 162
which indicates the octahedral geometry of [CuL(bpy)Cl2] with small
distortions.
3.7.1. Electronic Absorption Titrations
Electronic absorption titration is an effectual method to examine the
binding mode and the binding affinity of metal complexes with DNA. It
is generally accepted that the interaction of metal complexes with DNA
generally takes place via both covalent and non-covalent ways. In case
of covalent interaction, the labile ligand of the complexes is replaced
by nitrogen base of DNA such as guanine N7 while the non-covalent
DNA binding with intercalative, electrostatic and groove binding of
metal complexes arise in the outside of DNA double helix [28]. The
absorption spectra of the metal complexes in the presence and absence
of DNA are given in Fig. 1. As shown in Fig. 1, the complexes displayed
absorption bands around 300–309 nm, assigned to intraligand transi-
tion. Upon increasing amounts of DNA, the absorption bands of the
complex were affected. In specific hypochromism and red shift were ob-
tained in all the complexes after escalating DNA concentration. While
escalating DNA concentration, the observed hypochromisms are 12.4%,
8.7%, 9.3% and 7.2% for [CuL(bpy)Cl2] (309 nm), [CoL(bpy)Cl2]
(304 nm), [NiL(bpy)Cl2] (306 nm) and [ZnL(bpy)Cl2] (305 nm) respec-
tively. These considerable hypochromism and red shifts reveal that
complexes can interact with CT DNA through intercalation mode of
binding which may occur by the strong stacking interaction between
the aromatic chromophore of the ligand and the base pairs of DNA [29].
In order to compare the CT DNA-binding strength of the studied
complexes quantitatively, their intrinsic binding constants were
evaluated by the changes associated with absorption for the complexes
with increasing amounts of CT DNA. The electronic absorption data are
given in Table 1. The determined intrinsic binding constants for
[CuL(bpy)Cl2], [CoL(bpy)Cl2], [NiL(bpy)Cl2] and [ZnL(bpy)Cl2] are
3.6. Mass Spectra
The mass spectra of the Schiff base ligand (L) and its [CuL(bpy)Cl2]
complex are given in supplementary file (Fig. S5). The mass spectrum
of Schiff base ligand (L) showed molecular ion peak at m/z = 138
corresponding to [C8H9O2]+ ion. In addition, the ligand (L) exhibits
peaks for the fragments at m/z 241, 165 and 106 corresponding to
[C15H14NO2]+, [C9H10O2N]+ and [C7H6O]+ respectively (Fig. S3a).
Moreover, the mass spectrum of [CuL(bpy)Cl2] complex showed peaks
with m/z 659 (Base peak), 557, 539, 468, 405, 289 and 218 correspond-
ing to [C34H32ClCuN4O4]+, [C26H23Cl2CuN4O2]+, [C24H24Cl2CuN2O2]+
,
[C24H24CuN2O4]+ [C24H24N2O4]+, [C10H8Cl2CuN2]+ and [C10H8CuN2]+
respectively (Fig. S3b). The m/z of all the fragments of ligand (L) and
its metal complex prove the stoichiometry of the complexes of the
type [ML(bpy)Cl2]. The mass spectra of ligand (L) and its complexes
show molecular ion peaks which are in good agreement with the struc-
ture suggested by elemental analysis, magnetic and spectral studies.
3.7. Biological Studies
3.24
× , 2.55 × , 2.28 ×
106 M−1 106 M−1 106 M−1 and
In order to confirm the possible binding mode of the complexes with
CT DNA, many techniques were used. The DNA cleavage and anti-
pathogenic screening experiments have also been carried out. In
addition to that, molecular docking calculations have been performed
to understand the nature of binding of the complexes with DNA. They
are discussed below.
2.08 × 106 M−1, respectively. Interestingly, the observed Kb value of
the present organometallic complexes is lower than that for a classical
intercalator, such as EB and higher than those of some Schiff base
metal complexes [30] indicating that the present complexes strongly
bind with DNA through an intercalation mode at the double helix
structure of DNA.
Fig. 1. Absorption spectra of (a) [CuL(bpy)Cl2], (b) [CoL(bpy)Cl2], (c) [NiL(bpy)Cl2] and [ZnL(bpy)Cl2] complexes in buffer pH = 7.2 at 25 °C in the presence of increasing amount of DNA.
Arrow indicates the changes in absorbance upon increasing the DNA concentration.

T. Arun et al. / Journal of Photochemistry & Photobiology, B: Biology 154 (2016) 67–76
71
3.7.2. EB Fluorescence Displacement Assay
pulse voltammetries (CV and DPV) were also carried out on metal
complexes by increasing the DNA concentration. These methods
are considered to be the very sensitive analytical techniques to find
out the changes in oxidation state of the metallic species in the pres-
ence of biomolecules. It has been already reported that, when the
metal complexes interact with DNA via intercalation mode, the
peak potential presents a positive shift. It presents a negative shift
for electrostatic interaction [35]. In the present study, CV and DPV
techniques have been employed to find out the nature of DNA bind-
ing mode. In all of the experiments, the binding of complexes with CT
DNA were carried out in double distilled water with Tris–HCl
[(hydroxymethyl)aminomethane] (Tris–HCl, 5 mM) and NaCl
(50 mM) and adjusted to pH 7.2 with hydrochloric acid.
The cyclic voltammograms of [Cu(bpy)Cl2], [Co(bpy)Cl2],
[Ni(bpy)Cl2] and [Zn(bpy)Cl2] have been collected both in the absence
and presence of DNA. The effect of varying DNA concentration on cyclic
voltammograms of the studied complexes is presented in Fig. 4.
Incremental addition of DNA effectively changes both potentials and
currents of anodic as well cathodic peaks. This confirms the interaction
between the complex and DNA [36].
In order to further confirm the intercalation mode between the
complexes with DNA has been studied by EB fluorescence displacement
assay. EB is one the most sensitive fluorescent probes, does not have any
appreciable emission intensity in Tris–HCl-buffer solutions, due to
fluorescence quenching of free EB by the solvent molecules. Wilson
et al., have been reported that EB is a distinctive indicator of intercala-
tion while it can show enhanced emission intensity in the presence of
DNA, due to its strong intercalation of the planar of EB phenanthridine
ring between the adjacent base pairs of DNA [31]. This enrichment of
fluorescence intensity can be quenched by the addition of another mol-
ecule, which can bind DNA via intercalative mode by displacing EB. The
EB fluorescence displacement spectra of EB bound to DNA both in the
absence and presence of [CuL(bpy)Cl2] and [NiL(bpy)Cl2] are shown in
Fig. 2. Upon increasing amount of complex to the EB-bound DNA causes
a slow decrease in fluorescence intensity due to interaction between the
complexes and EB-DNA and it further leads to the quenching in the
fluorescence intensity of EB-DNA system. The reason for the decrease
in emission intensity is the replacement of EB from EB-DNA system by
metal complexes. It suggests the intercalative mode of binding [32].
Fluorescence quenching data were further analyzed by means of the
following Stern–Volmer equation,
The parameters received from CV of all the metal complexes in
DMSO are given in Table 2. The CV and DPV voltammograms of the
complexes in 5 mM Tris HCl/50 mM NaCl buffer (pH = 7.2) were
recorded in the range of +1 to −1 with scan rate of 0.1 Vs−1
.
I0=I ¼ 1 þ Ksv½Qꢀ
In the absence of DNA, CV of [Cu(bpy)Cl2] reveals four characteristic
peaks. Among them, two are anodic (Epa: −0.489 and −0.668 V) and
two are cathodic (Epc: 0.689 and −1.068 V). From the two separate an-
odic and cathodic peaks, two redox couples are assumed as follows:Á
where I0 and I are the fluorescence intensities in the absence and pres-
ence of quencher, respectively; [Q] is the concentration of the quencher;
Ksv is the Stern–Volmer quenching constant, which is obtained from the
slope of plot (I0/I vs [Q]). The quenching constant (Ksv) values of the
metal complexes are 2.08 × 104 M−1 and 1.27 × 104 M−1, respectively.
The results are consistent with the absorption spectral studies.
À
CuðIIÞ → CuðIÞ Epa ¼ −0:489 V; Epc ¼ 0:689 V; ΔEp ¼ 0:179 V and E1=2 ¼ −0:089 V ;
À
Á
CuðIÞ → Cuð0Þ Epa ¼ −0:668 V; Epc ¼ −1:068 V; ΔEp ¼ 1:759 V and E1=2 ¼ −0:189 V :
3.7.3. Viscosity Measurements
[Co(bpy)Cl2] shows only one redox couple in the absence of DNA
and its details are given below:
The mode of binding between the complexes and DNA was also con-
firmed by viscosity measurements. The previous report explains that, in
classical intercalation, the intercalating agents (e.g., EtBr) are expected
to elongate the DNA double helix and base pairs are divided to accom-
modate the intercalated ligands and thus outcome is the enhancement
of DNA's viscosity. In contrast partial or non-classical intercalation of
the ligand could twist or kink the DNA double helix, reduce its effective
length and decrease the viscosity [33]. Therefore, a hydrodynamic mea-
surement such as viscosity is considered as the least ambiguous and
most critical experiment for binding mode of the complexes with
DNA. The changes in the viscosity of CT DNA in the presence of increas-
ing amounts of the complexes are given in Fig. 3. As shown in this Fig. 3,
ethidium bromide (EtBr) usually increases the relative viscosity due to
lengthening of DNA double helix because of intercalation. Upon increas-
ing amount of complexes the relative specific viscosities of DNA in-
creased steadily, which is confirmed the metal complexes bound to
DNA by intercalation [34].
À
Á
CoðIIÞ → CoðIÞ; Epa ¼ 1:119 V; Epc ¼ 0:533 V; ΔEp ¼ 0:585 V; and E1=2 ¼ 0:826 V :
Likewise [Ni(bpy)Cl2] shows one redox couple in the absence of DNA
and it is detailed below:
À
Á
NiðIIÞ → NiðIÞ; Epa ¼ 0:642 V; Epc ¼ −0:824 V; ΔEp ¼ 1:466 V; and E1=2 ¼ −0:090 V :
CV study of [Zn(bpy)Cl2] in the absence of DNA also results in one
redox couple and the details are provided below:
À
Á
ZnðIIÞ → Znð0Þ; Epa ¼ 0:541 V; Epc ¼ −1:016 V; ΔEp ¼ 1:557 V; and E1=2 ¼ 0:778 V :
Irrespective of the complexes, all the above redox couples are found
to have approximately unity peak current ratio which is credited that
the reactions of the complexes in glassy carbon are quasi-reversible
redox process.
DPV of the metal complexes both in the absence and presence of
DNA is given in Fig. 5. Upon mounting the amounts of DNA, the formal
peak potential as well as the current intensity are reduced. The change
in the potential shift is related to the ratio of binding constant:
3.7.4. Electrochemical Titrations
The transition metal complexes of high oxidation state play a cru-
cial function in bioinorganic chemistry, because of their pharmaco-
logical importance as redox enzyme models. Cyclic and differential
À
Á
E0b−E0 ¼ 0:0591 log K =K
f
½redꢀ
½oxiꢀ
Table 1
Change in electronic absorption parameters for the complexes on interaction with CT DNA
in 5 mM Tris HCl/50 mM NaCl buffer (pH 7.2).
where E0b and E0f are peak potentials of complex in the presence
(bound) and absence (free form) of DNA, respectively. In our case,
metal complexes show one electron transfer reaction during the
above said redox process and value of K[red]/K[oxi] ratio is less than
unity and this value strongly favors a quasi-reversible process. The
synthesized metal complexes give both the anodic and cathodic peak
potential shifts (Table 2). These shifts indicate the intercalating mode
of DNA binding with metal complexes [37].
Compound
λ max
Δλ (nm)
H%
K )
b × 106 (M−1
Free
Bound
[CuL(bpy)Cl2]
[CoL(bpy)Cl2]
[NiL(bpy)Cl2]
[ZnL(bpy)Cl2]
309
304
306
305
311
305
308
306
2
1
2
1
12.4
8.7
9.3
3.24
2.55
2.28
2.08
7.2

72
T. Arun et al. / Journal of Photochemistry & Photobiology, B: Biology 154 (2016) 67–76
Fig. 2. Fluorescence quenching curves of EB bound to DNA: (a) [CuL(bpy)Cl2] and (b) [NiL(bpy)Cl2] [DNA] = 10 μL, [EB] = 5 μL, [Complex] = 0–120 μL.
3.8. DNA Cleavage Activity
(Lane 1) and DNA presence of L (Lane 2) do not show any cleavage
ability. However, the cleavage of super coiled (form-1) to the open
circular (form-II) and liner (form-III) is migrated by all the metal
complexes. These observations highlight that a combination of a metal
complex and oxidant H2O2 is very necessary to show more efficient
cleavage of DNA. This cleavage activity may be due to the formation of
hydroxyl radical (OH.). The formation of hydroxyl free radical (OH.) is
due to the reaction between the metal complex and oxidant. The
metal complexes in the presence of H2O2 may produce reactive hydroxyl
radical (OH.) that can damage the deoxyribose moiety [39].
The cleavage of pUC19 DNA examined by transition metal
complexes has been curiosity of biochemists. This can be attained by
targeting the essential components of DNA such as phospho-di-ester
bonds, deoxyribose sugar moiety or nucleobases. The DNA cleavage
activity of the heterocyclic ligand bridged metal complexes has been
investigated by incubating the complexes with pUC19 DNA in the
absence of an external agent in substrate medium of Tris–acetate–
EDTA (TAE) (pH = 7.2) buffer at 37 °C by gel electrophoresis, in
which migration from naturally going on, covalently closed circular
form to the open circular form and linear form was scrutinized [12]. If
the reaction mixture is placed under the influence of electric field,
DNA is negatively charged species, it will migrate towards anode and
this migration depends upon the gel concentration, size of DNA, electric
potential and buffer nature. It has been already reported by Brissos et al.,
when pUC19 DNA is subjected to gel electrophoresis three phenomena
may be observed, (I) relatively fast migration will be generated for the
super coiled form (Form I): (II) if scission occurs on one strand, the
super coiled system will relax to generate a slower moving open circular
form (Form II): and (III) if both strands are cleaved, the linear form
(Form III) will be produced, which migrates at an intermediate rate [38].
The oxidative DNA damage ability the heterocyclic ligand and its
mixed ligand metal complexes is studied with pUC19 DNA in the
absence and presence of H2O2 as an oxidizing agent. The pUC19 DNA
cleavage activity of the L and its complexes has been carried out in the
presence of H2O2 as shown in Fig. 6. The DNA cleavage activity is
significantly enhanced by incorporation of metal ion in the respective
oxidant. As seen from Fig. 6, the control experiments with DNA alone
3.9. Antimicrobial Screening
Biological investigations of the newly synthesized Schiff base ligand
(L) and its metal complexes were screened for in-vitro anti-pathogenic
activity against sensitive organisms such as two Gram-positive bacteria
(Staphylococcus aureus and Bacillus subtilis), two Gram-negative
bacteria (Escherichia coli and Salmonella typhi) and three fungi
(Fusarium solani, Aspergillus niger and Candida albicans) by dilution
method. Gentamicin and Fluconazole were used as standards for
bacteria and fungi respectively. The minimum inhibitory concentration
(MIC) values of measured in antibacterial and antifungal studies of the
complexes are given in Tables 3 and 4.
The results in the above Tables 3 and 4, indicate that the inhibition
growth of the Schiff base ligand becomes more enhanced when
chelation with the metal ions. This enhancement in anti-pathogenic
activity is rationalized on the basis of the metal chelates by possessing
an additional azomethine (HCN) linkage which imports in elucidating
the mechanism of transamination and resamination reactions in

T. Arun et al. / Journal of Photochemistry & Photobiology, B: Biology 154 (2016) 67–76
73
Fig. 3. Cyclic voltammograms of (a) [CuL(bpy)Cl2] and (b) [NiL(bpy)Cl2] in buffer (pH =
7.2) at 25 °C in the presence of increasing amount of DNA.
biological system. Moreover, the presence of azomethine group
(\\CHN) and chelation effect with central metal enhances the anti-
pathogenic activity [40]. This increasing anti-pathogenic activity of
these mixed ligand metal complexes may be explained by Overtone
concept and Tweedy's chelation theory [41]. On chelation, the polarity
of the metal ion gets reduced to a greater extent, due to the overlap of
the ligand orbital and partial sharing of its positive charge with donor
groups. In addition, it enhances the π-electron delocalization on the
whole chelating ring which results enhance in the liphophilicity of the
complexes. Consequently, the metal complexes easily penetrate into
the lipid membrane and blocking of the metal binding sites in the
enzymes of organisms. While chelation is not the only criterion for
anti-pathogenic activity, there are other several factors which also
increase the activity are nature of metal ion, ligand, geometry of the
metal complexes, the lipophilicity and the presence of co-ligands,
the steric and pharmacokinetic factors. These mixed ligand metal
complexes also disturb the respiration process of the cell wall and
thus block the synthesis of the proteins which restricts further growth
of organisms. From the antimicrobial results, it is observed that the
Gram-positive bacteria are more sensitive to metal complexes than
Gram-negative bacteria. It is due to the presence of cell wall in Gram-
negative bacteria which is impermeable lipid based bacterial outer
membrane.
Fig. 4. Differential pulse voltammograms of (a) [CuL(bpy)Cl2] and (b) [CoL(bpy)Cl2] in
buffer (pH = 7.2) at 25 °C in the presence of increasing amount of DNA.
the suitable binding site and preferred orientation of the molecules
inside DNA [43]. It is universally accepted that, if the binding free energy
is low, then the binding affinity is more potent in between the receptor
(DNA) and “the ligand” (Schiff base and its metal complexes) molecules.
For these reasons, molecular docked model calculations were presented
and the most probable molecular docked poses are depicted in Fig. 7.
This figure obviously shows that the Schiff base ligand and its metal
complexes interact with DNA through an intercalation mode involving
outside edge stacking interaction with oxygen atom of the phosphate
backbone. As seen from Fig. 7, it is clear that the L and metal complexes
fit well into the intercalation mode of targeted DNA [44]. Moreover,
resulting structures are stabilized by van der Waals interaction and
hydrophobic contacts [45] with DNA functional group that define the
stability of intercalations. The resulting docking scores of molecular
docked L and metal complexes are found to be −226.30 (L), −270.12
[CuL(bpy)Cl2], −269.85 [CoL(bpy)Cl2], −269.69 [NiL(bpy)Cl2] and
−269.57 [ZnL(bpy)Cl2] KJ mol−1 respectively. It is fascinating to note
Table 2
Redox potential profiles for interaction of DNA with metal complexes.
Compound
E1/2(V)a
Free
bΔEp(V)
Ipa/Ipc
3.10. Molecular Docking with DNA
K[red]/K[oxd]
Bound
Free
Bound
Molecular docking is an effective technique to know the drug–
nucleic acid interactions for the progress of modern drug design and
finding, as well as to find out the accurate binding site available at the
molecular target DNA predominantly in a non-covalent interaction
[42]. Computer aided molecular docking studies of Schiff base ligand
(L) and metal complexes with DNA duplex of sequence d(CGCGAATT
CGCG)2 dodecamer (PDB ID: 1BNA) were carried out in order to offer
CuL(bpy)Cl2]
−0.089
−0.189
0.826
−0.090
0.778
0.482
−0.061
0.832
0.351
0.912
0.179
1.759
0.585
1.466
1.557
0.965
1.782
0.482
2.011
1.627
0.76
0.89
0.96
0.82
0.69
0.654
[CoL(bpy)Cl2]
[NiL(bpy)Cl2]
[ZnL(bpy)Cl2]
0.546
0.364
0.327
Data from cyclic voltammetric measurements: aE1/2 is calculated as the average of anodic
(EPa) and cathodic (Epc) peak potentials; Ea1/2 = EPa + Epc/2; bΔEp = Epa − Epc
.

74
T. Arun et al. / Journal of Photochemistry & Photobiology, B: Biology 154 (2016) 67–76
Table 3
Minimum inhibitory concentration of the synthesized compounds against the growth of
bacteria (μM). Minimum inhibitory concentration (MIC) (× 104 μM).
Compound
S. aurous
B. subtilis
E. coli
S. typhi
[L]
16.2
5.8
6.2
7.1
8.1
3.1
17.9
6.2
7.1
7.4
8.4
3.4
16.5
6.6
8.2
8.2
8.7
3.9
18.8
7.1
8.3
8.4
9.3
4.1
[CuL(bpy)Cl2]
[CoL(bpy)Cl2]
[NiL(bpy)Cl2]
[ZnL(bpy)Cl2]
Gentamicin
Table 4
Minimum inhibitory concentration of the synthesized compounds against the growth of
fungi (μM).Minimum inhibitory concentration (MIC) (× 104 μM).
Compound
F. solani
A. niger
C. albicans
[L]
18.7
7.3
7.8
10.1
11.2
3.1
19.1
6.9
6.7
9.5
11.7
2.8
18.8
5.7
8.2
8.7
10.2
2.7
[CuL(bpy)Cl2]
[CoL(bpy)Cl2]
[NiL(bpy)Cl2]
[ZnL(bpy)Cl2]
Gentamicin
Fig. 5. Effect of increasing amounts of [EB] ( ), [CuL(bpy)Cl2] ( ), [CoL(bpy)Cl2] ( ),
[NiL(bpy)Cl2] ( ) and [ZnL(bpy)Cl2] ( ) on the relative viscosity of CT DNA. Plot of
relative viscosity (η/ηo)1/3 vs [complex]/[DNA].
that the binding energy of the [CuL(bpy)Cl2] is higher than that of other
complexes and the order of binding energy is as follows:
[CuL(bpy)Cl2] N [CoL(bpy)Cl2] N [NiL(bpy)Cl2] N [ZnL(bpy)Cl2]. The bio-
logical efficacy including DNA binding, DNA cleavage and antimicrobial
screening data is in the following order: [CuL(bpy)Cl2] N [CoL(bpy)Cl2] N
[NiL(bpy)Cl2] N [ZnL(bpy)Cl2]. It clearly indicates that the docking score
decreases when the biological activity increases. Hence, it shows that
the examined docking score of the present complexes is higher than
that of a few Schiff base metal complexes available in the literature [46].
voltammetry, differential pulse voltammetry and molecular docking
analysis. These studies prove that CT DNA interaction of the complexes
follows intercalation mode. The DNA cleavage activity of all the metal
complexes with pUC19 DNA under aerobic conditions is efficient cleav-
age in the presence of oxidizing agent (H2O2). The anti-pathogenic
screening indicates that these complexes are good antimicrobial agents
against different organisms and standards. These biological findings
from our study would be helpful in perceptive of DNA interaction
exposed by metal complexes and may lead to develop novel metal
based therapeutic drugs.
4. Conclusion
Acknowledgments
A new and innovative hetrocylic Schiff base ligand which is capable
of making metal complexes with enhanced pharmacological properties
has been synthesized and characterized. A series of metal (Cu(II), Co(II),
Ni(II) and Zn(II)) complexes have been synthesized from L and charac-
terized by physicochemical, spectral and electrochemical techniques.
The role of 2,2′-bipyridine as ancillary ligand would hopefully take the
studied complexes as more bio-relevant. All the complexes have
octahedral geometry except Cu(II) complex which has the same with
distortion. The interaction of the complexes with DNA has been
effectively examined and explored by electronic absorption, ethidium
bromide displacement assay, viscosity measurements, cyclic
The authors express their heartfelt thanks to the College Managing
Board, Principal, and Head of the Department of Chemistry, VHNSN
College for providing necessary research facilities to complete this
work successfully. The characterization facilities afforded by IIT Bombay
and CDRI, Lucknow are gratefully acknowledged by all authors.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.jphotobiol.2015.11.011.
Fig. 6. The gel electrophoretic separation of plasmid pUC19 DNA treated with metal complexes. Lane 1; DNA control; Lane 2: DNA + ligand + H2O2; Lane 4: DNA + [CuL(bpy)Cl2] + H2O2;
Lane 5: DNA + [CoL(bpy)Cl2] + H2O2; Lane 6: DNA + [NiL(bpy)Cl2] + H2O2; Lane 7: DNA + [ZnL(bpy)Cl2] + H2O2.

T. Arun et al. / Journal of Photochemistry & Photobiology, B: Biology 154 (2016) 67–76
75
Fig. 7. Molecular docked model of L and its complexes with DNA.
substituent dependent selectivity and colour change, Sensors Actuators B Chem.
206 (2015) 524–530.
[7] A.Y. Lukmantara, D.S. Kalinowski, N. Kumar, D.R. Richardson, Synthesis and biologi-
cal evaluation of 2-benzoylpyridine thiosemicarbazones in a dimeric system:
structure–activity relationship studies on their anti-proliferative and iron chelation
efficacy, J. Inorg. Biochem. 141 (2014) 43–54.
References
[1] M. Nath, P.K. Saini, Chemistry and applications of organotin(IV) complexes of Schiff
bases, Dalton Trans. 40 (2011) 7077–7121.
[2] P.G. Cozzi, Metal–salen Schiff base complexes in catalysis: practical aspects, Chem.
Soc. Rev. 33 (2004) 410–421.
[3] N. Ananthi, U. Balakrishhnan, S. Velmathi, K.B. Manjunath, G. Umesh, Synthesis,
characterization and third order non linear optical properties of metalloorganic
chromophores, Opt. Photon. 2 (2012) 40–45.
[4] R. Antony, S.T.D. Manickam, P. Kollu, P.V. Chandrasekar, K. Karuppasamy, S.
Balakumar, Highly dispersed Cu(II), Co(II) and Ni(II) catalysts covalently
immobilized on imine-modified silica for cyclohexane oxidation with hydrogen
peroxide, RSC Adv. 4 (2014) 24820–24830.
[5] G.G. Mohamed, M.M. Omar, A.A. Ibrahim, Biological activity studies on metal
complexes of novel tridentate Schiff base ligand. Spectroscopic and thermal
characterization, Eur. J. Med. Chem. 44 (2009) 4801–4812.
[8] C. Rajarajeswari, M. Ganeshpandian, M. Palaniandavar, A. Riyasdeen, M.A. Akbarsha,
Mixed ligand copper(II) complexes of 1,10-phenanthroline with tridentate
phenolate/pyridyl/(benz)imidazolyl Schiff base ligands: covalent vs non-covalent
DNA binding, DNA cleavage and cytotoxicity, J. Inorg. Biochem. 140 (2014) 255–268.
[9] H. Zafar, A. Ahmad, A.U. Khan, T.A. Khan, Synthesis, characterization and antimicro-
bial studies of Schiff base complexes, J. Mol. Struct. 1097 (2015) 129–135.
[10] R.R. Zaky, K.M. Ibrahim, H.M.A. El-Nadr, S.M. Abo-Zeid, Bivalent transition metal
complexes of (E)-3-(2-benzylidenehydrazinyl)-3-oxo-N-(p-tolyl)propanamide:
spectroscopic, computational, biological activity studies, Spectrochim. Acta A Mol.
Biomol. Spectrosc. 150 (2015) 40–53.
[6] A. Kundu, P.S. Hariharan, K. Prabakaran, S.P. Anthony, Developing new Schiff base
[11] M. Shamsi, S. Yadav, F. Arjmand, Synthesis and characterization of new transition
molecules for selective colorimetric sensing of Fe3+ and Cu2+ metal ions:
metal {Cu(II),Ni(II) and Co(II)}L-phenylalanine–DACH conjugate complexes:

76
T. Arun et al. / Journal of Photochemistry & Photobiology, B: Biology 154 (2016) 67–76
in vitro DNA binding, cleavage and molecular docking studies, J. Photochem.
Photobiol. B 136 (2014) 1–11.
[31] W.D. Wilson, L. Ratmeyer, M. Zhao, L. Strekowski, D. Boykin, The search for
structure-specific nucleic acid-interactive drugs: effects of compound structure on
RNA versus DNA interaction strength, Biochemistry 32 (1993) 4098–4104.
[32] K. Nagaraj, G. Velmurugan, S. Sakthinathan, P. Venuvanalingam, S. Arunachalam,
Influence of self-assembly on intercalative DNA binding interaction of double-
chain surfactant Co(III) complexes containing imidazo[4,5-f][1,10]phenanthroline
and dipyrido[3,2-d:2′-3′-f]quinoxaline ligands: experimental and theoretical
study, Dalton Trans. 43 (2014) 18074–18086.
[12] P. Gurumoorthy, D. Mahendiran, D. Prabhu, C. Krulvasu, A.K. Rahiman, Magneto-
structural correlation, antioxidant, DNA interactions and growth inhibition activities
of new chloro-bridged phenolate complexes, RSC Adv. 4 (2014) 42855–42872.
[13] G. Dehghan, J.E.N. Dolatabadi, A. Jouyban, K.A. Zeynali, S.M. Ahmadi, S. Kashanian,
Spectroscopic studies on the interaction of quercetin–terbium(III) complex with
calf thymus DNA, DNA Cell Biol. 30 (2010) 95–201.
[14] G. Zuber, J.C. Quada, S.M. Hecht, Sequence selective cleavage of
a
DNA
[33] V.A. Kawade, A.A. Kumbhar, A.S. Kumbhar, C. Nather, A. Erxleben, U.B. Sonawane,
R.R. Joshi, Mixed ligand cobalt(II) picolinate complexes: synthesis, characterization,
DNA binding and photocleavage, Dalton Trans. 40 (2011) 639–650.
[34] T. Arun, S. Packianathan, M. Malarvizhi, R. Antony, N. Raman, Bio-relevant
complexes of novel N2O2 type heterocyclic ligand: synthesis, structural elucidation,
biological evaluation and docking studies, J. Photochem. Photobiol. B Biol. 149
(2015) 93–102.
[35] M. Cory, D.D. Mckee, J. Kagan, D.W. Henry, J.A. Miller, Design, synthesis and DNA
binding properties of bifunctional intercalators. Comparison of polymethylene and
diphenyl ether chains connecting phenanthridine, J. Am. Chem. Soc. 107 (1985)
2528–2536.
octanucleotide by chlorinated bithiazoles and bleomycins, J. Am. Chem. Soc. 120
(1998) 9368–9369.
[15] N. Shahabadi, S. Mohammadi, Synthesis characterization and DNA interaction
studies of a new Zn(II) complex containing different dinitrogen aromatic ligands,
Bioinorg. Chem. Appl. 2012 (2012) 571913–571921.
[16] S. Anbu, S. Shanmugaraju, M. Kandaswamy, Electrochemical, phosphate hydrolysis,
DNA binding and DNA cleavage properties of new polyaza macrobicyclic dinickel(II)
complexes, RSC Adv. 2 (2012) 5349–5357.
[17] L.S. Lerman, Structural considerations in the interaction of DNA and acridines, J. Mol.
Biol. 3 (1961) 18–14.
[18] Z.H. Chohan, M. Arif, M. Sarfraz, Metal-based antibacterial and antifungal amino acid
derived Schiff bases: their synthesis, characterization and in vitro biological activity,
Appl. Organomet. Chem. 21 (2007) 294–302.
[19] E. Mrkalić, A. Zianna, G. Psomas, M. Gdaniec, A. Czapik, E.C. Argyropoulou, M.L.
Kantouri, Synthesis, characterization, thermal and DNA-binding properties of new
zinc complexes with 2-hydroxyphenones, J. Inorg. Biochem. 134 (2014) 66–75.
[20] Z. Li, E. Badaeva, A. Ugrinov, S. Kilina, W. Su, Platinum chloride complexes containing
6-[9,9-Di(2-ethylhexyl)-7-R-9H-fluoren-2-yl]-2,2′-bipyridine ligand (RNO2, CHO,
Benzothiazol-2-yl,n-Bu, Carbazol-9-yl, NPh2): tunable photophysics and reverse
saturable absorption, Inorg. Chem. 52 (2013) 7578–7592.
[36] T. Arun, R. Subramanian, S. Packianathan, N. Raman, Fluorescence titrations of bio-
relevant complexes with DNA: synthesis, structural investigation, DNA binding/
cleavage, antimicrobial and molecular docking studies, J. Fluoresc. 25 (2015)
1127–1140.
[37] M.T. Carter, M. Rodriguez, A.J. Bard, Voltammetric studies of the interaction of metal
chelates with DNA. 2. Tris-chelated complexes of cobalt (III) and iron (II) with 1, 10-
phenanthroline and 2, 2′-bipyridine, J. Am. Chem. Soc. 111 (1989) 8901–8911.
[38] R.F. Brissos, E. Torrents, F.M.S. Mello, W.C. Pires, E.P. Silveira-Lacerda, A.B. Caballero,
A. Caubet, C. Massera, O. Roubeau, S.J. Teat, P. Gamez, Highly cytotoxic DNA-
interacting copper(II) coordination compounds, Metallomics 6 (2014) 1853–1868.
[39] P. Mendu, C.G. Kumari, R. Ragi, Synthesis, characterization, DNA binding, DNA cleav-
age and antimicrobial studies of Schiff base ligand and its metal complexes, J.
Fluoresc. 25 (2015) 369–378.
[21] W.E. Estes, D.P. Gavel, W.B. Hatfield, D.J. Hodgson, Magnetic and structural
characterization of dibromo- and dichlorobis(thiazole)copper(II), Inorg. Chem. 17
(1978) 1415–1421.
[22] R.K. Parashar, R.C. Sharma, Stability studies in relation to IR data of some Schiff base
complexes of transition metals and their biological and pharmacological studies,
Inorg. Chim. Acta 151 (1988) 201–208.
[23] M.B. Halli, R.B. Sumathi, Synthesis, spectroscopic, antimicrobial and DNA cleavage
studies of new Co(II), Ni(II), Cu(II), Cd(II), Zn(II) and Hg(II) complexes with
naphthofuran-2-carbohydrazide Schiff base, J. Mol. Struct. 1022 (2012) 130–138.
[24] M.S. El-Shahawi, A.F. Shoair, Synthesis, spectroscopic characterization, redox
properties and catalytic activity of some ruthenium(II) complexes containing
aromatic aldehyde and triphenylphosphine or triphenylarsine, Spectrochim. Acta
A Mol. Biomol. Spectrosc. 60 (2004) 121–127.
[40] A.A. El-Sherif, M.M. Shoukry, M.M.A. Abd-Elgawad, Synthesis, characterization,
biological activity and equilibrium studies of metal(II) ion complexes with
tridentate hydrazone ligand derived from hydralazine, Spectrochim. Acta A Mol.
Biomol. Spectrosc. 98 (2012) 307–321.
[41] B.G. Tweedy, Plant extracts with metal ions as potential antimicrobial agents, Phyto-
pathology 25 (1964) 910–914.
[42] J. Lakshmipraba, S. Arunachalam, R.V. Solomon, P. Venuvanalingam, A. Riyasdeen, R.
Dhivya, M.A. Akbarsha, Surfactant–copper(II) Schiff base complexes: synthesis,
structural investigation, DNA interaction, docking studies, and cytotoxic activity, J.
Biomol. Struct. Dyn. 33 (2015) 877–891.
[25] R. Antony, S.T.D. Manickam, K. Karuppasamy, P. Kollu, P.V. Chandrasekar, S.
Balakumar, Organic–inorganic hybrid catalysts containing new Schiff base for envi-
ronment friendly cyclohexane oxidation, RSC Adv. 4 (2014) 42816–42824.
[26] D. Kivelson, R. Neiman, ESR studies on the bonding in copper complexes, J. Chem.
Phys. 35 (2004) 149–155.
[27] B.J. Hathaway, D.E. Billing, The electronic properties and stereochemistry of mono-
nuclear complexes of the copper(II) ion, Coord. Chem. Rev. 5 (1970) 143–207.
[28] S. Tabassum, M. Zaki, F. Arjmand, New modulated design and synthesis of
quercetin- CuII/ZnII–SnI2V scaffold as anticancer agents: in vitro DNA binding profile,
DNA cleavage pathway and -I activity, Dalton Trans. 42 (2013) 10029–10041.
[29] H. Farrokhpour, H. Hadadzadeh, F. Darabi, F. Abyar, H.A. Rudbari, T. Ahmadi-Bagheri,
A rare dihydroxo copper(II) complex with ciprooxacin; a combined experimental
and ONIOM computational study of the interaction of the complex with DNA and
BSA, RSC Adv. 4 (2014) 35390–35404.
[43] S. Tabassum, S. Yada, Investigation of diorganotin(IV) complexes: Synthesis,
characterization, in vitro DNA binding studies and cytotoxicity assessment of di-n-
butyltin(IV)complex, Inorg. Chim. Acta 423 (2014) 204–214.
[44] G. Barone, A. Terenzi, A. Lauria, A.M. Almerico, J.M. Leal, L. Busto, B. García, DNA-
binding of nickel(II), copper(II) and zinc(II) complexes: structure–affinity relation-
ships, Coord. Chem. Rev. 257 (2013) 2848–2862.
[45] P. Arthi, S. Shobana, P. Srinivasan, L. Mitu, A.K. Rahiman, Synthesis, characterization,
biological evaluation and docking studies of macrocyclic binuclear manganese(II)
complexes containing 3,5-dinitrobenzoyl pendant arms, Spectrochim. Acta A Mol.
Biomol. Spectrosc. 143 (2015) 49–58.
[46] A. Lauria, R. Bonsignore, A. Terenzi, A. Spinello, F. Giannici, A. Longo, A.M. Almerico,
G. Barone, Nickel(II), copper(II) and zinc(II) metallo-intercalators: structural details
of the DNA-binding by a combined experimental and computational investigation,
Dalton Trans. 43 (2014) 6108–6119.
[30] T. Kiran, V.G. Prasanth, M.M. Balamurali, C.S. Vasavi, P. Munusami, K.I.
Sathyanarayanan, M. Pathak, Inorg. Chim. Acta 433 (2015) 26–34.