Interaction of palladium(II) coordination compounds with calf thymus DNA and their antibacterial activity

Article abstract of DOI:10.1016/j.inoche.2012.04.011

The palladium(II) complexes of the formulations [Pd(clpbpy)Cl₂], [Pd(clcpbpy)Cl₂], [Pd(brcpbpy)Cl₂], [Pd(clmpbpy)Cl ₂] {clpbpy=4-(4-chlorophenyl)-6-phenyl-2,2′-bipyridine (L ¹), clcpbpy=4,6-bis(4-chlorophenyl)-2,2′-bipyridine (L ²), brcpbpy=4-(4-bromophenyl)-6-(4-chlorophenyl)-2,2′- bipyridine (L³) and clmpbpy=6-(4-chlorophenyl)-4-p-tolyl-2,2′- bipyridine (L⁴)} have been synthesized and characterized using elemental analysis, electronic spectra, IR and mass spectra. The study on the interaction of the palladium complexes with calf thymus DNA (CT DNA) has been performed using absorption titration, viscosity measurements and thermal denaturation techniques. The study suggests the intercalative mode of DNA binding. Gel electrophoresis assay demonstrates the ability of the complexes to cleave the pUC19 DNA. The antibacterial activity of complexes has been screened against two Gram^{(+ ve)} and three Gram^{(- ve)} microorganisms.

Full text of DOI:10.1016/j.inoche.2012.04.011

Inorganic Chemistry Communications 21 (2012) 61–64
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Interaction of palladium(II) coordination compounds with calf thymus DNA and their
antibacterial activity
⁎
Mohan N. Patel , Promise A. Dosi, Bhupesh S. Bhatt
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar–388 120, Gujarat India
a r t i c l e i n f o
a b s t r a c t
Article history:
The palladium(II) complexes of the formulations [Pd(clpbpy)Cl2], [Pd(clcpbpy)Cl2], [Pd(brcpbpy)Cl2], [Pd
(clmpbpy)Cl2] {clpbpy═4-(4-chlorophenyl)-6-phenyl-2,2′-bipyridine (L1), clcpbpy═4,6-bis(4-chlorophenyl)-
2,2′-bipyridine (L2), brcpbpy═4-(4-bromophenyl)-6-(4-chlorophenyl)-2,2′-bipyridine (L3) and clmpbpy═6-(4-
chlorophenyl)-4-p-tolyl-2,2′-bipyridine (L4)} have been synthesized and characterized using elemental analysis,
electronic spectra, IR and mass spectra. The study on the interaction of the palladium complexes with calf thymus
DNA (CT DNA) has been performed using absorption titration, viscosity measurements and thermal denaturation
techniques. The study suggests the intercalative mode of DNA binding. Gel electrophoresis assay demonstrates
the ability of the complexes to cleave the pUC19 DNA. The antibacterial activity of complexes has been screened
against two Gram(+ve) and three Gram(−ve) microorganisms.
Received 24 January 2012
Accepted 18 April 2012
Available online 25 April 2012
Keywords:
Palladium(II) complexes
Nucleolytic activity
Intercalation
Thermal denaturation
© 2012 Elsevier B.V. All rights reserved.
Due to resounding success of cisplatin, as antitumor drug, and
closely related platinum complexes, a large number of platinum
and palladium complexes containing not only amine based ligands,
but also various other ligands have become the subject of intensive re-
search [1]. Palladium(II) complexes have been investigated for develop-
ing the new antitumor agents, [2] because palladium(II) has a similar
coordination mode and chemical properties to platinum(II). Some pal-
ladium complexes with aromatic N-containing ligands, e.g. derivatives
of pyridine, quinoline, pyrazole and 1,10-phenanthroline, have shown
very promising antitumor characteristics [3].
DNA plays a fundamental role in the storage and expression of
genetic information in a cell. Studies on the interaction of transition
metal complexes with DNA have been pursued in recent years [4].
Investigations of the interactions of DNA with transition metal com-
plexes are basis to design new types of the pharmaceutical molecules
to elucidate the mechanism and principles involved in the site specif-
ic recognition of DNA [5]. In general, most components have three
distinct modes of non-covalent interaction with DNA, i.e. intercalative
association, DNA groove binding and electrostatic attraction [6].
In continuation of our earlier work [7], herein we report the
synthesis, characterization and biological behaviours of four Pd(II)
complexes (Scheme 1). The DNA binding properties have been studied
by absorption titration, viscosity measurement and DNA melting tem-
perature. The cleavage properties of the complexes have been investi-
gated using gel electrophoresis technique. The antibacterial activity of
the complexes has been performed against five microorganisms.
The IR spectral data of complexes are shown in Table 1. The
Infrared spectral measurement of bipyridines showed the bands
at ~1550 cm−1 and ~1470 cm−1, correspond to C═C and C═N ring
stretching, whereas the bands at ~740 cm−1 and ~3050 cm−1 are
due to aromatic C―H out of plane bending and stretching respective-
ly. Major change that occurs after complexation are the shifting of
bands for C═C and C═N ring stretching from ~1550, ~1470 to ~1590,
~1530 cm−1, respectively, which supports the binding of metal via
nitrogen atoms of bipyridines skeleton. The data are further supported
by appearance of a sharp band at ~550 cm−1, characteristic bands of
Pd―N bond. Stretching mode of Pd―Cl is to be expected in the region
less than 400 cm−1 i.e. ~360 cm−1 [8].
In the synthesized Pd(II) complexes, only one d―d band at ~426 nm
was observed, which is assigned to dz2 dx2− y2 transition, whereas other
two bands are related to charge transfer bands [9]. The bands observed
at ~271 and ~250 nm correspond to the charge transfer transitions. All
these bands point toward the low spin complex of d8 system with
square planar geometry.
FAB-mass spectra of all complexes were obtained using m–nitro
benzyl alcohol as matrix. A mass spectrum of complex [Pd(clpbpy)
Cl2] (1) is shown in Fig. 1. Peaks at 136, 137, 154, 289 and 307 m⁄z
values are of matrix. Peak at 519 m/z is assigned to molecular
ion peak of complex 1. The peaks at 482 and 448 m/z correspond
to the fragments [Pd(clpbpy)Cl]+ and [Pd(clpbpy)]+, respectively
and peak at 342 m/z corresponds to [clpbpy]. The fragmentation of
p-chloro phenyl group from clpbpy molecule gives peak at 231 m/z.
There exist doublets at 519:521, 482:484 and 448:450 m/z values,
which correspond to isotopic patterns of chlorine atom. The isotopic
pattern due to the presence of chlorine and palladium appeared in
the spectrum.
⁎
Corresponding author. Tel.: +91 2692 226856*218.
E-mail address: jeenen@gmail.com (M.N. Patel).
1387-7003/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2012.04.011

62
M.N. Patel et al. / Inorganic Chemistry Communications 21 (2012) 61–64
Scheme 1. General reaction scheme for synthesis of ligand and complex [Pd(Ln)Cl2].
Table 1
IR spectra of complexes (cm−1).
The complexes were screened for in-vitro antibacterial activity
against three Gram(−ve) i.e. E. coli, P. aeruginosa, S. marcescens and two
Gram(+ve) i.e. S. aureus, B. subtilis, microorganisms using MIC method.
The antimicrobial activity data reveals that all the complexes are more
active against all the species compared to metal salt and bipyridine
derivatives (Table 2). The complexes show better antibacterial activity
than Pd(II) complexes reported by Guerra et al. [10] and Juribašić
et al. [11], while they have similar antibacterial with Pd(II) complexes
Compounds ν(C―H) ν(C C) ν(C N) ν(C―Cl) ν(C―Br) δ(C―H) ν(M―N)
[Pd(L1)Cl2] 3063
1597
1592
1597
1589
1545
1525
1535
1553
1042
1042
1045
1044
–
733
748
741
710
555
555
545
532
[1]
[Pd(L2)Cl2] 3101
–
[2]
[Pd(L3)Cl2] 3055
1022
[3]
[Pd(L4)Cl2] 3055
–
[4]
Fig. 1. FAB-mass spectrum of complex 1, i.e. [Pd(clpbpy)Cl2], obtained using m–nitro benzyl alcohol.
Table 2
Antibacterial activities of metal salt, ligands and palladium complexes in terms of min-
imum inhibitory concentration (MIC in μM).
Compounds
S. aureus B. subtilis S. marcescens P. aeruginosa E. coli
Na2PdCl4
2238.00
610
540
610
690
2490.00
705
510
595
715
72
42
55
81
2145.00
670
585
650
720
81
49
66
93
1940.00
645
490
625
720
75
43
58
88
2820.00
675
535
635
725
79
46
59
89
Ligand 1 [L1]
Ligand 2 [L2]
Ligand 3 [L3]
Ligand 4 [L4]
[Pd(L1)Cl2] [1] 71
[Pd(L2)Cl2] [2] 47
[Pd(L3)Cl2] [3] 57
[Pd(L4)Cl2] [4] 82
Fig. 2. Absorption titration curve of [Pd(clpbpy)Cl2] [1] in absence and in presence of
increasing amount of DNA; 50–150 μM in Tris–HCl buffer (50 mM Tris–HCl, pH 7.2),
[complex]=15 μM, with incubation period of 30 min. at 37 °C, Inset: Plot of [DNA]/
(εa −εf) versus [DNA].

M.N. Patel et al. / Inorganic Chemistry Communications 21 (2012) 61–64
63
Fig. 4. Effect on relative viscosity of DNA under the influence of increasing amount of
complexes at 27 0.1 °C in Tris–HCl buffer (50 mM Tris–HCl, pH 7.2).
Fig. 3. Melting curves of CT DNA in the absence and presence of complexes 1–4.
Table 3
Absorption titration data of ligands and palladium(II) complexes upon addition of CT
DNA.
Table 4
Gel electrophoresis data.
Compounds
λmax (nm)
Δλ
Binding
Compounds
% SC
% OC
% Cleavage
constants
Free
Bound
Kb (M−1
)
DNA control
DNA+metal salt
DNA+L1
DNA+L2
DNA+L3
DNA+L4
DNA+1
DNA+2
DNA+3
DNA+4
75
68
59
56
58
62
23
17
20
28
25
32
41
44
42
38
77
83
80
72
–
Ligand 1 [L1]
Ligand 2 [L2]
Ligand 3 [L3]
Ligand 4 [L4]
[Pd(L1)Cl2] [1]
[Pd(L2)Cl2] [2]
[Pd(L3)Cl2] [3]
[Pd(L4)Cl2] [4]
259
261
258
266
293
292
288
288
261
263
260
268
294
293
290
290
2
2
2
2
1
1
2
2
1.29×103
2.48×103
1.68×103
1.01×103
3.85×104
6.57×104
4.47×104
1.58×104
9.33
21.33
25.33
22.67
17.33
69.33
77.33
73.33
62.66
reported by Safronova et al. [12]. This increase in biocidal activity may
be due to light of Overtone's concept, chelation theory or may be due
to the effect of the metal ion on the normal cell process [13].
is strongly related to the stability of the double-helical structure. A
change of Tm may be observed when an interaction of complexes
with DNA occurs. Thus the study of the thermal behavior of DNA
in the presence of complexes provides information on the conforma-
tional changes and the strength of the DNA-complex interactions.
The thermal denaturation experiment carried out for CT DNA in
absence of the Pd(II) complexes revealed a Tm of 74.2 °C under experi-
mental conditions (Fig. 3). In presence of the complexes, Tm of DNA
increased to 78.3, 79.8, 79.1 and 77.7 °C for complexes 1–4 respectively.
The increase in Tm is comparable to some classical intercalators [17].
For further investigation regarding binding mode of complexes,
viscosity measurements on solutions of calf thymus DNA incubated
with the complexes were carried out, because the viscosity of a DNA
solution is sensitive to the presence of intercalating molecules [18].
In the relative specific viscosity measurement, with increase in ratio
of [Complex]/[DNA] from 0.02 to 0.2, the increase in relative specific
viscosity is observed (Fig. 4) due to increase in the length of DNA
helix, in which base pairs are separated to accommodate the classical
intercalators [19].
The absorption spectrum is one of the most important techniques
for measuring strength of the complexes to bind with the DNA [14].
The absorption spectra of complex 1 in the absence and presence of
CT DNA are shown in Fig. 2. In the UV region, complex 1 exhibits
intense absorption band around 294 nm, which is assigned to π–π*
transition of the chromophore. With increasing the concentration
of CT DNA, the absorption band shows hypochromism and red shift
(bathochromism). The binding constant value of palladium complexes
are found in the range of 1.58×104–6.57×104 M−1 (Table 3). The
complexes show similar binding constant value with Pd(II) complexes
reported by Yodoshi et al. and Mansouri-Torshizi et al. [15,16]. This
phenomenon is attributed to the strong stacking interaction between
the aromatic group and base pairs of CT DNA, when the complexes
intercalate to the CT DNA.
The double-helical structure of DNA is remarkably stable due to
hydrogen bonding and base stacking interactions. Upon increasing
the temperature, the double-helix dissociates to single strands since
heat harms chemical forces. The temperature at which half of DNA
sample melt is known as the DNA melting temperature (Tm), and
The DNA cleavage ability of the complexes was investigated by gel
electrophoresis technique using pUC19 DNA. The electrophoretic
migration patterns for the cleavage of pUC19 DNA are usually
Fig. 5. Photogenic view of interaction of pUC19 DNA (300 μg/mL) with of palladium(II) complexes (200 μM) using 1% agarose gel containing 0.5 μg/mL ethidium bromide. All
reactions were incubated in TE buffer (pH 8) in a final volume of 15 μL, for 3 h. at 37 °C.: Lane 1, DNA control; Lane 2, Na2PdCl4; Lane 3, L1; Lane 4, L2; Lane 5, L3; Lane 6, L4;
Lane 7, [Pd(clpbpy)Cl2] [1]; Lane 8, [Pd(clcpbpy)Cl2] [2]; Lane 9, [Pd(brcpbpy)Cl2] [3]; Lane 10, [Pd(clmpbpy)Cl2] [4].

64
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Acknowledgements
Authors thank Head, Department of Chemistry, Sardar Patel
University, India for making it convenient to work in laboratory and
U.G.C. for providing financial support under “UGC Research Fellowship
for Meritorious Students” scheme.
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