Synthesis, characterization, covalent binding, and degree of unwinding of platinum(II) bipyridine complexes

Article abstract of DOI:10.1002/zaac.201100466

The complexes [Pt(3''-clpbpy)Cl₂] (1) [3''-clpbpy = 4-(3''-chlorophenyl)-6-phenyl-2, 2'-bipyridine], [Pt(4''-clpbpy)Cl₂] (2) [4''-clpbpy = 4-(4''-chlorophenyl)-6-phenyl-2, 2'-bipyridine], [Pt(3''-brpbpy)Cl₂] (3) [3''-brpbp

Full text of DOI:10.1002/zaac.201100466

ARTICLE
DOI: 10.1002/zaac.201100466
Synthesis, Characterization, Covalent Binding, and Degree of Unwinding of
Platinum(II) Bipyridine Complexes
Mohan Patel,*[a] Deepen S. Gandhi,[a] Pradhuman A. Parmar,[a] Bhupesh S. Bhatt,[a] and
Anshul P. Patidar[a]
Keywords: 2,2Ј-Bipyridines; Binding constant; Covalent binding; Plasmid unwinding; Cytotoxicity; Platinum
Abstract. The complexes [Pt(3ЈЈ-clpbpy)Cl2] (1) [3ЈЈ-clpbpy = 4-(3ЈЈ- DNA by covalent mode. The intrinsic binding constant Kb of the com-
chlorophenyl)-6-phenyl-2,2Ј-bipyridine], [Pt(4ЈЈ-clpbpy)Cl2] (2) [4ЈЈ-
clpbpy
4-(4ЈЈ-chlorophenyl)-6-phenyl-2,2Ј-bipyridine], [Pt(3ЈЈ- and 2.31ϫ105 (4) M–1. The slight depression in relative specific vis-
brpbpy)Cl2] (3) [3ЈЈ-brpbpy = 4-(3ЈЈ-bromophenyl)-6-phenyl-2,2Ј-bi- cosity was observed, which also attributes to covalent binding of com-
pyridine], and [Pt(4ЈЈ-brpbpy)Cl2] (4) [4ЈЈ-brpbpy 4-(4ЈЈ-bro- plexes with DNA bases. Gel electrophoresis assay demonstrated the
plexes with HS DNA is 8.76ϫ104 (1), 9.89 ϫ104 (2), 1.52ϫ105 (3),
=
=
mophenyl)-6-phenyl-2,2Ј-bipyridine] were synthesized and charac-
terized. The binding of the complexes with herring sperm DNA (HS
DNA) was investigated by absorption titration and viscosity measure-
ability of the complexes to unwind negatively supercoiled pUC19 plas-
mid by 14° (1), 13° (2), 13° (3), and 11° (4). The in vitro cytotoxic
property of the synthesized metal complexes was also carried out
ments. It was found that the complexes have ability of interaction with against brine shrimp bioassay.
DNA under the influence of complexes were considered. Gel
electrophoresis technique was used to determine the influence
of complexes on pUC19 DNA.
1. Introduction
In the recent years, the transition-metal complexes have
been utilized to investigate the structure of nucleic acids and
to develop DNA cleaving agents,[1,2] DNA photoprobes,[3,4]
DNA-molecular light switches,[5,6] and so on. Lippard laid the
foundation stone in field of the square-planar PtII complexes
interacting with DNA.[7] Platinum-based anticancer drugs have
been in research focus for more than two decades particularly
for studying molecular mechanics and it is well accepted that
the binding of complexes to DNA, whether partly or fully,
triggers the death of tumor cells. In case of cis-platin, the Pt-
DNA adduct is formed by cross-linking of adjacent guanines
on same strand, whereas trans-platin cross-links the guanines
of opposite strands,[8] On the bases of structural activity rela-
tionships (SAR), it was concluded that the NH moiety is essen-
tial for interaction with DNA through hydrogen bonding,[9]
though involvement of NH groups is not completely clari-
fied.[10–12] Very little but detail about the binding of plati-
num(II) complexes with DNA by groove or intercalation are
known.[13–15]
2. Results and Discussion
2.1 Thermal and Electronic Behavior
TGA data of the complexes show no weight loss between
80 to 340 °C, which indicates absence of coordinated or lattice
water molecules. For low-spin square-planar d8-metal com-
plexes, three d–d spin-allowed transitions are expected corre-
sponding to the transitions from three lower lying d levels to
the empty dx2–y2 orbital. These transitions are designated as dz2
(a1g) Ǟ dx2–y2 (b1g), dxy (b2g) Ǟ dx2–y2 (b1g) and dxz,yz (eg) Ǟ
dx2–y2 (b1g).[6–9] The electronic spectral band corresponding to
dz2 (a1g) Ǟ dx2–y2 (b1g) transition appears at ca. 400 nm (ca.
25000 cm–1).[17] The charge-transfer bands appear at ca.
350 nm (ca. 28500 cm–1) and below.[18] In the synthesized PtII
bipyridine complexes, only one d–d band at ca. 436 nm (ca.
22,830 cm–1) is observed, which is assigned to dz2 Ǟ dx2–y2
transition, whereas the other two bands are masked by high
intensity charge-transfer bands. The bands observed at ca. 362
(ca. 27400 cm–1) and ca. 330 nm (30120 cm–1) correspond to
charge transfer transitions. All these bands (Table 1) points
towards a low spin complex of d8 system with square-planar
arrangement.
In continuation of our previous work,[16] we report the syn-
thesis and characterization of the bipyridine ligands (3ЈЈ-
clpbpy, 4ЈЈ-clpbpy, 3ЈЈ-brpbpy, and 4ЈЈ-brpbpy) and their com-
plexes {[Pt(3ЈЈ-clpbpy)Cl2] (1), [Pt(4ЈЈ-clpbpy)Cl2] (2),
[Pt(3ЈЈ-brpbpy)Cl2] (3), and [Pt(4ЈЈ-brpbpy)Cl2] (4)}. The
complexes were checked for their DNA interacting behavior.
Changes in spectroscopic and hydrodynamic behavior of HS
* Prof. Dr. M. N. Patel
2.2 IR Spectroscopy
E-Mail: jeenen@gmail.com
[a] Department of Chemistry
Sardar Patel University
The IR spectroscopic measurements of the ligands show
bands at ca. 1578 cm–1 and 1419 cm–1 corresponding to C=C
Vallabh Vidyanagar–388 120 Gujarat, India
838
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2012, 638, (5), 838–843

Platinum(II) Bipyridine Complexes
Table 1. Electronic spectroscopic data for the platinum(II) complexes.
λmax /nm
Charge transfer bands
2
2
2
dz Ǟ dx –y
[Pt(3ЈЈ-clpbpy)Cl2] (1)
[Pt(4ЈЈ-clpbpy)Cl2] (2)
[Pt(3ЈЈ-brpbpy)Cl2] (3)
[Pt(4ЈЈ-brpbpy)Cl2] (4)
330
328
333
331
360
364
361
365
435
431
438
441
and C=N ring stretching, whereas the bands at 718 cm–1 and 2.4 Interaction between Complexes and DNA
3065 cm–1 are due to aromatic C–H out-of-plane bending and
stretching, respectively. The sharp bands with medium inten-
sity around 1600 and 1497 cm–1 are characteristic of aromatic
2.4.1 Spectrophotometric Titration
Electronic absorption titration is employed universally to de-
ring stretching. The band for C–H out-of-plane ring defor-
mation appears at ca. 763 cm–1 and C–Cl stretching appears at
1038 cm–1. Major changes occur in the bands of C=C and C=N
ring stretching as they are shifted from 1578, 1419 to 1650,
and 1491 cm–1, respectively, supporting binding of metal
atoms by nitrogen atoms of bipyridine skeleton, which was
further supported by a sharp band at 589 and 479 cm–1, charac-
teristic bands of symmetric and asymmetric stretching of the
Pt–N bond.[19] The stretching mode of Pt–Cl is expected in the
region below 400 cm–1 i.e. ca. 320 cm–1.[20]
termine the binding mode of complexes with DNA. Small per-
turbation in the ligand-to-metal charge transfer (LMCT) band
i.e. hyperchromism along with a red shift of 2 nm results from
covalent binding of the complex to DNA.[21,22] Whereas partial
intercalation of the complex to DNA generally results in hy-
pochromism and a red shift.[23,24] Absorption spectra of syn-
thesized PtII complexes with increasing concentration of HS
DNA are shown in Figure 2. As the concentration of DNA is
increased, hyperchromism in the LMCT band of each complex
occurs. In addition to that, a red shift of about 2 nm is also
observed (Table 2). These results suggest a covalent interaction
mode of complexes with DNA. Quantitative comparison of the
DNA-binding affinities of the complexes was done by calculat-
ing the intrinsic binding constants (Kb) of the complexes with
HS DNA. The Kb values obtained for complexes 1, 2, 3, and
4 are 9.89ϫ104, 8.76ϫ104, 2.31ϫ105, and 1.52ϫ105 m–1,
respectively. Though these values are lower or similar to the
value observed for [Pt(dmphen)(CO3)]·H2O (dmphen = 2,9-
dimethyl-1,10-phenanthroline) (1.8ϫ105 m–1).[25] The lower-
ing in Kb value can be accredited to non-planarity of bipyr-
idines, i.e. phenyl and substituted phenyl at 6- and 4-position
respectively.[26] The substitution on 6-positions of one pyridine
ring of 2,2Ј-bipyridine is similar to the substitution on either 2-
or 9-position of 1,10-phenanthroline ligand, which may cause
2.3 Mass Spectrometry
FAB-mass spectra of all complexes were obtained by using
m-nitro benzyl alcohol as matrix. Peaks at 136, 137, 154, 289,
and 307 m⁄z value are due to the use of matrix. Complexes 1
and 2 show peaks at 748, 575, 540 and 343 m⁄z corresponding
to fragments [M + 136 + 3H]+, [M – Cl + H]+, [M – 2Cl +
2H]+, and [3ЈЈ/4ЈЈ-clpbpy], respectively. Complexes 3 and 4
show peaks at 659, 619, 583, and 387 corresponding to frag-
ments [M + 6H]+, [M – Cl + H]+, [M – 2Cl + H]+ and [3ЈЈ/
4ЈЈ-brpbpy], respectively. The pattern in the mass spectrum is
in close proximity to the isotopic pattern calculated from
ChemBioDraw Ultra 11.0 software. Hence, from all above ex-
perimental facts it can be concluded that the synthesized com-
plexes have square-planar arrangement. The proposed structure
of 1 is shown in Figure 1.
Figure 2. Absorption spectral traces of complex 1 with increasing
amount of DNA in phosphate buffer (Na2HPO4/NaH2PO4, pH 7.2).
[complex] = 50 μM, [DNA] = 0–63.3 μM with incubation period of
15 min at 37 °C. Plots of [DNA]/(εa – εf) versus [DNA] for the titration
of DNA with complex 1, which give εb = 5343 M–1·cm–1 at [DNA] =
Figure 1. Proposed structure for the synthesized complex
[Pt(3ЈЈ-clpbpy)Cl2].
1
63.3 μM (εa
= 26760; εf = 22032, 21168, 21336, 21371,
21416 M–1·cm–1).
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M. Patel et al.
ARTICLE
Table 2. Absorption titration data upon addition of Herring Sperm DNA.
λmax /nm
Hyperchromism H /%a)
Binding constant Kb /M–1
Free
Bound
Δλ
1
2
3
4
288
288
288
288
290
289
290
289
2
1
2
1
24.7
23.3
22.8
21.6
8.76ϫ104
9.89ϫ104
1.52ϫ105
2.31ϫ105
a) H /% = 100ϫ(Abound – Afree)/Afree
.
severe steric constraints near the metal core when the complex (present as a minor impurity in our plasmid preparations). The
approaches the DNA base pairs. Thus, due to constriction of compound that unwinds the DNA duplex reduces the number
phenyl at the intercalation site, binding ability of complex is of supercoils in DNA so their number decreases. This decrease
reduced.[27] On internal comparison, it is found that the elec- upon binding of unwinding agents causes a decrease in the rate
tron-withdrawing substituent on 4th position has high DNA of migration through agarose gel, beyond the point, where both
binding affinity compared to that on 3rd position.[28]
forms co-migrate, the migration rate begins to increase again
as positive supercoils are induced. This makes it possible to
observe and quantify the mean value of unwinding. Under the
present experimental conditions, σ was calculated to be –0.055
on the basis of the data of cis-platin, for which the rb(c) was
determined in this study and a Φ value of 13° was assumed.
Using this approach the DNA unwinding angles were deter-
mined (Table 3). The unwinding angle observed for new ana-
logues of cis-platin i.e. 1, 2, 3, and 4 are 14°, 13°, 13°, and 11°,
respectively, which are close to those observed for [PtCl2(1,4-
DACH)] (15Ϯ2°).[33]
2.4.2 Viscosity Measurement
To investigate the mode of binding for the series of plati-
num(II) complexes, the viscosity of a linearized DNA (approx.
200 bp) solution was measured in the presence of compounds
is regarded as the least ambiguous and most critical test of a
DNA binding model,[29] and affords a stronger argument for
DNA binding mode.[30] During the relative specific viscosity
measurement with increase in ratio of [complex]/[DNA] from
0.02 to 0.2, the slight depression in relative specific viscosity
was observed (Figure 3) resulting from bending or kinking of
the DNA helix, which attributes to covalent binding of com-
plexes with DNA bases.[31]
Figure 4. Unwinding of supercoiled pUC19 plasmid DNA modified
by [Pt(3ЈЈ-clpbpy)Cl2]. Lane 1: control, unmodified DNA; Lane 2–8,
rb = 0.02, 0.03, 0.04, 0.05, 0.06, 0.07, 0.08, respectively. The upper
band corresponds to the open circular form (OC) and the bottom one
corresponds to supercoiled form of pUC19 (SC). Arrow shows the
motion of the negatively charged supercoiled pUC19 toward the posi-
tive electrode.
Table 3. Data for determining the unwinding angle of pUC19 DNA
under influence of complexes.
Complex-to-
Unwinding
nucleotide ratioa)
Reference
angle Φ
rb(C)
Figure 3. Effect on relative viscosity of DNA under the influence of
increasing amount of ethidium bromide and complexes at 27Ϯ0.1 °C
in phosphate buffer(Na2HPO4/NaH2PO4, pH 7.2).
1
2
3
4
0.070
0.075
0.075
0.090
–
14
13
13
11
15
This work
This work
This work
This work
[40]
[PtCl2(1,4-DACH)]
a) Complex-to-nucleotide ratio at which supercoiled and nicked forms
comigrates.
2.4.3 Gel Electrophoresis
Electrophoresis on agarose gel bed is used to determine the
unwinding induced in negatively supercoiled pUC19 plasmid
by monitoring the degree of supercoiling[32] (Figure 4). The
rate of migration of closed circular DNA in an agarose gel
depends on its topological state. The supercoil form shows a
2.5 Cytotoxicity
Brine shrimp lethality bioassay is a recent development in
higher mobility than the completely uncoiled relaxed form the assay procedure of bioactive compound, which indicates
840 www.zaac.wiley-vch.de
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2012, 838–843

Platinum(II) Bipyridine Complexes
phenyl)-1-phenylprop-2-en-1-one (n = 3 or 4 and X = Cl or Br) were
prepared via aldol condensation between acetophenone and substituted
aldehydes.
cytotoxicity as well as a wide range of pharmacological activi-
ties (such as anticancer, antiviral, insecticidal, pesticidal, AIDS
etc) of the compounds.[34,35] All the synthesized compounds
were screened for their cytotoxicity (brine shrimp bioassay)
using the protocol of Meyer et al.[34] From the data obtained,
it is evident that complexes 1–4 displayed LC50 values of
20.46, 15.07, 24.14, and 17.19 μM, respectively against Arte-
mia Cysts. The data show no resemblance with the data ob-
tained in DNA unwinding experiments and hence we can con-
clude that cytotoxic properties of complexes are not due to
unwinding of DNA helix.
3. Conclusions
Complexes with electron withdrawing groups on 4ЈЈ posi-
tion show higher DNA binding affinity than complexes with
electron withdrawing groups on 3ЈЈ position. All the synthe-
sized complexes bind to DNA in covalent mode but complexes
2 and 4 have a higher binding affinity to DNA compared to
complexes 1 and 3, respectively. So, the nature of the group is
not sufficient to increase the binding affinity but the position
of group also plays an important role in DNA binding phenom-
ena. Cytotoxic studies of metal complexes 2 and 4 show potent
lethality (LC50) against brine shrimp compared to complexes
1 and 3, respectively.
Scheme 1. General scheme for the synthesis of 2,2Ј-bipyridine.
4.2.1 Synthesis of 3ЈЈ-clpbpy
4. Experimental Section
4.1 Materials and Instrumental Details
Potassium tetrachloroplatinate (Chemport India PVT LTD), Acetophe-
none, 3-chlorobenzaldehyde, 4-chlorobenzaldehyde, 3-bromobenzal-
dehyde, 4-bromobenzaldehyde, 2-acetylpyridine (Spectrochem, Mum-
bai, India), ethidium bromide (EB), bromophenol blue, agarose, her-
ring sperm DNA (Sigma chemical), were purchased commercially and
were used as such. C, H, and N elemental analyses were performed
with a model 240 Perkin–Elmer elemental analyzer. TGA was carried
out with a 5000/2960 SDTA, TA instrument (USA) operated at a heat-
ing rate of 10 K·min–1 in the range of 20–800 °C in a nitrogen atmo-
sphere. Infrared spectra were recorded with a Fourier transform IR
(FTIR) Shimadzu spectrophotometer as KBr pellets in the range 4000–
The ligand was synthesized by using 3-(3-chlorophenyl)-1-phen-
ylprop-2-en-1-one. Yield 61%, m.p. 120 °C. Anal. for C22H15ClN2
(342.82): C 76.48 (calcd. 77.08); H 4.45 (calcd. 4.41); N 8.08 (calcd.
8.17)%. FT-IR (KBr): ν˜ = (C–H) 3067 (m), ν(C=C) 1570 (s), ν(C=N)
1418 (s), δ(C–H) 711 (s), ν(C–Cl) 1040 (s) cm–1. 1H NMR (400 MHz,
CDCl3): δ = 8.780–8.708 (complex, 3 H, H3,3Ј,6Ј), 8.225 (d, 2 H,
H2ЈЈЈ,6ЈЈЈ), 7.954 (complex, 2 H, H5,4Ј), 7.840 (s, 1 H, H2ЈЈ), 7.760 (dd,
1 H, H6ЈЈ), 7.564 (complex, 2 H, H4ЈЈ,5ЈЈ), 7.514–7.450 (complex, 3 H,
H3ЈЈЈ,4ЈЈЈ,5ЈЈЈ), 7.427 (dt, 1 H, H5Ј) ppm. 13C NMR (100 MHz, CDCl3):
δ = 157.51 (C2), 155.78 (C2Ј), 148.96 (C6), 148.48 (C4), 143.56 (C6’),
140.59 (C1ЈЈ), 139.12 (C1ЈЈЈ), 137.62 (C4Ј), 135.16 (C3ЈЈ), 130.39 (C4ЈЈ),
129.32 (C5ЈЈ), 129.07 (C2ЈЈ), 128.81 (C2ЈЈЈ,6ЈЈЈ), 127.45 (C4ЈЈЈ), 127.16
(C3ЈЈЈ,5ЈЈЈ), 125.54 (C6ЈЈ), 124.08 (C5Ј), 121.91 (C3Ј), 118.61 (C3),
117.65 (C5) ppm.
1
400 cm–1. The H NMR and 13C NMR were recorded with a Bruker
Avance (400 MHz). The fast atomic bombardment mass spectra (FAB
MS) were recorded with a Jeol SX 102/Da-600 mass spectrophotome-
ter/data system using Argon/Xenon (6 kV, 10 mA) as the FAB gas.
The accelerating voltage was 10 kV and spectra were recorded at room
temperature. The electronic spectra were recorded with a UV-160A
UV/Vis spectrophotometer, Shimadzu (Japan). Photo quantization of
the gel after electrophoresis was carried out with AlphaDigiDocTM RT.
Version V.4.0.0 PC-Image software.
4.2.2 Synthesis of 4ЈЈ-clpbpy
The ligand was synthesized by using 3-(4-chlorophenyl)-1-phen-
ylprop-2-en-1-one. Yield 54%, m.p. 140 °C. C22H15ClN2 (342.82): C
76.40 (calcd. 77.08), H 4.97 (calcd. 4.41), N 8.30 (calcd. 8.17)%. FT-
IR (KBr): ν˜ = (C–H) 3068 (m), ν(C=C) 1577 (s), ν(C=N) 1419 (s),
δ(C–H) 722 (s), ν(C–Cl) 1032 (s) cm–1. 1H NMR (400 MHz, CDCl3):
δ = 8.754–8.710 (complex, 2 H, H3Ј,6Ј), 8.659 (s, 1 H, H3), 8.220 (d,
2 H, H2ЈЈЈ,6ЈЈЈ), 7.960 (s, 1 H, H5), 7.910 (t, 1 H, H4Ј), 7.789 (d, 2 H,
4.2 Synthesis of Ligands
The substituted bipyridines i.e. 4-(nЈЈ-halophenyl)-6-phenyl-2,2Ј-bi-
pyridines were synthesized by reaction of 1-(pyridylcarbonylethyl)pyr-
H2ЈЈ
, 6ЈЈ), 7.578–7.474 (complex, 5 H, H3ЈЈ,5ЈЈ,3ЈЈЈ,4ЈЈЈ,5ЈЈЈ), 7.386 (t, 1 H,
H5Ј) ppm. 13C NMR (100 MHz, CDCl3): δ = 157.43 (C2), 155.86
idinium iodide and 3-(n-X phenyl)-1-phenylprop-2-en-1-one (enone) (C2Ј), 150.22 (C6), 149.09 (C4), 142.22 (C6Ј), 139.28 (C1ЈЈ), 138.25
as the method reported by Nerve et al. (Scheme 1).[36] The reactant 1- (C1ЈЈЈ), 137.08 (C4Ј), 129.92 (C4ЈЈЈ), 129.30 (C2ЈЈЈ,6ЈЈЈ), 128.80 (C2ЈЈ,6ЈЈ),
(pyridylcarbonylethyl)pyridinium iodide was synthesized using 2-acet-
128.55 (C3ЈЈЈ,5ЈЈЈ), 127.10 (C3ЈЈ,5ЈЈ), 124.08 (C4ЈЈ), 121.78 (C5Ј), 118.40
ylpyridine, iodine and pyridine,[37] and the another reactants 3-(n-X (C3Ј), 117.50 (C3), 102.75 (C5) ppm.
Z. Anorg. Allg. Chem. 2012, 838–843
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841

M. Patel et al.
ARTICLE
instead of 3ЈЈ-clpbpy. Yield 74% (0.163 g, 0.249 mmol), m.p.:
Ͼ300 °C. C22H15Cl2BrN2Pt (653.26): C 40.12 (calcd. 40.45), H 2.35
(calcd. 2.31), N 4.33 (calcd. 4.29)%. FAB MS: m⁄z = 659 [Pt(3ЈЈ-
brpbpy)Cl2 + 6H]+, 619 [Pt(3ЈЈ-brpbpy)Cl + H]+, 583 [Pt(3ЈЈ-brpbpy)
+ H]+, 387 [3ЈЈ-brpbpy]. FT-IR (KBr): ν˜ = (C–H) 3070 (m), ν(C=C)
1651 (s), ν(C=N) 1488 (s), δ(C–H) 765 (s), ν(M–N)sym 589 (s), ν(M–
N)asym 481 (s) cm–1.
4.2.3 Synthesis of 3ЈЈ-brpbpy
The ligand was synthesized by using 3-(3-bromophenyl)-1-phen-
ylprop-2-en-1-one. Yield 59%, m.p. 132 °C. C22H15BrN2 (387.27): C
67.98 (calcd. 68.23), H 3.93 (calcd. 3.90), N 7.13 (calcd. 7.23)%. FT-
IR (KBr): ν˜ = (C–H) 3065 (m), ν(C=C) 1582 (s), ν(C=N) 1422 (s),
δ(C–H) 715 (s), ν(C–Br) 1044 (s) cm–1. 1H NMR (400 MHz, CDCl3):
δ = 8.763–8.714 (complex, 2 H, H3Ј,6Ј), 8.650 (s, 1 H, H3), 8.231 (d,
2 H, H2ЈЈЈ,6ЈЈЈ), 7.990 (s, 1 H, H5), 7.960 (s, 1 H, H2ЈЈ), 7.912 (t, 1 H,
H4Ј), 7.780 (d, 1 H, H6ЈЈ), 7.620 (d, 1 H, H4ЈЈ), 7.565 (t, 2 H, H3ЈЈЈ,5ЈЈЈ),
7.496 (t, 1 H, H4ЈЈЈ), 7.436–7.376 (complex, 2 H, H5Ј,5ЈЈ) ppm. 13C
NMR (100 MHz, CDCl3): δ = 157.60 (C2), 154.92 (C2Ј), 149.00 (C6),
148.44 (C4), 143.19 (C6Ј), 140.70 (C1ЈЈ), 139.10 (C1ЈЈЈ), 135.76 (C4Ј),
132.08 (C3ЈЈ), 130.68 (C2ЈЈ), 130.25 (C5ЈЈ), 129.40 (C4ЈЈ), 128.83
(C2ЈЈЈ,6ЈЈЈ), 127.13 (C4ЈЈЈ), 126.10 (C3ЈЈЈ,5ЈЈЈ), 124.28 (C6ЈЈ), 123.23 (C5Ј),
122.23 (C3Ј), 118.78 (C3), 117.93 (C5) ppm.
4.3.4 Synthesis of [Pt(4ЈЈ-brpbpy)Cl2] (4)
Compound 4 was synthesized using using the same procedure as de-
scribed for 1 with the difference that 4ЈЈ-brpbpy (0.131 g, 0.337 mmol)
used instead of 3ЈЈ-clpbpy. Yield 79% (0.174 g, 0.266 mmol), m.p.:
274 °C. C22H15Cl2BrN2Pt (653.26): C 40.09 (calcd. 40.45), H 2.33
(calcd. 2.31), N 4.26 (calcd. 4.29)%. FAB MS: m⁄z = 659 [Pt(4ЈЈ-
brpbpy)Cl2 + 6H]+, 619 [Pt(4ЈЈ-brpbpy)Cl + H]+, 583 [Pt(4ЈЈ-brpbpy)
+ H]+, 387 [4ЈЈ-brpbpy]. FT-IR (KBr): ν˜ = (C–H) 3066 (m), ν(C=C)
1646 (s), ν(C=N) 1494 (s), δ(C–H) 762 (s), ν(M–N)sym 586 (s), ν(M–
N)asym 477 (s) cm–1.
4.2.4 Synthesis of 4ЈЈ-brpbpy
The ligand was synthesized by using 3-(4-bromophenyl)-1-phen-
ylprop-2-en-1-one. Yield 59%, m.p. 146 °C. C22H15BrN2 (387.27); C
68.13 (calcd. 68.23), H 3.84 (calcd. 3.90), N 7.29 (calcd. 7.23)%. FT-
IR (KBr): ν˜ = (C–H) 3061 (m), ν(C=C) 1584 (s), ν(C=N) 1417 (s),
4.4 Interaction between Complexes and DNA
δ(C–H) 723 (s), ν(C–Br) 1037 (s) cm–1. H NMR (400 MHz CDCl3):
1
4.4.1 Binding of Complex with Herring Sperm DNA
δ = 8.882 (s, 1 H, H3), 8.828–8.806 (complex, 2 H, H3Ј,6Ј), 8.201 (dd,
The binding behavior of the PtII complexes toward herring sperm DNA
was assessed by monitoring the transformation in the absorptive nature
of complex (5ϫ10–5 m) brought by varying the concentration of DNA,
effect of increasing amount of DNA was nullified by adding same
aliquots of DNA to reference cell. Absorption data were utilized to
calculate intrinsic binding constant (Kb) for the complexes using fol-
lowing equation:
2 H, H2ЈЈЈ,6ЈЈЈ), 8.091 (t, 1 H, H4Ј), 8.009 (d, 1 H, H5), 7.807 (d, 2 H,
H2ЈЈ,6ЈЈ), 7.683 (d,
2 H, H3ЈЈ,5ЈЈ), 7.578–7.482 (complex, 4 H,
H5Ј,3ЈЈЈ,4ЈЈЈ,5ЈЈЈ) ppm. 13C NMR (100 MHz CDCl3): δ = 157.83 (C2),
154.43 (C2Ј), 149.48 (C6), 164.61 (C4), 143.55 (C6Ј), 139.75 (C1ЈЈ),
138.85 (C1ЈЈЈ), 137.00 (C4Ј), 132.36 (C2ЈЈЈ,6ЈЈЈ), 129.48 (C4ЈЈЈ), 128.94
(C2ЈЈ,6ЈЈ), 128.87 (C3ЈЈЈ,5ЈЈЈ), 127.11 (C3ЈЈ,5ЈЈ), 124.52 (C4ЈЈ), 123.87 (C5Ј),
122.92 (C3Ј), 118.92 (C3), 118.26 (C5) ppm.
[DNA] / (εa – εf) = [DNA] / (εb – εf) + 1 / Kb(εb – εf)
(1)
4.3 Synthesis of Complexes
4.3.1 Synthesis of [Pt(3ЈЈ-clpbpy)Cl2] (1)
where εa, εf and εb correspond to Aobs ⁄ [complex], the extinction coeffi-
cient for free complex and the extinction coefficient for the complexes
in the fully bound form, respectively and [DNA] is the concentration
of DNA in terms of base pairs. A plot of [DNA] / (εa – εf) versus
[DNA] gives Kb, as the ratio of slope to the intercept.[38]
Complex 1 was synthesized by heating 1:1 ratio of 3ЈЈ-clpbpy (0.116 g,
0.337 mmol) and K2PtCl4 (0.140 g, 0.337 mmol) in water/ethanol
(50 mL) at 80 °C with a drop of HCl (free acid is to avoid displacement
of Cl by OH) until the watery solution became colorless (0.5–5 h).
The reaction mixture was allowed to cool till room temperature is
attained. The obtained product was washed with hot water and dried
in vacuum. Yield 83% (0.170 g, 0.279 mmol), m.p.: Ͼ300 °C, Anal.
for C22H15Cl3N2Pt (608.80): C 43.01 (calcd. 43.40), H 2.50 (calcd.
2.48), N 4.65 (calcd. 4.60)%. FAB MS: m⁄z = 748 [Pt(3ЈЈ-clpbpy)Cl2
+ 136 + 3H]+, 575 [Pt(3ЈЈ-clpbpy)Cl + H]+, 540 [Pt(3ЈЈ-clpbpy) +
2H]+, 343 [3ЈЈ-clpbpy]. FT-IR (KBr): ν˜ = (C–H) 3067 (m), ν(C=C)
1650 (s), ν(C=N) 1489 (s), δ(C–H) 763 (s), ν(M–N)sym 592 (s), ν(M–
N)asym 482 (s) cm–1.
4.4.2 Viscosity Measurements
An Ubbelohde viscometer maintained at a constant temperature of
27Ϯ0.01 °C in a thermostatic jacket, was used to measure the flow
time of DNA in phosphate buffer (Na2HPO4/NaH2PO4, pH 7.2) with
accuracy of 0.01 sec. and precision of 0.1 sec. DNA sample used was
having approximate average length of 200 base pairs prepared by son-
ication to minimize the complexities arising from its flexibility.[39]
Flow time for each sample was measured in triplicate and an average
flow time was calculated. Representation of data was done in terms of
(η⁄η0)1⁄3 vs. concentration ratio ([complex] ⁄ [DNA]),[40] where η is the
viscosity of DNA solution in the presence of complex and η0 is the
viscosity of solution of DNA alone. Viscosity values were calculated
using following equation.[41]
4.3.2 Synthesis of [Pt(4ЈЈ-clpbpy)Cl2] (2)
Compound 2 was synthesized using the same procedure as described
for 1 with the difference that 4ЈЈ-clpbpy (0.116 g, 0.337 mmol) was
used instead of 3ЈЈ-clpbpy. Yield 80% (0.164 g, 0.269 mmol), m.p.:
Ͼ300 °C. Anal. for C22H15Cl3N2Pt (608.80): C 43.04 (calcd. 43.40),
H 2.45 (calcd. 2.48), N 4.64 (calcd. 4.60)%. FAB MS: m⁄z = 748
[Pt(4ЈЈ-clpbpy)Cl2 + 136 + 3H]+, 575 [Pt(4ЈЈ-clpbpy)Cl + H]+, 540
[Pt(4ЈЈ-clpbpy) + 2H]+, 343 [4ЈЈ-clpbpy]. FT-IR (KBr): ν˜ = (C–H)
3072 (m), ν(C=C) 1653 (s), ν(C=N) 1493 (s), δ(C–H) 760 (s), ν(M–
N)sym 588 (s), ν(M–N)asym 476 (s) cm–1.
η = (t – t0)
(2)
where t0 is the flow time of buffer alone and t is the flow time for
buffer containing DNA.
4.4.3 Unwinding of Supercoiled pUC19 DNA
4.3.3 Synthesis of [Pt(3ЈЈ-brpbpy)Cl2] (3)
Unwinding of supercoiled pUC19 DNA was measured by agarose gel
electrophoresis mobility shift assay.[42] The unwinding angle Φ, in-
Compound 3 was synthesized using the same procedure as described
for 1 with the difference that 3ЈЈ-brpbpy (0.131 g, 0.337 mmol) used duced per Pt–DNA adduct was calculated by Φ = –18σ⁄rb(c), where σ
842 www.zaac.wiley-vch.de
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Z. Anorg. Allg. Chem. 2012, 838–843

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Acknowledgement
Authors thank Head, Department of Chemistry, Sardar Patel Univer-
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Published Online: February 22, 2012
Z. Anorg. Allg. Chem. 2012, 838–843
© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.zaac.wiley-vch.de
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