Antimicrobial and nuclease activity of mixed polypyridyl ruthenium(II) complexes

Article abstract of DOI:10.1016/j.inoche.2010.08.022

The ruthenium(II) complexes with 2,9-dimethyl-1,10-phenanthroline (dmphen) and terpyridine derivatives have been synthesized and screened for their antimicrobial potency. The interaction of these complexes with Herring Sperm DNA was investigated by therma

Full text of DOI:10.1016/j.inoche.2010.08.022

Inorganic Chemistry Communications 13 (2010) 1480–1484
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Inorganic Chemistry Communications
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Antimicrobial and nuclease activity of mixed polypyridyl ruthenium(II) complexes
a
a
b
Mohan N. Patel a, , Pradhuman A. Parmar , Deepen S. Gandhi , Vasudev R. Thakkar
⁎
a
Department of Chemistry, Sardar Patel University, Vallabh Vidyanagar-388 120, Gujarat, India
BRD School of Bioscience, Sardar Patel University, Vallabh Vidyanagar-388 120, Gujarat, India
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The ruthenium(II) complexes with 2,9-dimethyl-1,10-phenanthroline (dmphen) and terpyridine derivatives
have been synthesized and screened for their antimicrobial potency. The interaction of these complexes with
Herring Sperm DNA was investigated by thermal denaturation, viscosity measurements, gel electrophoresis
and spectrophotometric methods. The results indicate that the complexes bound to DNA via partial
intercalative mode. The salt-dependent binding of these complexes has been determined by UV–Vis
spectrophotometric titration. The contribution of the non-electrostatic binding free energy (ΔGot ) to the total
binding free energy change (ΔGo) is found to be ~88% at [Na+]=0.075 M. The large value suggests that the
stabilization of the DNA binding is mostly due to the contribution of non-electrostatic process.
© 2010 Elsevier B.V. All rights reserved.
Received 8 May 2010
Accepted 17 August 2010
Available online 24 August 2010
Keywords:
Terpyridine
Polypyridyl ruthenium complexes
Partial intercalation
Salt dependence
Thermal denaturation
pUC19 DNA
The interaction of ruthenium(II) polypyridyl complexes with DNA
has been a topic of major bioinorganic interest. Ruthenium poly-
pyridyl complexes have received special attention due to their strong
metal to ligand charge transfer (MLCT) absorption, their unique
emission characteristics, the perturbance of which could be exploited
to study their DNA binding properties [1]. An understanding of how
these small molecules bind to DNA will be potentially useful in the
design of new drugs, diagnostic reagents, highly sensitive spectro-
scopic and reactive probes [2,3]. Polypyridyl ruthenium(II) complexes
can bind to DNA by non-covalent interactions such as electrostatic
binding, groove binding [4], intercalative binding and partial inter-
calative binding [5,6]. The ancillary ligands can also play an important
role in governing DNA binding of the complexes in octahedral
polypyridyl RuII complexes. So it is significant and interesting to
find the effects of ancillary ligands on the interaction and binding
mode of the complexes to DNA. The strong absorbance caused by
metal to ligand charge transfer (MLCT), the spectroscopic character-
istics and their perturbations upon binding to DNA of the RuII
complexes provide practicable means to explore their DNA binding
mechanisms [7].
phenyl)-2,2′:6′,2″-terpyridine}. All the complexes have been screened
for their in-vitro antibacterial activity. The interaction of the complexes
with DNA has been investigated by spectroscopic, viscosity measure-
ments, thermal denaturation and gel electrophoresis technique.
The salt-dependent binding of the complexes has been also studied
using spectrophotometric titration at various concentrations of salt
(NaCl).
Terpyridines were synthesized by adding 2-acetylpyridine (2.42 g,
20.0 mmol) to 70 mL ethanolic solution of an aldehyde (10 mmol).
KOH pellets (1.4 g, 26 mmol) and aqueous NH3 (30 mL, 25%,
0.425 mol) were added to the solution and stirred at room
temperature for 8 h. An off-white solid formed which was collected
by filtration and washed with H2O (3×10 mL) and EtOH (2×5 mL).
Crystallization from CHCl3–MeOH gave white crystalline solid.
[RuII(2‴-pytpy)(dmphen)Cl](ClO4) (1) was synthesized by taking
[RuIII(2‴-pytpy)Cl3] (202 mg, 0.39 mmol), 2,9-dimethyl-1,10-phe-
nanthroline (93 mg, 0.45 mmol), excess LiCl (108 mg, 2.6 mmol)
and NEt3 (0.8 mL) in 40 mL of ethanol and the mixture was refluxed
for 2 h under a dinitrogen atmosphere. The initial dark brown color of
the solution gradually changed to a deep purple. The solvent was
then removed under reduced pressure. The dry mass was dissolved in
a minimum volume of acetonitrile, and an excess saturated aqueous
solution of NaClO4 was added to it. The precipitate was filtered off
and washed with cold ethanol followed by ice-cold water. The
product was dried in vacuum and purified using a silica column. The
complex was eluted by 2:1 CH2Cl2/CH3CN. Other complexes were
synthesized by the same procedure described above by using different
terpyridines.
Herein, we report the synthesis and characterization of four [RuII(2‴-
pytpy) (dmphen)Cl](ClO4), [RuII(3‴-pytpy)(dmphen)Cl](ClO4), [RuII(3-
boptpy)(dmphen)Cl](ClO4) and [RuII(4-boptpy)(dmphen)Cl](ClO4)
complexes {where 2‴-pytpy=4′-(2‴-pyridyl)-2,2′:6′,2″-terpyridine,
3‴-pytpy=4′-(3‴-pyridyl)-2,2′:6′,2″-terpyridine, 3-boptpy=4′-(3-
benzyloxyphenyl)-2,2′:6′,2″-terpyridine, 4-boptpy=4′-(4-benzyloxy-
TGA data of the complexes show no weight loss occurring in the
range 80–180 °C. So, there is an absence of coordinated or lattice
⁎
Corresponding author. Tel.: +91 2692 226856x220.
E-mail address: jeenen@gmail.com (M.N. Patel).
1387-7003/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2010.08.022

M.N. Patel et al. / Inorganic Chemistry Communications 13 (2010) 1480–1484
1481
Table 1
water molecule. Both the ligands decompose in one step in the
Electronic spectral data for the ruthenium(II) complexes.
max/nm (ε/dm3 mol−1 cm−1
π→π*
temperature range 360–600 °C. Magnetic moment value of all
complexes was found to be zero, which suggests low-spin configu-
ration with d2sp3 hybridization for octahedral RuII complexes.
Perchlorate as a counterion is confirmed by a very strong, broad peak
at around 1086 cm−1 and the strong, sharp peak at around 625 cm−1
[8]. In the spectra of complexes 3 and 4, band at ~1230 cm−1 can be
attributed to the asymmetric stretching of aromatic ether. A weak, broad
peak at around 3070 cm−1, characteristic of aromatic C–H stretching as
well as a sharp peak at around 2930 cm−1, characteristic of C–H
stretching of methyl. A sharp peak of medium intensity at around 1600
and 1498 cm−1, characteristic of aromatic ring stretching. An intense,
sharp peak at around 760 cm−1, characteristic of C–H out-of-plane
deformations, appears as expected from a structure including aromatic
rings. A weak, sharp band in the range 490–520 cm−1, characteristic of
Ru–N stretching mode. A Ru–Cl stretching mode would be expected in
the region less than 400 cm−1 [9]. The suggested structure of complex 1
is shown in Fig. 1.
The electronic spectra of the ruthenium complexes were recorded in
dimethylsulfoxide medium and the band maxima and molar extinction
coefficients are listed in Table 1. The UV–Vis spectrum of RuII complexes
exhibits a typical metal to ligand charge transfer (MLCT) transition due
to Ru(dπ)→terpy(π*) with λmax from 491 to 497 nm (ε=18 670–22
210 dm3 mol−1 cm−1). The spin allowed metal to ligand charge
transfer (MLCT) band in the visible spectral region undergoes an
increase in intensity and a red shift, regardless of the electron-donor or
electron-acceptor nature of the substituent. Complexes show two bands
at around 283 nm (ε=45 714–57 283 dm3 mol−1 cm−1) and 312 nm
(ε=42 356–56 132 dm3 mol−1 cm−1) due to the overlap of ligand
centered transitions dmphen(π)→dmphen(π*) and terpy(π)→terpy
(π*) [10].
Complexes
λ
)
MLCT
[RuII(2‴-pytpy)(dmphen)(Cl)]ClO4 (1) 281 (45 714), 309 (46 190) 491 (18 670)
[RuII(3‴-pytpy)(dmphen)(Cl)]ClO4 (2) 280 (46 545), 311 (42 356) 491 (18 750)
[RuII(3-boptpy)(dmphen)(Cl)]ClO4 (3) 282 (56 386), 312 (52 740) 495 (21 340)
[RuII(4-boptpy)(dmphen)(Cl)]ClO4 (4) 285 (57 283), 314 (56 132) 497 (22 210)
and E. coli but found less active in the case of P. aeruginosa. All the
complexes found less active than pefloxacin.
Binding of the compound with DNA through intercalation
generally results in hypochromism and bathochromism due to the
intercalative mode involving a strong stacking interaction of the
planar aromatic chromophore of the compound with the base pairs of
DNA [12]. Fig. 2 shows the absorption spectra of complex 1 in the
presence of increasing amounts of DNA at room temperature. With
increasing DNA concentration to complex 1, hypochromism of the
MLCT band at 493 nm was observed along with a 2 nm red shift. The
red shift observed in complexes 2, 3 and 4 is 2 nm, 1 nm and 1 nm,
respectively. Similar red shift of the MLCT band reported for [Ru
(phen)2NMIP]2+ and [Ru(tpy)(PHNI)]2+ complexes [13,14]. So, small
red shifting of the MLCT band suggests that the complexes may bind
with DNA via partial intercalative mode.
In order to further investigate the binding strength of the
complexes, the intrinsic binding constants (Kb) were determined by
monitoring change in absorbance of the MLCT band with increasing
concentration of DNA. The intrinsic binding constants (Kb) of
complexes 1, 2, 3, 4 were found to be 2.47×104, 2.64×104,
4.56×103, 5.38×103 M−1, respectively. The binding constants of
ligands were found in the range of 1.35×103 to 4.43×103 M−1, which
are lower than intrinsic binding constants of the complexes. So,
complexation increases the DNA binding affinity. The binding
constants of complexes are smaller than that observed for [Ru(bpy)2
(dppz)] (N106) [15]. The reason for the low binding constant is
the presence of methyl groups on the 2nd and 9th positions of
phenanthroline causing severe steric constraints near the RuII core
when the complex intercalates to the DNA [16]. Another reason for
low Kb is the nonplanarity of terpyridines.
FAB-mass spectra of all complexes were obtained using m-nitro
benzyl alcohol as matrix. Peaks at 136, 137, 154, 289 and 307 m/z
values are due to usage of matrix. For complex 1, the peak at 793 m/z
value is assigned as a molecular ion peak associated with matrix (136)
and two H+ ions without perchlorate ion [11]. The base peak observed
at m/z=209 is due to the dmphen ligand associated with one proton.
Several other fragments at m/z 758, 448, 413 and 310 values are
observed, attributed to fragments associated with matrix or H+ ions.
The isotopic pattern for ruthenium and chlorine atoms is observed in
the spectrum.
Fig. 3 shows the plots of [DNA]/(εa −εf) versus [DNA] for [RuII(2‴-
pytpy)(dmphen)Cl](ClO4) (20 μM) with increasing amount of DNA
(0–16.4 μM) in phosphate buffer (pH 7.2), containing various
concentrations of NaCl and incubated for 10 min at 37 °C. The non-
All the ligands and their ruthenium complexes were screened in-
vitro for their growth inhibitory activity against two Gram(+ve)
,
,
Staphylococcus aureus and Bacillus subtilis, and three Gram(−ve)
Serratia marcescens, Pseudomonas aeruginosa and Escherichia coli,
microorganisms. No zone of inhibition was observed for any of the
ligands with 100 μL solution having a concentration of 1 μg/μL. MIC
data denotes that all the ligands show poor antibacterial activity than
complexes. All the complexes found more active against S. marcescens
Fig. 2. Electronic absorption spectra of [RuII(2‴-pytpy)(dmphen)Cl](ClO4) with
increasing amount of [DNA] in phosphate buffer (Na2HPO4/NaH2PO4, pH 7.2),
[complex]=20 μM, [DNA]=0–16.4 μM with an incubation period of 10 min at 37 °C,
inset: plot of [DNA]/(εa −εf) versus [DNA]. Arrow shows the absorbance change upon
increasing DNA concentrations.
Fig. 1. Suggested structure of the complex [RuII(2‴-pytpy)(dmphen)Cl](ClO4) (1).

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M.N. Patel et al. / Inorganic Chemistry Communications 13 (2010) 1480–1484
Fig. 4. Salt dependence of binding constant (Kb) for the binding of [RuII(2‴-pytpy)
(dmphen)Cl](ClO4) to DNA. The slope of this plot corresponds to the SK quantity
presented in Table 3.
Fig. 3. Plots of [DNA]/(εa −εf) versus [DNA] for the spectrophotometric titration of [RuII
(2‴-pytpy)(dmphen)Cl](ClO4) with increasing amount of [DNA] in phosphate buffer
(Na2HPO4/NaH2PO4, pH 7.2) at 37 °C and concentrations of NaCl varying from 0.005 to
0.1 M.
free energy changes (ΔG0) were calculated based on the standard
Gibbs relation (Eq. (2)). The data are represented in Table 3.
ΔG0 = –RT ln Kb
ð2Þ
electrostatic binding constant (K0t ) calculated using Eq. (1) and the
data are summarized in Table 2.
where R is the gas constant and T is the absolute temperature.
The electrostatic free energy change (ΔG0pe) is calculated using
Eq. (3) [20].
ꢄ
h
iꢅ
ꢂ
ꢀ
ꢁꢃ
ln Kb = ln Kt0 + Zξ−1 ln γ δ + Zψ ln Mþ
ð1Þ
h
i
þ
ꢀ
ΔG0pe = ðSKÞRT ln Naþ
ð3Þ
where Zψ is the negative value of the slope obtained from the plots of
logKb versus log[M+] (Fig. 4), ψ is the fraction of counter ions
associated with each DNA phosphate (ψ=0.88 for double-stranded B-
form DNA), Z is the partial charge on the binding ligand, γ is the
mean activity coefficient at cation concentration, where as remaining
terms are constant for double-stranded B-form DNA, ξ=4.2 and
δ=0.56.
where salt dependence binding constant is defined as the slope of
Fig. 4 (Eq. (4)).
h
i
SK = δ log Kb = δ log Naþ = –Zψ
ð4Þ
Non-electrostatic free energy change (ΔG0t ) represents the binding
free energy independent of salt concentration, which is defined as the
difference of Gibbs free energy change ΔG0 and electrostatic free
energy change (ΔG0pe) and calculated using Eq. (5).
The magnitude of K0t is constant throughout the experiment
at various concentrations of NaCl with the average value of 8.593×
103 M−1 (RSD limiting to 15%) but its contribution to Kb increases
and reaches to 39.38% when the concentration of sodium ion reaches
0.1 M [17]. The salt concentration used during the course of study
ranges from 0.001 to 0.1 M because the laws of theories of
polyelectrolyte are only applicable if the concentration is higher
than 0.1 M [18]. The value for the non-electrostatic DNA binding
constant of synthesized complexes is about 36% at 0.075 M NaCl,
which is higher compared to [Fe(phen)2(dppz)]2+ and classical
intercalator ethidium bromide (EtBr) [19].
ΔG0t = ΔG0−ΔG0pe
ð5Þ
The melting of DNA is an important parameter to study the
interaction of transition metal complexes with nucleic acids. Metal
complexes bind to the double-stranded DNA, usually stabilize the
duplex structure to some extent lead to an increase in the melting
temperature (Tm) of DNA [21–23]. The melting temperature of DNA
varies depending on the interacting strength of the compounds. The
temperature at which 50% of the DNA has become single strand is
known as melting temperature. It can be determined from the thermal
denaturation curves of DNA by monitoring the absorption changes at
260 nm [24]. The Tm observed for DNA (100 μM) is 74.2 0.2 °C under
our experimental conditions (Fig. 5).
In the presence of complexes 1, 2, 3, 4, the melting temperature
(ΔTm) of DNA increases by 4.7, 5.8, 2.7 and 2.9 °C, respectively. These
results show that the interaction of complex 2 with DNA is the
strongest among all complexes.
In the absence of crystallographic study, hydrodynamic volume
measurement is the most critical test to knock interaction properties
[25]. A classical intercalation mode increases the viscosity of DNA
solution by lengthening the DNA helix, while partial intercalation
decreases the viscosity of DNA solution by reducing effective length
of DNA [26]. The effect of the increasing amount of EtBr and
complexes on the relative viscosity of DNA is shown in Fig. 6. EtBr is
Dissection of binding free energy change (ΔG0) during binding of
complexes to DNA into electrostatic (ΔG0pe) and non-electrostatic
(ΔG0t ) free energy change at 0.075 M NaCl are summarized in Table 3
along with the data of [Fe(phen)2(dppz)]2+ [19]. The total binding
Table 2
Equilibrium binding constant (Kb) and contribution of the non-electrostatic binding
constant (K0t ) at various concentrations of NaCl.
a
b
[NaCl] (M)
Kb (M−1
)
K0t (M−1
)
K0t /Kb (%)
0.005
0.025
0.05
0.075
0.1
7.9159×104
4.4279×104
3.1399×104
2.4858×104
2.0059×104
8.059×103
9.357×103
9.049×103
8.602×103
7.900×103
10.18
21.13
28.82
34.61
39.38
Average K0t (M−1)=8.593×103
(
0.624×103)
a
Kb values are calculated as per Eq. (1).
b
K0t values are calculated as per Eq. (2) with RSD less than 15%.

M.N. Patel et al. / Inorganic Chemistry Communications 13 (2010) 1480–1484
1483
Table 3
Thermodynamic parameters during the binding study of Herring Sperm DNA with complexes at 0.075 M concentration of NaCl.
Complex
Kb/104 (M−1
)
ΔG0
SK
ΔG0Pe
Kt0/103 (Kt0/Kb%)
ΔGt0 (%ΔGt0/ΔG0)
Reference
1
2
3
4
2.4858
2.6544
0.4896
0.5672
9.5000
−25.07
−25.23
−21.05
−21.41
−28.39
0.440
0.431
0.422
0.407
0.493
−2.86
−2.76
−2.70
−2.61
−3.16
8.602 (34.61)
9.519 (35.86)
1.793 (36.64)
2.153 (37.97)
29.90 (31.46)
−22.21 (88.59)
−22.47 (89.04)
−18.34 (87.13)
−18.80 (87.80)
−25.23 (87.5)
This work
This work
This work
This work
[19]
[Fe(phen)2(dppz)]2+
a well known classical intercalator, which increases the relative
viscosity of DNA solution. The decrease in relative viscosity of DNA
solution by all ligands and complexes indicates that they interact with
DNA via partial intercalation mode.
Fig. 7 shows the electrophoretic separation of pUC19 DNA upon
reaction with complexes under aerobic condition. When the plasmid
Fig. 7. Photograph of the interaction of pUC19 DNA (100 μg/mL) with series of
ruthenium(II) complexes (200 μM) using 1% agarose gel containing 0.5 μg/mL ethidium
bromide (EtBr). All reactions were incubated in TE buffer (pH 8) in a final volume of
20 μL, for 24 h at 37 °C: lane 1, DNA control; lane 2, RuCl3; lane 3, [RuII(2‴-pytpy)
(dmphen)Cl](ClO4); lane 4, [RuII(3‴-pytpy)(dmphen)Cl](ClO4); lane 5, [RuII(3-boptpy)
(dmphen)Cl](ClO4); lane 6, [RuII(4-boptpy)(dmphen)Cl](ClO4).
Fig. 5. Melting curves of DNA in the absence and the presence of the complexes 1, 2, 3, 4
([DNA]=100 μM and [Complex]=20 μM, respectively).
Fig. 8. Gel eletrophoretic data of the DNA cleavage study.
DNA was subjected to electrophoresis upon reacting with complexes,
the fastest migration was observed for super coiled (SC), the slowest
moving open circular (OC) will produce upon relaxing of SC, the
intermediate moving is the linear form (L) generated on the cleavage
of the circular form. The data of the cleavage are presented in Fig. 8.
From the data, it is clear that the complexes can cleave DNA more
efficiently when compared to the metal salt. The difference in DNA
cleavage efficiency of the complexes was due to the difference in
binding affinity of the complexes to DNA.
Acknowledgements
Authors thank the head of the Department of Chemistry of Sardar
Patel University, Vallabh Vidyanagar, India for making it convenient to
work in the laboratory and U.G.C. for providing the financial support
under “UGC Research Fellowship in Science for Meritorious Students”
scheme.
Fig. 6. Effect on the relative viscosity of DNA under the influence of increasing amount
of ethidium bromide (EtBr) and complexes at 27 0.1 °C in phosphate buffer
(Na2HPO4/NaH2PO4, pH 7.2), as a medium.

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M.N. Patel et al. / Inorganic Chemistry Communications 13 (2010) 1480–1484
[10] N. Yoshikawa, S. Yamabe, N. Kanehisa, Y. Kai, H. Takashima, K. Tsukahara, Inorg.
Appendix A. Supplementary data
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Supplementary data to this article can be found online at
doi:10.1016/j.inoche.2010.08.022.
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