Studies on synthesis, characterization, DNA interaction and cytotoxicity of ruthenium(II) Schiff base complexes

Article abstract of DOI:10.1016/j.saa.2012.03.035

The synthesis and characterization of three hexa-coordinated ruthenium(II) Schiff base complexes of the type [RuCl(CO)(B)L] (B = PPh₃/AsPh ₃/py and L = monobasic tridentate Schiff base ligand derived by the condensation of salicylald

Full text of DOI:10.1016/j.saa.2012.03.035

Spectrochimica Acta Part A 94 (2012) 210–215
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Studies on synthesis, characterization, DNA interaction and cytotoxicity of
ruthenium(II) Schiff base complexes
Gunasekaran Rajaa, Ray. J. Butcherb, Chinnasamy Jayabalakrishnana,∗
a Department of Chemistry, Sri Ramakrishna Mission Vidyalaya College of Arts and Science, Coimbatore 641 020, Tamil Nadu, India
b Department of Chemistry, Howard University, Washington, DC 20059, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 14 February 2012
Accepted 11 March 2012
The synthesis and characterization of three hexa-coordinated ruthenium(II) Schiff base complexes of
the type [RuCl(CO)(B)L] (B = PPh3/AsPh3/py and L = monobasic tridentate Schiff base ligand derived by
the condensation of salicylaldehyde with 4-aminoantipyrine) are reported. IR, electronic, NMR and mass
spectral data of the complexes are discussed. An octahedral geometry has been tentatively proposed
for all the complexes. DNA binding properties of the ligand and its ruthenium(II) complexes have been
investigated by electronic absorption spectroscopy. Two of the complexes were tested for DNA cleavage
property. Finally, in vitro study of the cytotoxicity of the ligand and the complex [RuCl(CO)(PPh3)L] on
HeLa were tested. The IC50 value for the ligand and the complex were 52.3 and 31.6 ␮m respectively.
© 2012 Elsevier B.V. All rights reserved.
Keywords:
Ruthenium(II) complexes
DNA binding
DNA cleavage
Cytotoxicity
1. Introduction
ruthenium(II) complexes containing a Schiff base ligand derived
from 4-aminoantipyrine and salicylaldehyde.
The compounds of transition elements of the platinum group
hold promise in the design of new anti cancer agents. Cisplatin rep-
resents one of the most active and clinically useful agents used in
the treatment of cancer, with high response rates in ovarian and
small lung cancer. However, in common with many other cyto-
toxic drugs, cisplatin induces normal tissue toxicity particularly to
the kidney. Therefore, alternative metal compounds are presently
being evaluated in clinical trials [1–3]. One of the most promis-
ing metal is ruthenium [4]. Ruthenium compounds are regarded
as promising alternatives to platinum compounds and offer many
approaches to innovative metalopharmaceuticals, the compounds
are known to be stable and to have predictable structures both
in the solid state and in solution: tuning of ligand affinities and
accompanied by a steadily increasing knowledge of the biologi-
cal effects of ruthenium compounds [5]. Many studies suggest that
DNA is the primary intracellular target of antitumor drugs because
the interaction between small molecules and DNA can cause DNA
damage in cancer cells [6,7]. Thus, in order to develop new poten-
tial DNA targeting antitumor drugs, the binding mechanisms of
metal complexes to DNA should be studied. Moreover, Schiff bases
of 4-aminoantipyrine and its complexes have a wide applications
in medicinal, analytical and pharmacological areas [8,9]. With this
considerations in mind, in this paper we report the synthesis, char-
acterization, DNA binding, DNA cleavage and anticancer studies of
2. Experimental
2.1. Materials and methods
All the reagents used were of analar grade. Solvents were
purified and dried according to the literature procedures [10].
RuCl3·3H2O was purchased from Loba chemie and used without
further purification. The starting complexes [RuHCl(CO)(PPh3)3]
[11], [RuHCl(CO)(AsPh3)3] [12], [RuHCl(CO)(py)(PPh3)2] [13] were
prepared by the literature methods. Elemental analyses were per-
formed at Sophisticated Test and Instrumentation centre (STIC),
Cochin University of Science and Technology, Kerala. Infrared spec-
tra were recorded using KBr pellets on a Perkin-Elmer FT-IR 8000
spectrometer RX1 model from 4000 to 400 cm−1. Electronic spectra
were recorded in dichloromethane with a Systronics-2202 double
beam spectrophotometer from 800 to 200 nm. The 1H NMR spec-
tra of the ligand and complexes were recorded with a Bruker WM
400 DCX MHz instrument using TMS as an internal standard. Mass
spectrum of the complex was recorded with JEOL GC mate instru-
ment at Indian Institute of Technology, Chennai. Melting points
were recorded with Veego DS Model apparatus and are uncor-
rected. Anti cancer studies was carried out at the Kovai Medical
Centre and Hospital Pharmacy College, Coimbatore, Tamil Nadu,
India.
2.2. Preparation of the Schiff base ligand (HL)
∗
To an ethanolic solution of 4-aminoantipyrine (2.033 g,
10 mmol), a solution of salicylaldehyde (1.05 cm3, 10 mmol) was
Corresponding author. Tel.: +91 9442001793.
E-mail address: drcjbstar@gmail.com (C. Jayabalakrishnan).
1386-1425/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2012.03.035

G. Raja et al. / Spectrochimica Acta Part A 94 (2012) 210–215
211
Table 1
room temperature gave a ratio of 1.8–1.9:1, indicating that the DNA
was sufficiently free of protein. The concentration of CT-DNA per
nucleotide was determined from its absorption intensity at 260 nm
with a molar extinction coefficient of 6600 M−1 cm−1 [15,16]. Stock
solutions were stored at 4 ◦C, and used within 3 days. Titration
experiments were carried out by varying the concentration of
CT-DNA while keeping the ruthenium(III) complex concentration
constant (10 ␮m). The mixture was allowed to equilibrate for 5 min
before spectra was recorded.
The DNA cleavage experiments were done by agrose gel elec-
trophoresis. CT-DNA in 5 mM Tris–HCl/50 mM NaCl buffer (pH 7.2)
was treated with the complexes in the absence of additives. The
samples were added with loading buffer. Then they were elec-
trophoresed for 1 h at 50 V on 1% agrose gel using TAE buffer.
After electrophoresis, bands were visualized by UV-A light and pho-
tographed.
Crystal data and structure refinement for ligand.
Ligand
Empirical formula
Formula weight
Crystal system
Space group
a
b
c
˛
C18H17N3O2
307.35
Monoclinic
P 21/n
7.5983(3) A
˚
7.4985(3) A
˚
˚
27.2796(9) A
90◦
ˇ
ꢀ
95.321(3)◦
90◦
3
˚
Volume
Z
1547.58(9) A
4
Absorption coefficient
Reflections collected
Independent reflections
Max. and min. transmission
Data/restraints/parameters
Final R indices [I > 2ꢁ(I)]
0.088 mm−1
21432
6439 [R(int) = 0.0402]
1.00000 and 0.88810
6439/0/211
R1 = 0.0675, wR2 = 0.1518
2.6. Cell treatment procedure
The human cervical cancer cell line (HeLa) was obtained from
National Centre for Cell Science (NCCS), Pune, and grown in Eagles
Minimum Essential Medium containing 10% fetal bovine serum
(FBS). All cells were maintained at 37 ◦C, 5% CO2, 95% air and 100%
relative humidity. Maintenance cultures were passaged weekly,
and the culture medium was changed twice a week. The mono-
layer cells were detached with trypsin–ethylenediaminetetraacetic
acid (EDTA) to make single cell suspensions and viable cells were
counted using a hemocytometer and diluted with medium with 5%
FBS to give final density of 1 × 105 cells/ml. one hundred microlitres
per well of cell suspension were seeded into 96-well plates at plat-
ing density of 10,000 cells/well and incubated to allow for cell
attachment at 37 ◦C, 5% CO2, 95% air and 100% relative humid-
ity. After 24 h the cells were treated with serial concentrations
of the extracts and fractions. They were initially dissolved in
neat dimethylsulfoxide (DMSO) and further diluted in serum free
medium to produce five concentrations. One hundred microlitres
per well of each concentration was added to plates to obtain final
concentrations of 100, 50, 25, 12.5 and 6.25 ␮g/ml. The final vol-
ume in each well was 200 ␮l and the plates were incubated at 37 ◦C,
5% CO2, 95% air and 100% relative humidity for 48 h. The medium
containing without samples were served as control. Triplicate was
maintained for all concentrations.
added with stirring. The mixture was stirred for about 1 h. The yel-
low solid formed was filtered and recrystallized from ethanol. The
purity of the ligand was checked by TLC. The analytical data, FT-IR
and single crystal XRD studies confirm the structure of the Schiff
base.
2.3. X-ray structure determination
Single crystals of the ligand were obtained by slow evaporation
of ethanol solution of the ligand. Selected crystal data are given in
Table 1. Data were collected, respectively, on an Oxford Diffraction
Gemini, a Bruker SMART Apex CCD area detector systems using
˚
graphite monochromated Mo Ka radiation (k = 0.71073 A). X-ray
data reduction, structure solution and refinement were done using
SHELXS-97 and SHELXL-97 programs [14]. The structures were
solved by the direct methods.
2.4. Preparation of the ruthenium(II) Schiff base complexes
All the reactions were carried out under strictly anhydrous
conditions. The monobasic tridentate Schiff base ligand (0.1 mM)
was added to a solution of [RuHCl(CO)(EPh3)2(B)] (where E = P/As;
B = PPh3/AsPh3/py) (0.1 mM) in ethanol–benzene (1:1) mixture and
it was refluxed for 6 h. The resulting solution was concentrated to
about 3 cm3. The complexes were precipitated by the addition of a
small quantity of petroleum ether (60–80 ◦C). The complexes were
then filtered off, washed with petroleum ether and dried under
vacuum (Scheme 1). Various attempts have been made towards
crystallization of the complexes were unsuccessful.
2.7. Assessment of cell viability by MTT
3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bro-
mide (MTT) is
a yellow water soluble tetrazolium salt. A
mitochondrial enzyme in living cells, succinate-dehydrogenase,
cleaves the tetrazolium ring, converting the MTT to an insoluble
purple formazan. Therefore, the amount of formazan produced is
directly proportional to the number of viable cells. After 48 h of
incubation, 15 ␮l of MTT (5 mg/ml) in phosphate buffered saline
(PBS) was added to each well and incubated at 37 ◦C for 4 h. The
medium with MTT was then flicked off and the formed formazan
2.5. DNA-binding and cleavage experiments
The UV-A at 260 nm and 280 nm of the Calf-thymus DNA (CT-
DNA) solution in 5 mM Tris–HCl, 50 mM NaCl buffer (pH 7.2) at
CO
N
O
N
B
N
O
N
Ru
Cl
Benzene-Ethanol
Reflux, 6 h
+
[RuHCl(CO)(EPh3)2(B)]
N
HC
N
HC
O
OH
Scheme 1. Formation of ruthenium(III) Schiff base complexes E = P or As or py; B = PPh3 or AsPh3 or py.

212
G. Raja et al. / Spectrochimica Acta Part A 94 (2012) 210–215
Fig. 1. ORTEP for HL.
frequency at 1595–1601 cm−1, indicating coordination of the keto
group to the metal. A strong band due to phenolic C O is observed
at 1272 cm−1 in the spectrum of the ligand, and is shifted to higher
frequency (1308–1319 cm−1) for the complexes, showing that the
other coordination site is the phenolic oxygen [20]. This is also
supported by the absence of a broad band around 3160 cm−1, due
to phenolic OH, in the spectra of the complexes. For all three
complexes, IR spectra showed a strong band at 1951–1952 cm−1
due to terminally coordinated free carbonyl. A medium inten-
sity band at 1029 cm−1 is characteristic of coordinated nitrogen
base for the complex [RuCl(CO)(py)L] [13]. Characteristic bands for
triphenylphosphine/arsine are also present in the expected region
1436–1437 cm−1 [21].
crystals were solubilized in 100 ␮l of DMSO and then measured
the absorbance at 570 nm using micro plate reader. The % cell
inhibition was determined using the following formula.
Abs (sample)
Abs (control)
% cell inhibition = 100 −
× 100
3. Results and discussion
A Schiff base derived from 4-aminoantipyrine and salicylalde-
hyde was characterized by analytical, spectroscopic and single
crystal XRD studies. The perspective view of the ligand is shown in
Fig. 1 and the crystal data are presented in Table 1. The ligand crys-
tallizes in the monoclinic crystal system with P 21/n space group.
The structure of the ligand agrees well with the proposed structure
as reported earlier [17].
3.2. Electronic spectra
Three air stable, mononuclear octahedral ruthenium(III) Schiff
base complexes of the type [RuCl(CO)(B)L] (where, B = PPh3 or
AsPh3 or py; L = monobasic tridentate Schiff base) have been pre-
pared by the reaction of [RuHCl(CO)(EPh3)2(B)] (where E = P/As;
B = PPh3/AsPh3/py) with the Schiff base in 1:1 molar ratio in
ethanol–benzene mixture. The analytical data (Table 2) for the
new complexes agree well with the proposed molecular formulae
(Scheme 1).
The electronic spectra of the free ligand and its complexes were
recorded in DMSO and listed in Table 3. The free ligand shows two
transitions at 309 and 368 nm due to ␲–␲* and n– ␲* transitions
involving the benzene ring, C N and C O groups. These bands were
shifted in the spectra of the complexes, supporting the involvement
of the imine group nitrogen and keto group oxygen in coordination.
The spectra of the complexes showed an additional transition at
394–423 nm which may be assigned to a charge transfer transition
of ruthenium(II) [22]. All Schiff base ruthenium complexes were
diamagnetic indicating the presence of ruthenium(II). The ground
state of ruthenium(II) in an octahedral environment is 1A1g, arising
3.1. IR spectra
IR spectrum of the free Schiff base was compared with those
of the complexes in order to infer the coordination mode. The
FTIR spectral data are summarized in Table 3. In the spectrum of
from the t2g6 configuration, and the excited states corresponding to
1
the t2g5eg configuration are 3T1g
,
3T2g
,
1T1g and 1T2g. Hence, four
the ligand, a sharp peak at 1598 cm−1 is may be due to the C
N
1
3
3
bands corresponding to the transitions A1g → T1g
,
1A1g → T2g
,
1
1
1
1A1g → T1g and A1g → T2g are possible in order of the increas-
ing energy. The pattern of the electronic spectra for the complexes
indicates the presence of an octahedral environment around the
ruthenium(II) ion similar to that of other ruthenium octahedral
complexes [23].
group. This band was shifted to lower frequencies in the spectra
of all three complexes (1575–1589 cm−1), indicating the involve-
ment of the C N nitrogen in coordination to the metal [18]. A
band at 1642 cm−1 in the spectrum of the ligand is due to the
keto group in the pyrazolone ring [19], which is shifted to lower
Table 2
Analytical data of the ligand and complexes.
Table 3
IR and electronic spectroscopic data of the ligands and complexes.
Ligand/complex
Color
Mol.
mass
Calculated (found) (%)
Ligand/complex
FT-IR (cm−1
)
UV–vis ꢂmax (nm)
C
H
N
ꢃ
ꢃPh
ꢃ
C
C
N
O
C
O
HL
Yellow
307
733
777
550
70.3(70.0)
60.6(60.1)
57.2(57.8)
52.4(52.8)
5.6(5.9)
4.3(4.7)
4.0(4.6)
3.9(3.1)
13.7(13.9)
5.7(6.0)
5.4(5.7)
HL
1598
1589
1585
1575
1272
1316
1308
1319
1642
1601
1595
1600
309, 368
308, 365, 394
311, 369, 380
311, 369, 423
[RuCl(CO)(PPh3)L] Yellow
[RuCl(CO)(AsPh3)L] Green
[RuCl(CO)(py)L]
[RuCl(CO)(PPh3)L]
[RuCl(CO)(AsPh3)L]
[RuCl(CO)(py)L]
Brown
10.2(10.8)

G. Raja et al. / Spectrochimica Acta Part A 94 (2012) 210–215
213
Aromatic protons of the HL appear as multiplets in the range
6.6–7.6 ppm. The azomethine proton of HL appears as a singlet
at 8.1 ppm. The Ph OH proton of the ligand appears at 9.1 ppm.
The N CH3 and CH3 proton appears as singlet at 2.6 and 1.9 ppm
respectively. On complexation, the aromatic protons appear as a
multiplet in the range 6.1–8.0 ppm. The azomethine proton appears
at 8.4–8.6 ppm, which gets shifted indicating the coordination of
the azomethine nitrogen atom to the metal. The N CH3 and CH3
protons appear in the range 2.3–2.5 ppm and 1.5–1.8 ppm respec-
tively. Moreover, on complexation the deprotonation of phenolic
group of the HL and coordination of ruthenium through oxygen is
indicated by the absence of resonance for Ph OH.
3.4. Mass spectra
Fig. 2. HMR spectra of the ligand.
The EI mass spectrum of the complex [RuCl(CO)(PPh3)L] was
used to check its composition (Supplementary material). The
molecular ion peak observed at 732.91 confirms the stoichiometry
of the complex.
3.5. DNA binding studies
The Electronic absorption titrations of the free ligand and ruthe-
nium(II) complexes with CT-DNA were carried out. The spectra are
shown in Fig. 6. The intensity of the bands of the complexes around
230 and 250 nm were found to increase with increasing concen-
trations of DNA. The free ligand and complexes [RuCl(CO)(PPh3)L],
[RuCl(CO)(AsPh3)L] and [RuCl(CO)(py)L] exhibited hyperchromism
of 15.3, 27.5, 28.1 and 24.6% respectively. The observed hyper-
chromic effect is indicative of strong binding of the complexes to
CT-DNA. However, no red shift was observed in the absorption
traces, which rules out coordinate binding with the N7 base moi-
eties of DNA. Rather the interaction involves the phosphates of DNA,
via. electrostatic attraction [24]. The intrinsic binding constant Kb of
the complexes were also determined using the following equation
[25]
Fig. 3. HMR spectra of the complex [RuCl(CO)(PPh3)L].
[DNA]
εa − εf
[DNA]
εb − εf
1
=
+
Kb(εb − εf )
Fig. 4. HMR spectra of the complex [RuCl(CO)(AsPh3)L].
where [DNA] is the concentration of DNA in base pairs, the appar-
ent absorption coefficients εa, εf and εb correspond to Aobs/[Ru], the
extinction coefficient for the free complex and in the fully bound
complex respectively. In plots of [DNA]/εa − εf versus [DNA], Kb is
given by the ratio of the slope to the intercept. The Kb values for free
HL, [RuCl(CO)(PPh3)L], [RuCl(CO)(AsPh3)L] and [RuCl(CO)(py)L] are
2.47 × 104, 5.63 × 105, 5.18 × 105, and 4.66 × 105 M−1 respectively.
Hence, HL shows weaker binding towards CT-DNA than the three
complexes, and [RuCl(CO)(PPh3)L]shows the highest binding affin-
ity. All of the intrinsic binding constants are much lower than that
of classical intercalator ethidium bromide whose binding constants
were in the order of 107 ruling out the possibility of intercalation
in the present case [26].
Fig. 5. HMR spectra of the complex [RuCl(CO)(py)L].
3.3. 1H NMR spectra
1H NMR spectra of the HL and the complexes were recorded
in DMSO-d6 and the 1H chemical shift ı (ppm) are referenced to
internal TMS. The NMR spectra of the ligand and the complexes are
shown in the Figs. 2–5 and the values are presented in Table 4.
3.6. DNA cleavage studies
The cleavage ability of the complexes [RuCl(CO)(PPh3)L]
and [RuCl(CO)(py)L] towards CT-DNA was investigated by gel
Table 4
1H-NMR data of ligand and Ru(II) Schiff base complexes.
Ligand/complex
1H NMR spectra
HL
6.6–7.6 (Ar, m), 8.1 (H
6.1–7.4 (Ar, m), 8.4 (H
7.3–7.9 (Ar, m), 8.6 (H
6.7–8.0 (Ar, m), 8.4 (H
C
C
C
C
N, s), 9.1 (Ph OH, s), 1.9 ( CH3, s), 2.6 ( N CH3, s)
[RuCl(CO)(PPh3)L]
[RuCl(CO)(AsPh3)L]
[RuCl(CO)(py)L]
N, s), 1.7 ( CH3, s), 2.4 (
N, s), 1.5 ( CH3, s), 2.3 (
N, s), 1.8 ( CH3, s), 2.5 (
N
N
N
CH3, s)
CH3, s)
CH3, s)

214
G. Raja et al. / Spectrochimica Acta Part A 94 (2012) 210–215
Fig. 6. Absorption spectral traces of the ligand HL and the complexes [RuCl(CO)(PPh3)L], [RuCl(CO)(AsPh3)L] and [RuCl(CO)(py)L] with increasing concentration of CT-DNA
in a Tris HCl–NaCl buffer (pH 7.2).
electrophoresis in Tris–HCl/NaCl buffer (pH = 7.2) at 37 ◦C. Both
compounds show efficient DNA cleavage activity (Fig. 7). The
amount of cleavage was enhanced with increasing concentration
of the complexes, showing their potential chemical nuclease activ-
ity. The complex [RuCl(CO)(PPh3)L] has more cleavage activity than
[RuCl(CO)(py)L].
for antitumor activity in vitro because it exhibited higher activity
than the other two complexes in our DNA binding and cleavage
studies. The cytotoxicity against human cervical carcinoma cell line,
(HeLa) was studied. Using the MTT assay the IC50 values were cal-
culated from the numbers of cells that survived at each particular
complex concentration, after exposure for 48 h. The IC50 values for
the free ligand and complex were 52.3 and 31.6 ␮m respectively.
Thus, complex is more cytotoxic than the ligand. However, these
values are slightly lower than the standard anticancer drug cisplatin
whose IC50 value is in the range 16.7 ␮m.
3.7. Cytotoxicity test
The free ligand and its complex [RuCl(CO)(PPh3)L] were tested
for antitumor activity. The complex [RuCl(CO)(PPh3)L]was selected
4. Conclusion
Taken together, three new mononuclear ruthenium(II)
Schiff base complexes were synthesized and characterized.
Based on various physico-chemical and spectroscopic meth-
ods, the complexes are tentatively assigned an octahedral
geometry. The binding affinities for DNA were in the order
[RuCl(CO)(PPh3)L] > [RuCl(CO)(AsPh3)L] > [RuCl(CO)(py)L] > HL,
and [RuCl(CO)(PPh3)L] has higher DNA cleavage activity than
[RuCl(CO)(py)L]. The cytotoxicities of the free ligand and
[RuCl(CO)(PPh3)L] are slightly lower than the standard anticancer
drug cisplatin.
Fig. 7. Cleavage of CT-DNA (50 ␮m) by [RuCl(CO)(PPh3)L] and [RuCl(CO)(py)L]
in Tris HCl–NaCl buffer (pH 7.2). Lane 1: DNA control; lane 2: DNA + 50 ␮m
[RuCl(CO)(PPh3)L]; lane 3: DNA + 100 ␮m [RuCl(CO)(PPh3)L]; lane 4: DNA + 150 ␮m
[RuCl(CO)(PPh3)L]; lane 5: DNA + 50 ␮m [RuCl(CO)(py)L]; lane 6: DNA + 100 ␮m
[RuCl(CO)(py)L]; lane 7: DNA + 150 ␮m [RuCl(CO)(py)L].

G. Raja et al. / Spectrochimica Acta Part A 94 (2012) 210–215
215
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