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Coordination Behavior of N,N′,N″-Trisubstituted Guanidine Ligands in Their Ru–Arene Complexes: Synthetic, DNA/Protein Binding, and Cytotoxic Studies
Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
pubs.acs.org/Organometallics
Coordination Behavior of N,N′,N″‑Trisubstituted Guanidine Ligands
in Their Ru−Arene Complexes: Synthetic, DNA/Protein Binding, and
Cytotoxic Studies
Kumaramangalam Jeyalakshmi,†,‡ Jebiti Haribabu,† Chandrasekar Balachandran,§
Srividya Swaminathan,† Nattamai S. P. Bhuvanesh,∥ and Ramasamy Karvembu*,†
†
Department of Chemistry, National Institute of Technology, Tiruchirappalli 620015, India
Department of Science and Humanities, M. Kumarasamy College of Engineering, Karur 639113, India
§
Department of Hematology, Fujita Health University, 1-98, Dengakugakubo, Kutsukake-cho, Toyoake, Aichi 470-1192, Japan
∥
Department of Chemistry, Texas A&M University, College Station, Texas 77842, United States
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‡
S Supporting Information
*
ABSTRACT: Ruthenium complexes are fascinating for
exploration as anticancer drugs after the entry of KP1019
and NAMI-A in phase II clinical trials for the treatment of
metastatic tumors. The reaction of guanidine ligands with
[RuCl(μ-Cl)(η6-p-cymene)]2 yielded monometallic Ru(II)
complexes with N,N-type (1) and O,N-type (2 and 3)
ligands, whereas both monometallic (O,N) (7) and bimetallic
Ru(II) (4−6) complexes were obtained when [RuCl(μCl)(η6-benzene)]2 was used as a precursor. The complexes
were characterized using analytical, spectroscopic (UV−vis,
FT-IR, NMR, and mass), and single-crystal X-ray crystallography techniques. The stability of the complexes was tested by
UV−visible spectroscopy. The complexes were investigated for their interaction with calf thymus (CT) DNA and bovine serum
albumin using various spectroscopic techniques. Spectroscopic and viscosity experiments revealed that the intrinsic DNA
binding affinity of the Ru−p-cymene complexes was greater than that of the analogous Ru−benzene complexes due to the
increased hydrophobicity of the p-cymene ring. The in vitro cytotoxicity of the complexes against HepG2, A549, and Vero cells
was evaluated using MTT assay. The results revealed that the complexes with O,N bidentate-type ligands, 2 and 3, showed good
activity against HepG2 cell lines with an IC50 value of 15.41 and 17.74 μM, respectively. The results were compared with
cisplatin, and it was inferred that complexes 2 and 3 showed better activity than cisplatin. The apoptosis mode of cell death was
confirmed by staining and flow cytometry methods.
■
RAPTA family8 containing 1,3,5-triaza-7phosphatricyclo[3.3.1.1]decane ligand and Ru−arene complexes containing bidentate ethylenediamine, which are at an
advanced preclinical trial (Figure 1).9
Complexes of the type [(η6-arene)Ru(X)(YZ)] (where YZ
is a bidentate chelating ligand and X is a good leaving group)
exhibit both in vitro and in vivo anticancer activity; some cases
are even comparable with cisplatin.10 The aqueous reactivity of
[(η6-arene)Ru(X)(YZ)] complexes is highly dependent on the
nature of X, YZ, and the arene. [(η 6 -Arene)Ru(ethylenediamine)Cl]+ complexes exhibit anticancer activity
both in vitro and in vivo against cisplatin-resistant cancer
cells.11a It is expected that Cl− always serves as a better leaving
group where hydrolysis takes place consequently. Therefore,
the aqueous reactivity is dependent on the choice of ligand
INTRODUCTION
Metallodrugs have become important compounds in cancer
therapy; in particular, platinum complexes are used worldwide
against many tumor types.1 Though cisplatin is used as an
anticancer drug, its importance has been thwarted by two main
disadvantages: it is inefficient against platinum-resistant tumors
and has severe side effects such as neuro-, hepto-, and
nephrotoxicity.2 Therefore, the quest for alternative drugs to
the well-known cisplatin and its derivatives is highly needed. In
this context, ruthenium complexes are noted as anticancer
agents due to similar ligand exchange kinetics with that of the
platinum(II) complexes.3,4 The Ru(III) complexes NAMI-A
and KP1019 have shown the most promising results in
preclinical and clinical studies.5−7 Most recently, organometallic ruthenium(II) complexes especially half-sandwich
Ru(II)−arene complexes are fascinating as therapeutics
because their biological activity can be tuned by varying the
ligands. The good examples for this type of complexes are
© XXXX American Chemical Society
Received: September 26, 2018
A
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Figure 1. Ru−arene anticancer complexes (a) RAPTA-C, (b) RAPTA-B, (c) [(η6-arene)Ru(ethylenediamine)Cl]+, and (d) [(η6benzene)Ru(metronidazole)Cl2].
Scheme 1. Structure of the Ligands
Scheme 2. Synthesis of the Ru(II)(η6-p-cymene) Complexes
system. In the complexes containing diimine ligands, the
higher stability of the Ru−Cl bond was related to the electronwithdrawing property of α-diimine ligand.11b
Guanidine is an important ingredient in both organic and
inorganic chemistry. Guanidine functionalities are found in a
variety of natural compounds, either in a cyclic form or as
terminal groups of pendant substituents. The guanidine moiety
is a part of the arginine molecule in the single protonated form,
and it is responsible for the majority of arginine’s noncovalent
interactions.12 Some of the naturally occurring guanidines were
screened for their nuclease activity and exhibited cytotoxic
properties. Guanidine is noted as a sterically and electronically
flexible ligand because of the Y-shaped CN3 unit present in it.
Guanidines have a broad spectrum of biological activities, such
as antitumor,13−15 antimalarial, anti-inflammatory, urease
inhibition,16 etc. This is further enhanced by coordination
with metal. The possible coordination modes of guanidines are
(i) neutral guanidines,17,18 (ii) monoanionic guanidinates
[guanidinates (−1)],19,20 and (iii) dianionic guanidinates
[guanidinates (−2)].21,22
The different coordination possibilities of the ligand when
interacting with transition metals will be helpful in tuning up
the pharmacological properties. The difference in the donor
atoms (N,N and N,O) will have an influence on the rate of
hydrolysis, which is crucial in the cytotoxicity of the
metallodrugs, particularly of the type [RuCl(arene)L].23
Fascinated by the biological importance of ruthenium−arene
complexes and guanidines, herein, we report the synthesis of
Ru−arene complexes containing trisubstituted guanidine
ligands. Moreover, the biological applications of complexes
of guanidines have not been explored much. This motivates us
to study the biological applications of guanidine-based Ru−
arene complexes. The novel complexes were characterized by
various spectroscopic techniques and investigated for their
biological applications. To our knowledge, this is the first
report on ruthenium bimetallic complexes containing trisubstituted guanidine ligands.
■
RESULTS AND DISCUSSION
Synthesis of the Ligands and Complexes. The ligands
(L1−L4) were synthesized by using the procedure reported
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Scheme 3. Synthesis of the Ru(II)(η6-benzene) Complexes
Figure 2. Thermal ellipsoid (50%) plot of L5.
were observed around 1611−1590 and 1571−1558 cm−1,
respectively. On complexation (complex 1), there was a
disappearance of one N−H band and a decrease in the imine
stretching frequency, indicating that the coordination occurred
through imine and N−H nitrogen atoms. In the FT-IR spectra
of complexes 2, 3, and 7, there was a disappearance of one N−
H band and shift in the carbonyl stretching frequency to a
lower value, which clearly indicated the O,N coordination of
the ligands. There was a disappearance of two N−H bands and
shift in CO stretching frequency in the FT-IR spectra of
complexes 4, 5, and 6, which indicated the coordination of
carbonyl oxygen, imine nitrogen, and two N−H nitrogen
atoms after deprotonation, forming bimetallic Ru−arene
complexes. Interestingly, this is a rare coordination mode of
guanidine.
1
H and 13C NMR spectral data for L5 and complexes 1−7
are summarized in the Experimental Section, and spectra are
depicted in the Supporting Information (Figures S1−S16). In
the spectra of the ligands, the N−H (attached to the aromatic
ring) peak appeared at 12.07−10.06 ppm and the signal due to
N−H (attached to the benzyl group) appeared at 5.25 ppm
earlier.24−26 L5 was prepared from N-furoyl-N′-phenylthiourea
by the guanylation method. The ligands used in this work are
shown in Scheme 1.
Interaction of ligands L1−L3 with [RuCl(μ-Cl)(η6-pcymene)]2 yielded monometallic ruthenium(η6-p-cymene)
complexes 1−3, where the donor atoms of guanidines were
different (N,N/N,O) when the substitution was changed
(Scheme 2). The reaction of ligands L1, L2, L4, and L5 with
[RuCl(μ-Cl)(η6-benzene)]2 ended up with the formation of
monometallic 7 and bimetallic complexes 4−6 (Scheme 3).
Spectroscopy. The synthesized Ru−arene complexes were
characterized by UV−vis, Fourier transform infrared (FT-IR),
NMR, and mass spectroscopic techniques. The band appeared
in the UV−vis spectra of the complexes in the 422−454 nm
region indicated d → d transition in the complexes. The band
at 319−334 nm in the spectra of complexes 3, 5, and 6 was due
to the metal to ligand charge transfer transition.
In the FT-IR spectra of the ligands, two bands (strong and
weak) were observed for the N−H group in the range of
3394−3145 cm−1. The weak band is due to the hydrogen
bonded N−H. The CO and CN stretching frequencies
C
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Figure 3. Thermal ellipsoid (50%) plot of 1. Hydrogen atoms are not shown for clarity.
(L2 and L3). Signals due to aromatic protons of the ligands
appeared in the region 8.25−6.45 ppm.24−26 In the spectra of
complexes 1−3, the peak due to N−H proton (12.07−10.30
ppm) disappeared, supporting the coordination of N−H
nitrogen after deprotonation. The new signals appeared around
1.23−1.12, 2.63−2.34, and 5.09−3.89 ppm in the spectra of
the complexes indicated the presence of the p-cymene
moiety.27 In the 1H NMR spectra of complexes 5, 6, and 7,
an intense signal at 5.98 ppm confirmed the presence of a
benzene moiety in the complexes. In the spectrum of complex
4, a resonance due to benzene ring protons appeared at 5.11
and 4.95 ppm. There was a disappearance of both the N−H
peaks in the spectra of complexes 4−6, which provided
evidence for the formation of bimetallic complexes, whereas in
complex 7, there was a disappearance of only one N−H peak
and another N−H proton was observed at 5.23 ppm. Chemical
shift values of all the other aromatic and aliphatic protons were
in the expected range. In the 13C NMR spectra of the
complexes, the signals at 176.6−164.2 and 159.5−156.1 ppm
were attributed to carbonyl and imine carbons, respectively.
The presence of p-cymene carbons was confirmed by the
signals at 101.5−99.9, 98.5−97.9, 80.8−79.1, 83.7−81.3,
31.2−30.9, 22.6−21.9, and 19−18.3 ppm (complexes 1−3).
The signals due to benzene carbons appeared at 88.1−82.3
ppm (complexes 4−7).
Crystal Structures of the Ligand and Complexes.
Single crystals suitable for X-ray diffraction analysis were
obtained for ligand L5 and complexes 1−7, and the structures
are shown in Figures 2−9. Crystal data and selected
interatomic bond lengths and angles are summarized in Tables
1−4. In complexes 1−3 and 7, Ru adopted a pseudooctahedral piano stool geometry, the arene [1−3 (η6-pcymene) and 7 (η6-benzene)] rings displayed the common
π-bonded η6-coordination mode, and the guanidine ligands
assumed a bidentate chelate coordination mode (κ2-N,N or κ2-
Figure 4. Thermal ellipsoid (50%) plot of 2. Only selected atoms are
labeled for clarity.
O,N). The remaining coordination site was occupied by a
chloride ion. Complexes 4−6 displayed the same pseudooctahedral geometry around both the Ru centers. Guanidine
acted as a bridging ligand through N,N and O,N donor atoms.
The Ru bonded to an arene (η6-benzene) ring and a chloride
ion in addition to bridged guanidine. The Ru−centroid
distance in the complexes fell in a narrow range (1.654−
1.666 Å) and was similar to that of alike complexes.28 The
Ru−C distances were in the range of 2.129−2.214 Å. The Ru−
Cl bond length lay in the range of 2.3916−2.4218 Å,29 which
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Figure 5. Thermal ellipsoid (50%) plot of 3. Only selected atoms are labeled for clarity.
Figure 6. Thermal ellipsoid (50%) plot of 4.
was longer than the Ru−N (2.068−2.225 Å) and Ru−O
(2.056−2.1017 Å) bond lengths. N−Ru−Cl and O−Ru−Cl
bond angles were in the range of 83.30−86.67 Å, and the O−
Ru−N bond angle was in the range of 84.17−86.01 Å
(complexes 2−7). The four-membered chelate ring in
complexes 1, 4, 5, and 6 made the N−Ru−N bond angle
smaller (60.1−61.83 Å).
Solubility of the Complexes. The solubility measurements revealed that the complexes were less soluble in water
and the solubility values were compared with the known
anticancer agents (Table S1).
Stability of the Complexes. The stability of complexes
plays a crucial role in the development of drugs. The stability
of the complexes in a 1% DMSO/water mixture was monitored
using UV−visible spectroscopy over a period of 24 h. The
complexes exhibited the characteristic bands in their UV−
visible spectra (Figure S17). There was no significant change in
the spectra of complexes 1, 2, 3, and 7. Complexes 4, 5, and 6
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Figure 7. Thermal ellipsoid (50%) plot of 5. Hydrogen atoms are not shown for clarity.
Figure 8. Thermal ellipsoid (50%) plot of 6. Hydrogen atoms are not shown for clarity.
DNA Interaction Studies. DNA binding of the synthesized complexes was evaluated by UV−visible, ethidium
bromide displacement, and viscosity studies. The studies
revealed that all of the complexes effectively bound with CT
DNA. The details are provided in the Supporting Information.
Protein Binding Studies. The molecular targeting and
mechanism of action of anticancer drugs are not always
exhibited shifts in their characteristic band(s). The stable
complexes 2, 3, and 7 were evaluated for their stability in a 1%
DMSO/PBS buffer mixture (Figure S18). There is no shift in
the absorption bands of the tested complexes, which clearly
indicated the stability of the complexes in the biological
medium.30
F
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Figure 9. Thermal ellipsoid (50%) plot of 7.
unambiguous. The cytotoxic complex [(η6-p-cymene)Ru(ethylenediamine)Cl]PF6 interacted with DNA, whereas the
relatively noncytotoxic antimetastasis compound [(η6-pcymene)Ru(1,3,5-triaza-7-phosphaadamantane)Cl2] preferentially formed adducts with the histone proteins.31 Therefore,
protein binding studies are also significant for predicting the
mechanism of action of anticancer drugs. The binding ability of
the complexes with bovine serum albumin (BSA) was
evaluated by absorption and fluorescence spectral studies.
Absorption Spectral Studies. The electronic absorption
studies are used to explore the type of quenching. The static
quenching leads to the perturbation of the fluorophore and
results in the absorption spectral changes,32−36 whereas in
dynamic quenching, appreciable change in absorption spectra
of the fluorophore is not expected. The absorption spectra of
BSA in the absence and presence of the complexes are depicted
in Figure S25. For BSA, there were two absorption bands; the
one at 213 nm reflected the framework conformation, and the
band at 280 nm corresponded to the aromatic amino acids
(Trp, Tyr, and Phe) of the protein. Hyperchromism in the
absorption band at 280 nm was observed on addition of
complexes 1−7 to BSA, which clearly indicated the static type
of quenching.
Fluorescence Spectral Studies. The fluorescence property
of BSA is mainly due to the presence of tryptophan and
tyrosine residues. Alteration in the emission spectrum arises
primarily from the tryptophan residue because of protein
conformational changes, subunit association, substrate binding,
or denaturation.32 The fluorescence titration of complexes 1−7
(0−20 μM) with BSA resulted in a decrease in the
fluorescence intensity [62.9% (1); 63.2% (2); 76.8% (3);
72.3% (4); 72.7% (5); 59.2% (6); 52.5% (7)] with appreciable
blue shift (2−4 nm) at 345 nm (excitation wavelength = 280
nm) (Figures 10 and S26). The ability of the complexes to
quench the emission intensity was calculated quantitatively
using Stern−Volmer equation F0/F = 1 + Kq[Q], where F0 and
F are fluorescence intensities in the absence and presence of a
quencher, respectively, Kq is a linear Stern−Volmer quenching
constant, and [Q] is the quencher concentration. The slope of
linear plot of F0/F versus [Q] yielded Kq (Figure 11). Further,
the equilibrium binding constant was evaluated using the
Scatchard equation, log[(F0 − F)/F] = log Kb + n log[Q],
where Kb is the binding constant and n is the number of the
binding site. The Kb values were derived from the graph
between log[(F0 − F)/F] and log[Q] (Figure 12). The values
of Kb and Kq revealed the enhanced binding ability of complex
3 compared to the other complexes under investigation (Table
5). The calculated values of n (0.8−1) suggested the existence
of a single binding site in BSA for all complexes.37 DNA/
protein binding ability of the complexes was comparable with
that of the known Ru−arene complexes.38−42
Conformational Investigation. To understand the structural changes occurred in BSA on titration with complexes 1−
7, synchronous fluorescence studies were carried out.43 When
the concentration of the compounds was increased, the
intensity of emission corresponding to tyrosine (304 nm, Δλ
= 15 nm) was found to decrease in the magnitude of 48.5,
52.0, 66.4, 60.8, 58.6, 43.5, and 42.6%, respectively, for
complexes 1−7 with 2 nm blue shift in the emission
wavelength (Figures 13 and S26). The tryptophan fluorescence
emission (340 nm, Δλ = 60 nm) showed a significant decrease
in its intensity [62.5 (1), 63.8 (2), 78.0 (3), 70.3 (4), 74.8 (5),
58.1 (6), and 53.1 (7)%] without any change in the position of
the band (Figures 14 and S27). These results indicated that,
although the compounds affected the microenvironments of
both tyrosine and tryptophan during the binding process, the
effect was more pronounced toward tryptophan.
In Vitro Cytotoxic Activity. The in vitro cytotoxic activity
of the Ru−arene complexes (1−7) was screened against
human liver (HepG2), lung carcinoma (A549), and Vero
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Table 1. Crystal Data and Structure Refinement for Ligand L5 and Complexes 1−3
L5
empirical formula
formula weight
temperature (K)
wavelength (Å)
crystal system
space group
unit cell dimensions
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
volume (Å3)
Z
density (calculated) (mg/m3)
absorption coefficient (mm−1)
F(000)
crystal size (mm3)
θ range for data collection (deg)
index ranges
1
2
3
C18H15N3O2
305.33
110.15
0.71073
monoclinic
P121/c1
C28H28ClN3ORuS
591.11
110.15
0.71073
monoclinic
P121/c1
C29H30.60ClN3O1.30RuS
610.54
110.15
1.54178
monoclinic
P121/c1
C32H34ClN3ORu
613.14
110.15
0.71073
monoclinic
P121/c1
5.8612(9)
21.505(3)
12.042(2)
90
93.001(2)
90
1515.7(4)
4
1.338
0.090
640
0.34 × 0.2 × 0.18
1.894 to 27.531
−7 ≤ h ≤ 7, −27 ≤ k ≤ 26,
−15 ≤ l ≤ 15
14845
3483 [R(int) = 0.0253]
99.9
9.3991(18)
14.047(3)
19.692(4)
90
90.440(2)
90
2599.9(9)
4
1.510
0.812
1208
0.25 × 0.2 × 0.18
1.781 to 27.508
−12 ≤ h ≤ 12, −18 ≤ k ≤ 18,
−24 ≤ l ≤ 25
27077
5937 [R(int) = 0.0259]
99.8
10.3394(4)
27.3435(11)
9.7085(4)
90
95.913(2)
90
2730.14(19)
4
1.485
6.483
1252
0.13 × 0.06 × 0.04
4.857 to 59.979
−11 ≤ h ≤ 11, −30 ≤ k ≤ 30,
−10 ≤ l ≤ 10
62334
4030 [R(int) = 0.0580]
99.4
10.2845(17)
26.776(4)
10.3907(17)
90
96.220(2)
90
2844.5(8)
4
1.432
0.675
1264
0.54 × 0.47 × 0.19
1.521 to 27.407
−13 ≤ h ≤ 13, −34 ≤ k ≤ 34,
−13 ≤ l ≤ 13
50886
6448 [R(int) = 0.0312]
100.0
reflections collected
independent reflections [R(int)]
completeness to
θ = 25.242/25.242/60.0/25.242°
absorption correction
semiempirical from
equivalents
max and min transmission
0.7456 and 0.6949
refinement method
full-matrix least-squares
on F2
data/restraints/parameters
3483/0/208
goodness-of-fit on F2
1.030
final R indices [I > 2σ(I)]
R1 = 0.0383, wR2 = 0.0892
R indices (all data)
R1 = 0.0468, wR2 = 0.0941
largest diff. peak and hole (e·Å−3) 0.244 and −0.266
semiempirical from
semiempirical from
semiempirical from
equivalents
equivalents
equivalents
0.7456 and 0.6793
0.7519 and 0.4168
0.7456 and 0.6320
full-matrix least-squares on F2 full-matrix least-squares on F2 full-matrix least-squares on F2
5937/21/319
1.070
R1 = 0.0273, wR2 = 0.0595
R1 = 0.0324, wR2 = 0.0618
0.532 and −0.499
4030/267/399
1.143
R1 = 0.0261, wR2 = 0.0663
R1 = 0.0284, wR2 = 0.0670
0.407 and −0.607
6448/0/347
1.250
R1 = 0.0417, wR2 = 0.0917
R1 = 0.0486, wR2 = 0.0954
0.946 and −0.754
Our group has reported the cytotoxic activity of Ru(η6arene) complexes against A549 and HepG2 cell lines. Monoand binuclear Ru(η6-p-cymene) complexes containing indole
thiosemicarbazone ligands showed appreciable activity against
HepG2 and A549 cell lines.40 Ru(η6-arene) complexes
containing aroylthiourea ligands showed moderate activity
against the same cell lines.41,42 The present complexes, notably
2, 3 and 7, showed activity greater than that of the previously
reported Ru−arene complexes against HepG2 and A549 cell
lines.45−47 The activity of complexes 2 and 3 was also
comparable with that of the reported ruthenium−arene
complexes48−51 against HepG2 and A549 cell lines. It is
evident from the comparison that our complexes showed
comparable activity with the previously reported ruthenium−
arene complexes (Figure 17).
Cell Apoptosis Analysis by Flow Cytometry and
Fluorescent Staining Methods. Apoptosis, or programmed
cell death, has been used to describe a form of cell death in an
active and inherently controlled manner that eliminates no
longer wanted cells.52 Cell and nuclear shrinkage, chromatin
condensation, formation of apoptotic bodies, and phagocytosis
by neighboring cells characterize the main morphological
changes of the apoptosis process.53 Cleavage of chromosomal
DNA into oligonucleosomal size fragments is a biochemical
(kidney cells of an African green monkey) cell lines, and the
results are provided in Tables 6 and S2−S5 and Figures 15, 16,
and S28 and S29. All of the complexes were tested from their
low concentration (3.9 μg/mL) to higher concentration (250
μg/mL). In vitro cytotoxic activity investigation revealed that
complexes 2 and 3 exhibited high activity against HepG2
cancer cell line at lower concentration of 15.6 μg/mL, with
IC50 values of 15.41 and 17.74 μM, respectively. Complex 2
was even better than cisplatin in its activity. Complex 7
possessed moderate activity with an IC50 value of 59.01 μM.
Among all the complexes, complex 2 showed better activity
with an IC50 value of 58.1 μM against A549 cancer cell line,
whereas the other complexes showed half inhibition above 100
μM. All the complexes were less toxic against normal Vero cell
line. The results revealed that the cytotoxic activity of the
Ru(II)−arene complexes could be correlated with the donor
atoms as (N,O) > (N,N).44 The results were also in good
agreement with the DNA binding efficacy as the (N,O)-type
complexes showed a binding ability better than that of the
(N,N)-type complexes. It is also important to note that the
stable complexes (2, 3, and 7) exhibited appreciable activity
against the cancer cell lines. Unfortunately, the bimetallic
complexes did not show marked inhibition.
H
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Table 2. Crystal Data and Structure Refinement for Complexes 4−7
4
empirical formula
formula weight
temperature (K)
wavelength (Å)
crystal system
space group
unit cell dimensions
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
volume (Å3)
Z
density (calculated) (mg/m3)
absorption coefficient (mm−1)
F(000)
crystal size (mm3)
θ range for data collection (deg)
index ranges
6
7
C32H27Cl2N3ORu2
742.60
110.15
0.71073
monoclinic
P121/c1
C30H25Cl2N3O2Ru2
732.57
110.15
1.54178
triclinic
P1̅
C25H22ClN3ORuS
549.03
110.15
0.71073
triclinic
P1̅
10.072(2)
21.642(5)
12.766(3)
90
91.367(2)
90
2781.7(10)
4
1.788
1.383
1488
0.42 × 0.04 × 0.04
1.853 to 27.582
−13 ≤ h ≤ 13, −28 ≤ k ≤ 28,
−16 ≤ l ≤ 15
24070
6397 [R(int) = 0.0501]
100.0
10.2152(15)
21.795(3)
12.7780(19)
90
90.860(2)
90
2844.5(7)
4
1.734
1.281
1480
0.25 × 0.21 × 0.17
1.848 to 27.602
−13 ≤ h ≤ 13, −28 ≤ k ≤ 28,
−16 ≤ l ≤ 16
36616
6551 [R(int) = 0.0350]
99.2
9.5165(4)
10.5078(4)
13.7238(5)
86.107(2)
78.282(3)
83.087(3)
1332.64(9)
2
1.826
11.304
728
0.14 × 0.06 × 0.05
3.292 to 60.622
−10 ≤ h ≤ 10,
−11 ≤ k ≤ 11, 0 ≤ l ≤ 15
3890
96.5
8.8195(15)
10.3836(18)
12.827(2)
83.215(2)
72.928(2)
83.662(2)
1111.5(3)
2
1.640
0.943
556
0.54 × 0.37 × 0.13
1.981 to 27.490
−10 ≤ h ≤ 11,
−13 ≤ k ≤ 13, 0 ≤ l ≤ 16
8195
8195
99.7
semiempirical from
equivalents
0.7456 and 0.6663
full-matrix least-squares on F2
semiempirical from
equivalents
0.461 and 0.146
full-matrix least-squares on
F2
3890/0/352
1.045
R1 = 0.0504, wR2 = 0.1308
R1 = 0.0639, wR2 = 0.1378
1.592 and −0.947
semiempirical from
equivalents
0.746 and 0.632
full-matrix least-squares on
F2
8195/31/300
1.045
R1 = 0.0230, wR2 = 0.0587
R1 = 0.0243, wR2 = 0.0596
0.630 and −0.468
reflections collected
independent reflections [R(int)]
completeness to
θ = 27.50/60.750/25.242/25.242°
absorption correction
semiempirical from
equivalents
max and min transmission
0.7456 and 0.6393
refinement method
full-matrix least-squares on F2
data/restraints/parameters
goodness-of-fit on F2
final R indices [I > 2σ(I)]
R indices (all data)
largest diff. peak and hole (e·Å−3)
5
C30H25Cl2N3ORu2S
748.63
110.15
0.71073
monoclinic
P121/c1
6397/0/352
1.059
R1 = 0.0334, wR2 = 0.0669
R1 = 0.0511, wR2 = 0.0740
0.960 and −0.512
6551/0/361
1.216
R1 = 0.0382, wR2 = 0.0905
R1 = 0.0411, wR2 = 0.0917
0.800 and −0.730
Table 3. Selected Bond Lengths (Å) and Angles (deg) for Complexes 1−3 and 7
Ru(1)−arene centroid
Ru(1)−Cl(1)
Ru(1)−N(1)
Ru(1)−N(3)/Ru(1)−O(1)/Ru(1)−O(1)/Ru(1)−O(1)
Ru(1)−C(19)/C(25)/C(28)/C(25)
Ru(1)−C(20)/C(20)/C(27)/C(20)
Ru(1)−C(21)/C(21)/C(26)/C(21)
Ru(1)−C(22)/C(22)/C(25)/C(22)
Ru(1)−C(23)
Ru(1)−C(24)
N(1)−Ru(1)−Cl(1)
N(3)−Ru(1)−Cl(1)/O(1)−Ru(1)−Cl(1)
N(3)−Ru(1)−N(1)/O(1)−Ru(1)−N(1)
1
2
3
7
1.654
2.4006(6)
2.1352(16)
2.1017(17)
2.1654(19)
2.197(2)
2.189(2)
2.179(2)
2.173(2)
2.151(2)
85.50(5)
85.39(5)
61.53(6)
1.666
2.4404(6)
2.081(2)
2.0719(16)
2.208(3)
2.174(3)
2.157(3)
2.165(2)
2.189(2)
2.154(3)
84.22(6)
84.85(5)
86.01(7)
1.655
2.4160(8)
2.081(2)
2.072(2)
2.188(3)
2.197(3)
2.183(3)
2.165(3)
2.170(3)
2.211(3)
84.22(6)
84.85(5)
86.01(7)
1.665
2.4180(6)
2.1009(17)
2.0986(15)
2.159(2)
2.197(2)
2.198(2)
2.189(2)
2.185(2)
2.173(2)
85.42(5)
85.76(4)
84.92(6)
hallmark of apoptosis.54 The mechanism of cell death was
analyzed using flow cytometry and by fluorescent staining
methods. Complexes 2 and 3, which showed efficient cytotoxic
activity, were used for the flow cytometric analysis. The flow
cytometry is one of the efficient and specific methods to
investigate the molecular and morphological events occurring
during cell death. The fractions of cell populations in different
quadrants were analyzed using quadrant statistics. The lower
left quadrant (R2), lower right quadrant (R3), upper right
quadrant (R4), and upper left quadrant (R5) contained the
living cells, early apoptosis cells, late apoptosis cells, and dead
cells, respectively. From this, we could infer that the complexes
induced apoptosis in accordance with their activity. The
population of living cells decreased, and that of the early and
late apoptotic cells increased after treatment with the
complexes in HepG2 cells. In the case of complex 2, 3.2% of
I
DOI: 10.1021/acs.organomet.8b00702
Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Table 4. Selected Bond Lengths (Å) and Angles (deg) for
Complexes 4−6
Ru(1)−arene centroid
Ru(2)−arene centroid
Ru(1)−Cl(1)
Ru(1)−N(2)/N(1)/N(1)
Ru(1)−O(1)
Ru(2)−Cl(2)
Ru(2)−N(1)/N(2)/N(2)
Ru(2)−N(3)
Ru(1)−C(19)/C(25)/C(19)
Ru(1)−C(20)/C(26)/C(20)
Ru(1)−C(21)/C(21)/C(21)
Ru(1)−C(22)
Ru(1)−C(23)
Ru(1)−C(24)
Ru(2)−C(25)/C(31)/C(25)
Ru(2)−C(26)/C(32)/C(26)
Ru(2)−C(27)
Ru(2)−C(28)
Ru(2)−C(29)
Ru(2)−C(30)
O(1)−Ru(1)−Cl(1)
O(1)−Ru(1)−N(2)/
N(1)/N(1)
N(2)/N(1)/
N(1)−Ru(1)−Cl(1)
N(1)/N(2)/
N(2)−Ru(2)−Cl(2)
N(3)−Ru(2)−Cl(2)
N(3)−Ru(2)−N(1)/
N(2)/N(2)
4
5
6
1.659
1.659
2.4388(9)
2.088(3)
2.083(2)
2.3916(9)
2.150(2)
2.068(2)
2.171(3)
2.162(3)
2.169(3)
2.164(3)
2.209(3)
2.163(3)
2.153(3)
2.213(3)
2.174(3)
2.177(4)
2.140(4)
2.151(4)
86.26(7)
85.00(9)
1.660
1.661
2.4410(11)
2.084(3)
2.075(3)
2.3984(10)
2.164(3)
2.064(3)
2.175(4)
2.214(4)
2.170(4)
2.183(5)
2.158(4)
2.174(4)
2.191(5)
2.210(5)
2.146(5)
2.170(5)
2.159(4)
2.180(4)
85.88(9)
84.17(12)
1.662
1.672
2.4218(18)
2.080(6)
2.056(6)
2.4055(19)
2.225(7)
2.073(7)
2.176(8)
2.168(8)
2.187(8)
2.201(8)
2.187(8)
2.188(8)
2.175(8)
2.203(10)
2.163(10)
2.148(9)
2.129(9)
2.169(9)
86.67(17)
85.0(2)
83.98(7)
84.53(10)
84.52(17)
86.11(7)
85.76(9)
83.73(18)
85.68(7)
61.66(10)
85.40(10)
61.61(13)
85.06(19)
60.1(3)
Figure 11. Stern−Volmer plot of the fluorescence titrations of the
complexes with BSA.
Figure 12. Scatchard plot of the fluorescence titrations of the
complexes with BSA.
Table 5. Protein Binding Constant (Kb), Quenching
Constant (Kq), and Number of Binding Sites (n) for
Complexes 1−7
Figure 10. Fluorescence quenching curves of BSA in the absence and
presence of 3. [BSA] = 1 μM and [complex] = 0−20 μM.
complex
Kb (M−1)
Kq (M−1)
n
1
2
3
4
5
6
7
3.54 × 10
5.06 × 104
2.72 × 105
1.32 × 105
1.51 × 105
2.06 × 104
1.20 × 104
8.07 × 10
8.71 × 104
1.66 × 105
1.31 × 105
1.93 × 105
6.94 × 104
5.33 × 104
0.92
0.96
1.05
0.98
1.02
0.89
0.86
4
4
treatment with the complexes. DAPI bound strongly to A-Trich regions in DNA and passed through an intact cell
membrane. FITC acted as a phosphatidyl serine tracer and
suggested the presence of apoptosis. PI could only penetrate
cells where the cell membrane had been compromised. The
results showed that significant morphological changes like
condensation and fragmentation were found after treatment of
the complexes with HepG2 and A549 cells, and apoptotic cells
were indicated with arrows in Figures 19 and 20.
early apoptosis and 58.5% late apoptosis were observed.
Complex 3 induced 15.4% early apoptosis and 69.5% late
apoptosis (Figure 18).
The mode of cell death was visualized using fluorescent
staining method. HepG2 and A549 cells treated with IC50
concentration of complexes 2 (HepG2, 9.33 μg/mL, and A549,
62.5 μg/mL) and 3 (HepG2, 10.89 μg/mL) were subjected to
confocal microscopic studies. In this study, 4′,6-diamidino-2phenylindole (DAPI, blue), fluorescein isothiocyanate (FITC,
green), and propidium iodide (PI) fluorescence (red) stains
were used to assess the morphological changes in the cell after
■
CONCLUSIONS
We have accomplished the synthesis of novel Ru(II)−arene
complexes containing guanidine ligands which exhibited
versatile coordination behavior to form monometallic (N,N/
J
DOI: 10.1021/acs.organomet.8b00702
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Organometallics
against HepG2 cell line; interestingly, 2 was more efficient than
cisplatin. The cell death mechanism of the active complexes 2
and 3 was found to be apoptosis as assessed by flow cytometry
and staining methods. The active complexes which showed
appreciable binding ability with biomolecules and possessed
remarkable cytotoxic activity might be a potential anticancer
drug.
■
Materials and Methods. Chemicals obtained from commercial
suppliers were used as received and were of analytical grade. The
melting points were determined on Lab India instrument and are
uncorrected. Elemental analyses were carried out using PerkinElmer
instrument. FT-IR spectra were obtained as KBr pellets using a
Nicolet-iS5 spectrophotometer. UV−visible spectra were recorded
using a Shimadzu-2600 spectrophotometer. Emission spectra were
measured on a Jasco V-630 spectrophotometer using 5% DMF in
buffer as the solvent. NMR spectra were recorded in CDCl3/DMSOd6 solvent using TMS as an internal standard on a Bruker 500/400
MHz spectrometer. ESI-MS spectra were recorded using a high
resolution Bruker maXis impact mass spectrometer.
Synthesis of the Ligands. The ligands L1−L4 were described
previously.24−26 The guanidine ligand (L5) was synthesized from Nfuroyl-N′-phenylthiourea by guanylation method.55 Thiourea (0.2462
g, 5 mmol) was mixed with aniline (0.456 mL, 5 mmol) in DMF and
triethylamine (1 mL, 10 mmol). The temperature was maintained
below 5 °C using an ice bath, and 1 equiv of mercuric chloride
(1.3576 g, 5 mmol) was added to the reaction mixture with vigorous
stirring. The ice bath was removed after 30 min, and the stirring was
continued overnight. The progress of the reaction was monitored
using TLC. After all the thiourea was consumed, 20 mL of chloroform
was added to the reaction mixture and the suspension was filtered
through a sintered glass funnel to remove the HgS residue. The
solvents were evaporated under reduced pressure, and the solid
residue was dissolved in 20 mL of CH2Cl2, then washed with water,
and the organic phase was dried over anhydrous Na2SO4. The residue
obtained after evaporation of the solvent was recrystallized from
ethanol to get the crystals of L5.
N,N′-Diphenyl-N″-furoylguanidine (L5). Yield: 63%, white solid;
mp 120 °C. Anal. Calcd for C18H15N3O2: C, 70.81; H, 4.95; N, 13.76.
Found: C, 70.65; H, 5.14; N, 13.60. ESI-MS (m/z): found 306.1262
(M + H)+; calcd for C18H15N3O2 305.1164. UV−vis (ethanol): λ, nm
(ε, dm3 mol−1 cm−1) 295 (20850), 286 (20400). FT-IR (KBr, cm−1):
3395, 3226 (N−H), 1590 (CO), 1558 (CN). 1H NMR (500
MHz, CDCl3): δ, ppm 10.06 (s, 1H), 8.39 (s, 1H), 7.74−7.69 (m,
2H), 7.63−7.53 (m, 1H), 7.42 (d, J = 8.1 Hz, 3H), 7.24 (d, J = 7.9
Hz, 2H), 7.17−7.01 (m, 4H), 6.50 (d, J = 12.4 Hz, 1H). 13C NMR
(125 MHz, CDCl3): δ, ppm 169.6 (CO), 156.4 (CN), 152.3,
145.9, 145.3, 137.2, 136.4, 129.8, 129.5, 128.9, 126.0, 124.1, 115.8,
111.6 (aromatic carbons).
Synthesis of the Ru−Arene Complexes. The ligand L1 (1
mmol) and NaOMe (0.054 g, 1 mmol) were dissolved in methanol
(20 mL) and stirred for 30 min at room temperature. Then [RuCl(μCl)(η6-p-cymene)]2 (0.6120 g, 1 mmol) was added, which turned the
reaction mixture into a red solution. After 6 h of reflux, the clear red
solution was evaporated to dryness, and the residue was dissolved in
chloroform, from which the precipitate was slowly formed, which was
filtered off and dried. Recrystallization of the crude product with
methanol yielded reddish orange crystals (1). For the synthesis of
complexes 2 and 3 from L2 and L3, respectively, the same procedure
was followed except the reaction mixture was stirred at reflux for 8−
10 h.
The ligands L1/L2/L4/L5 (1 mmol) and NaOMe (0.054 g, 1
mmol) were dissolved in methanol (20 mL) and stirred for 30 min at
room temperature. Addition of [RuCl(μ-Cl)(η6-benzene)]2 (0.5000
g, 1 mmol) to the above solution turned it red. After 6 h of stirring at
room temperature, an orange solid appeared. The solid was filtered
off, washed with methanol, and dried in vacuo. The solid was
Figure 13. Synchronous spectra of BSA (1 μM) as a function of
concentration of 3 (0−20 μM) with Δλ = 15 nm.
Figure 14. Synchronous spectra of BSA (1 μM) as a function of
concentration of 3 (0−20 μM) with Δλ = 60 nm.
Table 6. In Vitro Cytotoxic Activity of the Complexes in
HepG2, A549, and Vero Cell Lines
IC50 (μM)
complex
HepG2
A549
Vero
1
2
3
4
5
6
7
cisplatin
>250
15.41
17.74
>250
>250
>250
59.01
21.5
176.33
58.18
128.78
>250
>250
>250
143.85
18
>250
136.98
152.33
>250
>250
>250
>250
EXPERIMENTAL SECTION
N,O) Ru−p-cymene complexes and monometallic (O,N) and
bimetallic (N,N and N,O) Ru−benzene complexes. The
complexes were characterized by various spectroscopic and
X-ray crystallographic techniques. The binding efficacy of the
complexes to DNA has been investigated using spectroscopic
and hydrodynamic measurements. The intrinsic binding
constant revealed that the Ru−p-cymene complexes showed
higher binding ability than the Ru−benzene analogues. The
spectroscopic evidence also supported the binding of the
complexes to BSA. The in vitro cytotoxic studies revealed that
the complexes with the N,O bidentate ligand showed
appreciable activity than the N,N-type complexes against two
cancer cell lines. Complexes 2 and 3 were the most active
K
DOI: 10.1021/acs.organomet.8b00702
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Figure 15. Comparison of cytotoxic activity of complexes 1−7 against HepG2 cancer cells (24 h exposure). Data were calculated by mean ± SD of
three independent experiments with each experiment conducted in triplicate.
Figure 16. Comparison of cytotoxic activity of complexes 1−7 against A549 cancer cells (24 h exposure). Data were calculated by mean ± SD of
three independent experiments with each experiment conducted in triplicate.
p-cymene), 46.6 (aliphatic carbon in the ligand), 30.9, 21.9, 18.3
(aliphatic carbons in p-cymene).
[RuCl(η6-p-cymene)(L3-O,N)] (3). L3 (0.3434 g, 1 mmol) was
used. Yield: 76%, orange solid; mp 185 °C. Anal. Calcd for
C32H34ClN3ORu: C, 62.68; H, 5.59; N, 6.85. Found: C, 62.52; H,
5.77; N, 6.66. ESI-MS (m/z): found 577.1554 (M − Cl)+; calcd for
C32H34ClN3ORu 613.1434. UV−vis (ethanol): λ, nm (ε, dm3 mol−1
cm−1) 250 (31300), 319 (9200), 441 (1033). FT-IR (KBr, cm−1):
3419 (N−H), 1593 (CO), 1566 (CN). 1H NMR (500 MHz,
CDCl3): δ, ppm 8.20 (d, J = 4.3 Hz, 2H), 7.45 (d, J = 10.1 Hz, 3H),
7.37−7.17 (m, 4H), 7.14 (d, J = 5.0 Hz, 2H), 7.04 (t, J = 10.1 Hz,
3H), 5.30−3.89 (m, 7H, p-cymene phenyl-H, NH, CH2), 2.60−2.52
(m, 1H, p-cymene CH(CH3)2), 2.31 (s, 3H, p-cymene CH3), 2.15 (s,
3H), 1.14 (dd, J = 28.6, 4.0 Hz, 6H, p-cymene CH(CH3)2). 13C NMR
(125 MHz, CDCl3): δ, ppm 176.6 (CO), 159.4 (CN), 143.8,
137.3, 134.1, 130.2, 129.2, 129.0, 128.45, 127.61, 127.2, 124.4
(aromatic carbons), 99.9, 97.9, 81.0, 79.1 (aromatic carbons of pcymene), 46.7 (aliphatic carbon in the ligand), 31.0, 22.6 (aliphatic
carbons in p-cymene), 21.0 (aliphatic carbon in the ligand), 18.9
(aliphatic carbons in p-cymene).
[{Ru(η6-C6H6)Cl}2(L1-N,N,O,N)] (4). L1 (0.3213 g, 1 mmol) was
used. Yield: 76%, orange solid; mp 295 °C. Anal. Calcd for
C30H25Cl2N3ORu2S: C, 48.13; H, 3.37; N, 5.61; S, 4.28. Found: C,
48.32; H, 3.51; N, 5.46; S, 4.48. ESI-MS (m/z): found 500.0372 [M
− 2Cl − Ru(η6-C6H6)]+; calcd for C30H25Cl2N3ORu2S 748.9182.
UV−vis (DMF): λmax, nm (ε, dm3 mol−1 cm−1) 250 (11900), 269
(11757), 452 (1457). FT-IR (KBr, cm−1): 1592 (CO), 1543 (C
N). 1H NMR (400 MHz, CDCl3): δ, ppm 8.57 (dd, J = 3.7, 1.1 Hz,
1H), 7.46 (dd, J = 4.9, 1.1 Hz, 2H), 7.03 (dd, J = 4.9, 3.7 Hz, 2H),
6.82−6.77 (m, 6H), 6.66 (t, J = 7.3 Hz, 1H), 6.59 (t, J = 7.3 Hz, 1H),
5.11 (s, 6H, benzene), 4.95 (s, 6H, benzene). 13C NMR (100 MHz,
CDCl3): δ, ppm 166.1 (CO), 159.5 (CN), 157.7, 152.9, 152.0,
148.5, 144.5, 139.9, 132.0, 130.9, 130.0, 129.5, 127.4, 126.8, 126.5,
125.2, 123.6, 123.5, 123.0 (aromatic carbons), 83.7, 82.7 (aromatic
carbons of benzene).
[{Ru(η6-C6H6)Cl}2(L4-N,N,O,N)] (5). L4 (0.3153 g, 1 mmol) was
used. Yield: 70%, orange solid; mp 290 °C. Anal. Calcd for
C32H27Cl2N3ORu2: C, 51.75; H, 3.66; N, 5.66. Found: C, 51.91; H,
3.81; N, 5.83. ESI-MS (m/z): found 494.0798 [M − 2Cl − Ru(η6-
recrystallized in methanol/chloroform mixture to get red crystals (4−
7).
[RuCl(η6-p-cymene)(L1-N,N)] (1). L1 (0.3213 g, 1 mmol) was used.
Yield: 79%, orange solid; mp 185 °C. Anal. Calcd for
C28H28ClN3ORuS: C, 56.89; H, 4.77; N, 7.11; S, 5.42. Found: C,
56.73; H, 4.58; N, 7.29; S, 5.61. ESI-MS (m/z): found 556.0997 (M
− Cl)+; calcd for C28H28ClN3ORuS 591.0685. UV−vis (ethanol): λ,
nm (ε, dm3 mol−1 cm−1) 248 (32700), 292 (25200), 439 (1446). FTIR (KBr, cm−1): 3203 (N−H), 1610 (CO), 1559 (CN). 1H
NMR (500 MHz, CDCl3): δ, ppm 10.32 (s, 1H), 8.47 (d, J = 1.1 Hz,
1H), 7.53 (d, J = 4.7 Hz, 1H), 7.47−7.42 (m, 1H), 7.14 (t, J = 8.9 Hz,
1H), 6.96 (d, J = 3.4 Hz, 3H), 6.86 (t, J = 7.5 Hz, 3H), 6.74 (t, J = 7.3
Hz, 1H), 6.69 (d, J = 7.8 Hz, 2H), 5.09−5.00 (m, 4H, p-cymene
phenyl-H), 2.63−2.58 (m, 1H, p-cymene CH(CH3)2), 2.34 (s, 3H, pcymene CH3), 1.22 (d, J = 5.8 Hz, 6H, p-cymene CH(CH3)2). 13C
NMR (125 MHz, CDCl3): δ, ppm 170.3 (CO), 155.62 (CN),
145.2, 141.7, 135.4, 132.0, 130.2, 128.0, 127.9, 126.9, 124.0, 123.7,
122.8, 122.3 (aromatic carbons), 100.6, 98.5, 81.3, 80.8 (aromatic
carbons of p-cymene), 31.2, 22.4, 19.0 (aliphatic carbons in pcymene).
[RuCl(η6-p-cymene)(L2-O,N)] (2). L2 (0.3354 g, 1 mmol) was
used. Yield: 76%, orange solid; mp 174 °C. Anal. Calcd for
C29H30ClN3ORuS: C, 57.56; H, 5.00; N, 6.94; S, 5.30. Found: C,
57.71; H, 5.19; N, 6.79; S, 5.47. ESI-MS (m/z): found 570.1160 (M
− Cl)+; calcd for C29H30ClN3ORuS 605.0842. UV−vis (ethanol): λ,
nm (ε, dm3 mol−1 cm−1) 252 (18300), 289 (15700), 434 (1102). FTIR (KBr, cm−1): 3417 (N−H), 1571 (CO), 1532 (CN). 1H
NMR (500 MHz, CDCl3): δ, ppm 8.85 (t, J = 5.7 Hz, 1H), 8.43 (dd,
J = 3.6, 1.1 Hz, 1H), 7.73 (dd, J = 3.6, 1.1 Hz, 1H), 7.48 (dd, J = 4.9,
1.1 Hz, 1H), 7.43 (dd, J = 8.4, 7.5 Hz, 1H), 7.31 (dd, J = 5.0, 1.1 Hz,
1H), 7.24−7.22 (m, 2H), 7.18−7.16 (m, 2H), 7.12−7.08 (m, 1H),
7.02 (d, J = 6.6 Hz, 1H), 6.98 (dd, J = 5.0, 3.7 Hz, 1H), 5.14−4.93
(m, 7H, p-cymene phenyl-H, NH, CH2), 2.73−2.66 (m, 1H, pcymene CH(CH3)2), 2.21 (s, 3H, p-cymene CH3), 1.18 (d, J = 6.9
Hz, 6H, p-cymene CH(CH3)2). 13C NMR (125 MHz, CDCl3): δ,
ppm 175.7 (CO), 159.1 (CN), 155.4, 146.8, 142.2, 140.0, 137.1,
131.9, 130.1, 129.7, 128.4, 128.3, 127.3, 127.1, 127.1, 126.7, 125.8,
124.6 (aromatic carbons), 101.5, 97.9, 83.7, 80.4 (aromatic carbons of
L
DOI: 10.1021/acs.organomet.8b00702
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Organometallics
Figure 17. IC50 values of reported [RuCl(η6-arene)L]-type complexes (L is a bidentate ligand).
mol−1 cm−1) 244 (8400), 294 (5542), 422 (971). FT-IR (KBr): ν,
cm−1 3419 (N−H), 1561(CO), 1526 (CN). 1H NMR (500
MHz, DMSO-d6): δ, ppm δ 8.18 (d, J = 3.9 Hz, 1H), 7.60 (d, J = 12.7
Hz, 2H), 7.44−7.08 (m, 4H), 6.96−6.45 (m, 6H), 5.98 (s, 6H,
benzene), 5.28 (s, 1H), 4.83 (s, 2H). 13C NMR (125 MHz, DMSOd6): δ, ppm 164.2 (CO), 158.0 (CN), 155.0, 141.8, 131.1, 130.1,
128.8, 128.4, 127.9, 126.7, 121.5, 116.8 (aromatic carbons), 88.1
(aromatic carbon of benzene), 45.9 (aliphatic carbon in the ligand).
Solubility Measurements. The concentrations of saturated
solutions of the Ru(II) complexes were determined by UV−visible
studies by following a literature procedure.56
Stability Studies. The stability of complexes 1−7 in 1% DMSO/
water mixture was monitored over a period of 24 h by UV−visible
spectroscopy. The complexes which were stable in a 1% DMSO/water
mixture were monitored for their stability in biological medium. The
test was done by dissolving the complexes in minimum quantity of 1%
DMSO, and it was diluted with PBS buffer. The UV−visible spectra of
the resultant mixture were monitored over a period of 24 h.
X-ray Crystallography. A Bruker APEX2 [or GADDS (for 2 and
6)] X-ray (three-circle) diffractometer was employed for crystal
screening, unit cell determination, and data collection. Integrated
intensity information for each reflection was obtained by reduction of
the data frames with the program APEX2.57 The integration method
employed a three-dimensional profiling algorithm, and all data were
corrected for Lorentz and polarization factors, as well as for crystal
decay effects. Finally, the data were merged and scaled to produce a
suitable data set. The absorption correction program SADABS58 [or
C6H6)]+; calcd for C32H27Cl2N3ORu2 742.9618. UV−vis (DMF):
λmax, nm (ε, dm3 mol−1 cm−1) 243 (9785), 334 (3100), 454 (1128).
FT-IR (KBr, cm−1) 1594 (CO), 1552 (CN). 1H NMR (500
MHz, DMSO-d6): δ, ppm 8.18 (d, J = 3.9 Hz, 1H), 7.60 (d, J = 12.0
Hz, 2H), 7.43−7.21 (m, 4H), 7.00−6.45 (m, 8H), 5.98 (s, 12H,
benzene). 13C NMR (125 MHz, CDCl3): δ, ppm 176.6 (CO),
156.1 (CN), 144.9, 138.2, 135.2, 130.7, 129.0, 128.1, 128.0, 127.8,
124.4, 123.8, 122.7, 122.6 (aromatic carbons), 82.3 (aromatic carbons
of benzene).
[{Ru(η6-C6H6)Cl}2(L5-N,N,O,N)] (6). L5 (0.3053 g, 1 mmol) was
used. Yield: 66%, orange solid; mp 275 °C. Anal. Calcd for
C30H25Cl2N3O2Ru2: C, 49.18; H, 3.44; N, 5.74. Found: C, 49.02;
H, 3.63; N, 5.90. ESI-MS (m/z): Found 484.0583 [M − 2Cl −
Ru(η6-C6H6)]+; calcd for C30H25Cl2N3O2Ru2 732.9411. UV−vis
(DMF): λmax, nm (ε, dm3 mol−1 cm−1) 268 (18850), 332 (7975),
451 (1775). FT-IR (KBr, cm−1): 1579 (CO), 1544 (CN). 1H
NMR (400 MHz, DMSO-d6): δ 8.47 (d, J = 3.6 Hz, 1H), 8.09 (d, J =
5.0 Hz, 1H), 8.02 (d, J = 9.1 Hz, 1H), 7.98 (d, J = 9.1 Hz, 2H), 7.95
(d, J = 8.3 Hz, 2H), 7.63−7.57 (m, 3H), 7.32−7.28 (m, 3H), 5.98 (s,
12H, benzene). 13C NMR (100 MHz, DMSO-d6): δ, ppm 180.7
(CO), 162.2 (CN), 136.7, 135.3, 134.2, 133.7, 132.7, 128.7,
128.6, 128.3, 128.3, 127.3, 126.7, 126.3, 125.4, 124.6, 122.2 (aromatic
carbons), 87.6 (aromatic carbons of benzene).
[RuCl(η6-C6H6)(L2-N,O)] (7). L2 (0.3354 g, 1 mmol) was used.
Yield: 66%, reddish brown solid; mp 301 °C. Anal. Calcd for
C25H22ClN3ORuS: C, 54.69; H, 4.04; N, 7.65; S, 5.84. Found: C,
54.87; H, 4.24; N, 7.46; S, 5.99. UV−vis (DMF): λmax, nm (ε, dm3
M
DOI: 10.1021/acs.organomet.8b00702
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Article
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Figure 18. Apoptosis of HepG2 cells treated with IC50 concentrations (15.63 μg/mL) of 2 and 3 for 24 h. The percentage of apoptotic cells was
determined by flow cytometry. Values are the mean ± SD from at least three independent experiments. Significance was calculated by Tukey’s
multiple comparisons test (n = 3); *p < 0.05 indicates the significant differences from the control. The four areas in the diagrams represent four
different cell states: living cells (R2), early apoptotic cells (R3), late apoptotic cells (R4), and dead cells (R5).
Figure 19. DAPI (blue), FITC (green), and PI (red) fluorescence staining for the detection of apoptosis in HepG2 cells. Cells were treated with
complexes 2 and 3 at the IC50 concentration of 15.6 μg/mL. The fluorescent signals of DAPI, FITC, and PI were examined under a confocal laser
scanning microscope. Control: a1-DAPI, a2-FITC, a3-PI, and a4-merged. Treated: 2 b1-DAPI, b2-FITC, b3-PI, and b4-merged; 3 c1-DAPI, c2FITC, c3-PI, and c4-merged. Arrows indicate apoptotic cancer cells.
TWINABS58 (for 6 and 7)] was employed to correct the data for
absorption effects. Systematic reflection conditions and statistical tests
of the data suggested the space group. Solutions were obtained readily
using SHELXT (XT).59,60 Hydrogen atoms were placed in idealized
positions and were set riding on the respective parent atoms. All nonhydrogen atoms were refined with anisotropic thermal parameters.
N
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Figure 20. DAPI (blue), FITC (green), and PI (red) fluorescence staining for the detection of apoptosis in A549 cells. Cells were treated with
complex 2 at the IC50 concentration of 62.5 μg/mL. The fluorescent signals of DAPI, FITC, and PI were examined under a confocal laser scanning
microscope. Control: a1-DAPI, a2-FITC, a3-PI, and a4-merged. Treated: 2 b1-DAPI, b2-FITC, b3-PI, and b4-merged. Arrows indicate apoptotic
cancer cells.
The structures were refined (weighted least-squares refinement on F2)
to convergence.58−61 Olex2 was employed for the final data
presentation and structure plots.61
DNA Interaction Studies. The detailed experimental procedures
for DNA binding studies by UV−visible, fluorescence, and viscosity
measurements are given in the Supporting Information.
Protein Binding Studies. The binding of Ru−arene complexes
1−7 with BSA was studied using fluorescence spectra recorded at a
fixed excitation wavelength corresponding to BSA at 280 nm and
monitoring the emission at 335 nm. The excitation and emission slit
widths and scan rates were constantly maintained for all the
experiments. Stock solution of BSA was prepared in 50 mM
phosphate buffer (pH 7.2) and stored in the dark at 4 °C for further
use. Concentrated stock solutions of each test compound were
prepared by dissolving it in DMF−phosphate buffer (5:95) and
diluted with phosphate buffer to get required concentrations. Next,
2.5 mL of BSA solution was titrated by successive additions of the
complexes (2 μM). For synchronous fluorescence spectra measurements, the same concentration of BSA and the complexes were used
and the spectra were measured at two different Δλ (difference
between the excitation and emission wavelengths of BSA) values of 15
and 60 nm.
Cytotoxic Activity. Cytotoxic activity of the Ru(II)−arene
complexes was studied against HepG2, A549, and Vero cell lines.
The cells were maintained in Dulbecco’s modified Eagle’s medium
with 10% fetal bovine serum and 2 mM L-glutamine, along with
antibiotics (about 100 international unit/mL of penicillin, 100 μg/mL
of streptomycin) with the pH adjusted to 7.2. One hundred
microliters of medium containing 15000 cells/well and different
concentrations of the complexes were seeded in 96-well plates. The
complexes were dissolved in DMSO (10 mg/mL) to prepare stock
solution for cytotoxic studies. The cells were cultivated at 37 °C with
5% CO2 and 95% air in 100% relative humidity. After 24 h, an aliquot
of 100 mL of medium containing 1 mg/mL of 3-(4,5-dimethylthiazol2-yl)-2,5-diphenyltetrazolium bromide (MTT) was loaded into the
plate. The cells were cultured for 4 h, and then the solution in the
medium was removed. An aliquot of 100 μL of DMSO was added to
the plate which was shaken until the crystals were dissolved.62 The
activity against cancer cells was determined by measuring the
absorbance of the converted dye at 570 nm in an ELISA reader.
Cytotoxicity of each sample was expressed as IC50 value. The
percentage of growth inhibition was calculated using the following
formula:
mean OD of the untreated cells (control) − mean OD of the treated cells
× 100
mean OD of the untreated cells (control)
Apoptosis Determination by Flow Cytometry. Apoptosis was
analyzed using an Annexin V-FITC/PI detection kit (Biolegend, San
Diego). 63 HepG2 cells were harvested after treatment and
resuspended in binding buffer. Aliquots of 105 cells were mixed
with 5 μL each of annexin V-FITC and PI solution for 15 min at room
temperature in the dark. After incubation, 400 μL binding buffer was
added, and cells were analyzed by FACS Calibur flow cytometer
(Becton Dickinson).
Morphological Changes by Fluorescence Microscopy.
HepG2 cells in the absence and presence of complexes 2 and 3 at
IC50 concentration were used for confocal microscopic analysis.
Similarly, the morphological changes in A549 cells with and without
the addition of complex 2 (62.5 μg/mL) were viewed. After 24 h of
treatment, cells were washed twice with 0.01 M PBS and suspended in
binding buffer (ice-cold 1:1 methanol/acetone). Cells were incubated
with DAPI, FITC, and PI for 30 min at 4 °C in the dark. Cells were
then centrifuged, and pellets were smeared. DAPI, FITC, and PI
fluorescence were immediately observed under confocal laser
scanning microscope (ZEISS, LSM710, Germany).64a,b
■
ASSOCIATED CONTENT
S Supporting Information
*
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organomet.8b00702.
Figures depicting the graphs of stability/binding studies
and NMR spectra of L5 and all of the complexes (PDF)
Accession Codes
CCDC 1491584−1491591 contain the supplementary crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
O
DOI: 10.1021/acs.organomet.8b00702
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AUTHOR INFORMATION
Corresponding Author
*E-mail: kar@nitt.edu.
ORCID
Chandrasekar Balachandran: 0000-0002-0750-2316
Ramasamy Karvembu: 0000-0001-8966-8602
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
R.K. gratefully acknowledges SERB (EMR/2016/00321) for
the financial support. K.J. thanks the Department of Science
and Technology, Ministry of Science and Technology,
Government of India for doctoral fellowship under DSTINSPIRE programme. J.H. thanks the University Grants
Commission for the fellowship (F1-17.1/2012-13/RGNF2012-13-ST-AND-18716). Authors sincerely thank Prof. N.
Emi, Department of Hematology, Fujita Health University,
Japan for cytotoxicity studies.
■
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