1 Introduction Medicinal inorganic chemistry is a thriving interdisciplinary research area [1-4] which offers exciting possibilities for the design of novel metal-based therapeutic /diagnostic agents with unique mechanisms of action [5]. It is at the interface between medicine and inorganic chemistry, which includes metal-based drugs, metal sequestering or mobilizing agents, metal diagnostic aids and the medicinal recruitment of endogenous metal ions [6-8]. The design and synthesis of small metal complexes that bind and react at specific sequences of DNA become important as bioinorganic chemists begin to define on a molecular level, how genetic information is expressed. Understanding DNA targeting with specificity is therefore, potentially useful in developing design principles to guide the synthesis of improved chemotherapeutic agents, sensitive chemical probes of DNA structures in solution and tools for the molecular biologist to dissect genetic systems. Small DNA binding agents have attracted substantial interest in the field of chemotherapy against cancer [9-14]. The well recognized success of Barnett Rosenberg in 1960 with the discovery of cisplatin [cis-(PtCl 2 (NH 3 ) 2 )] as the first successful inorganic anticancer drug [15] opened new vistas in the frontier areas of research in chemical and biological sciences. The platinum based cisplatin and the second generation alternative derivatives viz. carboplatin, oxaliplatin, etc. are still the most widely used chemotherapeutic agents for treating solid malignancies like testicular, ovarian, bladder, lung, head and neck carcinomas (Figure 1) [16]. These compounds appear to act by forming adducts with DNA, thereby interfering with transcription and DNA replication to trigger apoptosis of the cell [17]. However, platinum based anticancer agents are non-specific resulting in significant toxicity (severe side effects including nephrotoxicity and gastrointestinal toxicity) and activity to act in restricted spectrum
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1
Introduction
Medicinal inorganic chemistry is a thriving interdisciplinary research area [1-4] which
offers exciting possibilities for the design of novel metal-based therapeutic /diagnostic
agents with unique mechanisms of action [5]. It is at the interface between medicine
and inorganic chemistry, which includes metal-based drugs, metal sequestering or
mobilizing agents, metal diagnostic aids and the medicinal recruitment of endogenous
metal ions [6-8]. The design and synthesis of small metal complexes that bind and
react at specific sequences of DNA become important as bioinorganic chemists begin
to define on a molecular level, how genetic information is expressed. Understanding
DNA targeting with specificity is therefore, potentially useful in developing design
principles to guide the synthesis of improved chemotherapeutic agents, sensitive
chemical probes of DNA structures in solution and tools for the molecular biologist to
dissect genetic systems. Small DNA binding agents have attracted substantial interest
in the field of chemotherapy against cancer [9-14].
The well recognized success of Barnett Rosenberg in 1960 with the discovery of
cisplatin [cis-(PtCl2(NH3)2)] as the first successful inorganic anticancer drug [15]
opened new vistas in the frontier areas of research in chemical and biological
sciences. The platinum based cisplatin and the second generation alternative
derivatives viz. carboplatin, oxaliplatin, etc. are still the most widely used
chemotherapeutic agents for treating solid malignancies like testicular, ovarian,
bladder, lung, head and neck carcinomas (Figure 1) [16]. These compounds appear to
act by forming adducts with DNA, thereby interfering with transcription and DNA
replication to trigger apoptosis of the cell [17]. However, platinum based anticancer
agents are non-specific resulting in significant toxicity (severe side effects including
nephrotoxicity and gastrointestinal toxicity) and activity to act in restricted spectrum
2
of tumors as well as inherent or acquired resistance. Hence in the field of
metallopharmaceutical research, there has been a continuous quest for the new metal-
based antitumor drugs exhibiting lesser toxic side effects and broader range of
antitumor activity, particularly, based on increased understanding of the biochemical
differences between normal and cancerous tissues.
These metal based compounds have been classified as ‘classical therapeutics and non-
classical therapeutics’. Classical therapeutics refer to the drugs that target DNA and
owe their anticancer activity to nonrepairable interaction with DNA and make use of
fast replication and mitotic processes of malignant cells [18,19]. Classical drugs based
on other metals can address the problems associated with platinum drug-toxicity and
their DNA binding mode of action has attracted increasing interest; however, non-
classical metal-based drugs are those that are able to target specific proteins or
enzymes. These drugs are capable to target cellular signaling pathways overexpressed
in cancer cells.
Figure 1. Platinum (II) complexes in worldwide clinical use, cisplatin (left), carboplatin (middle), oxaliplatin (right). Transition metals appear more appealing for this purpose due to their unique
properties, such as redox transfer, electron shuttling, thermodynamic and kinetic
stability and versatile coordination geometries arising from various oxidation states
that go beyond sp, sp2 and sp3 hybridizations of carbon [20-23]. Additionally, the
formation of a transition metal complex alters the solubility and lipophilicity of the
H3N
H3NH3N
O
Pt H3N
OCl
Cl
O
OPt
N
N
H2
H2O
O
O
OPt
3
drug, resulting in changes in pharmacokinetics, biodistribution and biotransformation
[24] and tunes the mode of binding and their reactivity towards biomolecules.
A vast number of ligands can be readily ‘plugged’ in and out of a wide variety of
metal centers. This unique property of metal complexes provides system that utilizes
strength of both synthetic organic chemistry and transition metal complexes.
Any metal ion or complex is subject to the potential limitations in the Bertrand
diagram, (Figure 2) which is usually used in discussing the essentiality of elements
[25].
Figure 2. Bertrand diagram indicating the relationship between benefit/detriment from an element and its concentration. Great variations are found in each region depending on the nature of the element.
The area of optimum physiological response will vary greatly according to the
element, its speciation, oxidation state and biochemistry of the specific compound in
which it is found. Therefore, the areas of deficiency, toxicity and optimum
physiological response can be dramatically varied by considering a combination of
these variables, as well as design features of the potential ligands which may be
altered to tune the delivery of that metal ion into the biological system [26]. Thus the
refinement of biological properties of metal complexes by well tailored,
multifunctional ligands offer exciting possibilities and can play an integral role in
4
muting the potential toxicity of the metallo-drug to have a positive impact in areas of
diagnosis and therapy.
Ligands can modify the reactivity, lipophilicity, oral/systematic bioavailability of
metal ions, stabilization of oxidation state, substitutional inertness depending on the
requirements for chemotherapy [27]. Recent advances in ligand design have resulted
in potent antitumor compounds that are active in cisplatin resistant cell lines, and also
include additional features to allow for an increased understanding of the mechanism
of action of the drug.
Amino acids have proven to play a significant role in the synthesis of novel drug
candidate with the use of non-proteinogenic, natural and unnatural amino acids. The
relevance of amino acids lies in their biological importance, not only as they form the
building blocks of peptides and proteins but also play essential roles in catalysis,
molecular recognition, information transfer and other biological functions [28,29].
Furthermore, the amino acids are very important compounds for the transfer inside the
cell of biologically active alkylating agents and especially first of them was glycine
[30]. Amino acids are involved in binding with target molecules and achieve higher
efficiency and specificity by the combination of interacting groups. The properties of
the side groups of aromatic amino acids in metal complexes are particularly
interesting, because they can be involved in interactions with central metal ion as well
as other aromatic rings. Various side groups of amino acids are found to have a
potential to recognize the specific base sequence through hydrogen bond formation
with nucleic acids in DNA [31,32]. High biological importance, chirality and
amphiphilicity combined with a low molecular weight and relative simplicity of
molecular structures make amino acids, the most suitable candidates for drug scaffold
representing typical features of natural bioactive substances [33-35].
5
The synthesis of artificial amino acids in particular, chiral phenyl glycines is of great
interest because of their use in medicinal chemistry [36]. The demand for D-para
hydroxy phenyl glycine and D-phenyl glycine have significantly increased in the area
of synthesis of new drugs such as aspoxicillin, cefbuperazone, β- lactams, etc. [37,38].
Although converting a molecule from one enantiomer to other seems like a small
change in the structure, it can provide paramount impact on the way the molecule
interacts with its surroundings, especially other chiral compounds. Enantiomeric
forms of drugs have different therapeutic or adverse effects and may cause them to be
metabolize in different ways [39]. The development and marketing of single
enantiomeric drugs has grown rapidly following the guidelines of new US FDA that
recommends the use of enantioselective identity and stability tests to determine the
contributions of individual enantiomers to pharmacological and toxicological
response [40]. The search for design of single enantiomeric drug candidate is a very
important task in medicinal chemistry. Amino acids exhibit their coordination
behavior towards metal ions through the amino carboxylato groups in a fixed
aromatic interactions and other non- covalent interactions (weak interactions)
involving the amino acid side chain groups encountered by these complexes can affect
their conformations, electron density, etc. and a wide variety of possibilities regarding
structures and functions of metal-amino acid complexes can be expected [41]. Amino
acid complexes of Cu (II) and Zn (II) are known to be important for metal ion
transport in blood. A ternary Cu (II) complex, Cu(his)(thr) (his = histidine, thr =
threonine) was isolated from human blood serum by Sarkar et al. [42] and tracer
studies indicated that ternary Cu (II)-amino acid complexes composed of histidine
6
(his), asparagine (asn) and glutamine (gln) are preferentially formed in blood plasma
[43].
Mixed ligand ternary metal complexes of amino acids and nucleic acid constituents
play an important role in biochemical processes, for example, genetics and molecular
biology [44], as nucleic acids and proteins recognize each other by very specific and
selective interactions through amino acid side chain and nucleic acid constituents
[45]. Mixed ligand ternary complexes of nucleic acid bases and nucleotides such as
thymine, cytidine, 2-thiouracil and amino acids viz. L-alanine, L-phenylalanine and
L-tryptophan were synthesized and characterized by various spectroscopic techniques
[46-49].
Gudasi et al. [50] have synthesized complexes of phenylglycine hydrazide with
transition metal ions Cu (II), Co (II) and Zn (II) (Figure 3 and 4). These complexes
Figure 3. Proposed structure for the ligand phenylglycine ester (pge) and phenyl glycine hydrazide (pgh).
Figure 4. Proposed structure for the complexes of phenylglycine hydrazide (pgh).
7
were thoroughly characterized on the basis of elemental analysis, magnetic moments
and spectral (IR, NMR and UV-visible) studies. Previously, Mylonas et al. [51] have
prepared Pt (II) and Pd (II) metal complexes of amino acid derivatives of D,L-
phenylglycine, D,L-phenylglycine methyl esters and D,L-phenylglycine-O-benzyl
ester. The microanalysis and spectroscopic data of the complexes supported the
unidentate-coordination of these amino acid derivatives being bound to the metal
atom through amino group only, further these complexes were tested for antitumor
activities.
Gao et al. in 2009 [52], studied the interaction of some new Pd (II) and Pt (II) (Figure
5) complexes of phenylglycine with FS-DNA (Fish-sperm DNA) adenosine 5’
triphosphate and adenine. These investigations were carried out by UV-visible
absorption spectra and fluorescence studies.
Figure 5. The crystal structure of complex [C20H28N2O6PtS2]. In the presence of complex [C20H28N2O6PtS2], the FS-DNA binding studies show
Figure 6. Absorbance spectra of DNA in the absence and presence of increasing amounts of [C20H28N2O6PtS2], DNA-12.25 X 10-3M. Arrows indicate the change in absorbance upon increasing concentration of the complex.
8
hypochromism and red shift as shown by the isosbestic points maintained until the
end of UV-titrations. The UV-visible absorption spectra of FS-DNA, in the absence
and presence of the complex [C20H28N2O6PtS2] are given in figure 6. The spectra
reveal two similar bands at 210 and 260 nm and with increasing complex
concentrations, the hypochromism increased upto 9.0% at 258 nm with isosbestic
points at 220 and 257 nm.
Copper-amino acid complexes are an important and interesting class of biologically
relevant molecules. Many low molecular weight copper complexes with amino acids
and their derivatives act as anti-inflammatory, anti-ulcers, anti-convulsant, anti-cancer
and radiation protection agents [53]. They can assume a variety of coordination
geometries from distorted square planar, flattened tetrahedral, distorted square
pyramidal to distorted octahedral as observed in their experimental crystal structures.
Copper (II) ions chelated with amino acids are capable of efficient DNA cleavage
activity by either oxidative or hydrolytic cleavage depending on the amino acid
involved [54]. A recent study carried out by Lin et al. [55] investigated the effect of
pH, concentration of both amino acid and hydrogen peroxide and the type of amino
acids (glycine, lysine, L-alanine) on the kinetics of OH· radical formation from the
reaction of copper (II)-amino acid mixtures with hydrogen peroxide.
Chiral amino alcohols are a promising class of chiral sources employed for the
synthesis of chiral coordination compounds [56-59]. Although chiral amino alcohol
complexes are practically known for their application in asymmetric catalysis [60-62],
however number of structurally characterized chiral amino acid based ligands is
scarce in literature.
First report of a structurally characterized mononuclear chiral Mn (IV) complex
derived from S-2-[2-hydroxy-1-phenylethylimino)methyl]phenol(S)-2-phenylglycinol
9
and salicyaldehyde appeared in research article by Zacharias et al. [63] and its
catalytic activity for the oxidation of olefin using iodosobenzene as an oxidant was
described. In another attempt, they prepared dichloro-bridged dinuclear copper (II)
complex with the same ligand [64] (Figure 7 and 8). However, DNA binding profile
of these complexes or their derivatives is still unraveled.
CHO
OH HO
NH2
HMeOH
OH
N
OH
Ph H
+R. T.
Figure 7. Synthesis of S-2-[2-hydroxy-1-phenylethylimino)methyl]phenol].
Figure 8. The ORTEP drawing of mononuclear chiral Mn (IV) and dichloro-bridged dinuclear copper derived from S-2-[2-hydroxy-1-phenylethylimino)methyl]phenol].
Das et al. [65] synthesized and structurally characterized two new dinuclear copper
(II) complexes of the formulation [Cu2(µ-Cl)2(HL1)2].C2H5OH and [Cu2(µ-
Cl)2(HL2)2].CH3OH (Figure 9 and 10) where HL1 and HL2 are derived from the chiral
amino alcohols (S)-(-)-2-amino-3-phenyl-1-propanol and (S)-(+)2- phenylglycinol.
10
Figure 9. Synthesis of the chiral ligand H2L1 and H2L2.
Figure 10. Thermal ellipsoid plot (20%) of complexes [Cu2(µ-Cl)2(HL1)2].C2H5OH and [Cu2(µ-Cl)2(HL2)2].CH3OH showing atom labeling scheme. The solvent molecule is omitted. The X-band EPR measurements for the complexes [Cu2(µ-Cl)2(HL1)2].C2H5OH and
[Cu2(µ-Cl)2(HL2)2].CH3OH were performed at liquid nitrogen temperature in
methanol solutions. The EPR spectra of both the complexes [Cu2(µ-
Cl)2(HL1)2].C2H5OH and [Cu2(µ-Cl)2(HL2)2].CH3OH were similar and typical of a
mononuclear square planar/square-pyramidal Cu (II) complex with a dx2-y2 ground-
state doublet as shown in figure 11. The data suggest that the bis(µ-halo)-bridged
structures [(HL)Cu(µ-Cl)2Cu(HL)] complexes, [Cu2(µ-Cl)2(HL1)2].C2H5OH and
[Cu2(µ-Cl)2(HL2)2].CH3OH rather getting dissociated in solution presumably due to
interaction with the solvent molecules. This observation was also consistent with the
fact that the chloride bridges in the dinuclear complexes were rather weak as shown
11
by the large Cu–Cl distances (2.8802 Å, 2.6872 Å, etc.) in their crystal structures.
Similar behavior has also been observed for some analogous chloro bridged dimeric
copper complexes [66,67]. The detailed EPR parameters and magnetic moments are
given in table 1.
Figure 11. X-band EPR spectra of complexes [Cu2(µ-Cl)2(HL1)2].C2H5OH and [Cu2(µ-Cl)2(HL2)2].CH3OH in methanol at liquid nitrogen temperature. Table 1. EPR data and magnetic moments for complexes [Cu2(µ-Cl)2(HL1)2].C2H5OH and [Cu2(µ-Cl)2(HL2)2].CH3OH
Deoxyribonucleic acid (DNA) is the primary cellular target molecule for most
anticancer therapies according to cell biology. Binding studies of small molecules
with DNA are very important in the development of new therapeutic reagents and
DNA molecular probes [68]. Nucleic acids under physiological conditions are
polyanions composed of heterocyclic bases linked to a sugar-phosphate backbone.
Thus, they are quite amenable to probe with positively charged transition metal ions.
DNA is a flexible, dynamic and polymorphic in structure that can assume a variety of
12
interconverting forms [69]. Each form has its own biological role in the regulation of
the life of cell. Selective stabilization of one of these forms can help to discover and
better understand their biological roles. Genetic information is provided by the DNA
in at least two different ways. First, the sequence of its nucleotide determines the
primary structure of the proteins. Second, DNA can regulate gene expression through
its shape [70].
Structurally DNA is characterized in A, B and Z forms [71]. Double stranded DNA
commonly adapts a right-handed helical conformation that of B- and A- form,
however, they differ in the conformation of sugar (C2’-endo for B-DNA and C3’-
endo for A-DNA, and in helical parameters). There are two well defined right handed
grooves, termed as the major and minor grooves and each has characteristic width and
depth which together result into the distinctive shape associated with helical form
[72]. The major groove is a shallow, almost convex surface and the minor groove is a
narrow crevice, zigzagging in a left handed fashion along the side of the major groove
(Figure 12).
Figure 12. Depiction of major and minor grooves within B-DNA as viewed from top to bottom of the duplex. Grooves are defined with respect to the glycosyl linkage of each base to its respective deoxyribose. Coordination compounds offer many binding modes to DNA, including outer sphere
non-covalent binding, metal coordination to nucleobases and phosphate backbone
13
sites, as well as strand cleavage induced by oxidation using redox-active metal centers
(Figure 13). In covalent binding the labile ligand of the complexes is replaced by a
nitrogenous base of DNA such as guanine N7. On the other hand, the non-covalent
DNA interactions include intercalation, electrostatic interaction, hydrogen bonding
and groove (surface) binding of cationic metal complexes along outside of DNA
helix, along major or minor groove. Intercalation involves the partial insertion of
aromatic heterocyclic rings of ligands between the DNA base pairs.
Regarding the mechanistic features of the therapeutic action of the drug, one of the
important process is DNA condensation which is the essential precondition to
transport a therapeutic gene to its target position [73]; Schlepping with negative
charges and grooves on its main chain [74-77], DNA always interact with substrates
possessing positive charge which are implanted to both major and minor grooves of
DNA. Metal complexes with cationic character have a natural aptitude to overcome
the repulsive forces of negatively charged DNA segments by charge neutralization to
form densely packed DNA condensates (figure14) [78]. Packing of DNA into a
condensed structure involves overcoming the coulombic barrier related to the
negatively charged phosphates on the DNA. Other energetic barriers arise from the
14
loss in configurational entropy when organizing the extended DNA molecule into
well-defined structures and the bending of the stiff double helix. Binding of a
multivalent cationic ligand to DNA is an exchange reaction where counter ions are
released both from the DNA and the ligand, causing an increase in the overall entropy
[79,80].
Figure 14. Schematic image of formation of DNA-metal condensates.
Meng et al. [81] has reported the first dinuclear metal complexes of the formulation,
[Cu2(dtpb)Cl2]Cl2 and [Cu2(dtpb)Cl2(H2O)](NO3)2 (Figure15) derived from
benzimidazole ligand that can induce the metal-DNA condensation, however,
condensation of mononuclear cobalt (II) complex was earlier reported [82].
Figure 15. ORTEP view of the dinuclear Cu (II) complexes [Cu2(dtpb)Cl2]Cl2 and [Cu2(dtpb)Cl2(H2O)](NO3)2). For clarity, solvent molecules, hydrogen atoms and counter anions are omitted.
The two dinuclear Cu (II) complexes [Cu2(dtpb)Cl2]Cl2 and
[Cu2(dtpb)Cl2(H2O)](NO3)2 induced DNA condensation and were examined by TEM.
The TEM images showed that the dinuclear complexes first led to a collapse of the
Complex metal ion
DNA-metal complex
15
extended linear or supercoiled DNA into a loose aggregate (Figure 16a) and then
further into a compactly globular inclusion with the incubation time at pH 7.4 (Figure
16b). The DNA molecules are arranged in a tree growth ring like fashion in the
inclusions, indicating the formation of the compact inclusions by the entanglement of
DNA around a core. The double-stranded DNA has been clearly observable in this
stage. As the incubation time was prolonged, the nanoparticles could further assemble
into an amorphous state (Figure 16c) and a regularly ellipsoid-like structure (Figure
16d) in which the DNA is invisible. These large and dense structures are on the
micrometer-scale in size. The dose-dependent DNA condensation was also examined
and was similar to the time-dependent process.
Figure 16. Visualization of DNA condensers under TEM. The DNA nanoparticles were formed by incubation of 5.6 µM λDNA with 2 (10 µM) for 30 (a, b), 60 (c), and 120 min (d) in 20 mM Tris-HCl buffer (pH 7.4).
16
The condensed DNA was not well-consistent with the reported rod like and toroidal
DNA condensates in morphology and exhibited a new morphology of DNA
condensates. The TEM images, together with the size measurements performed in
solutions, indicated that DNA is condensed into nanometer-to micrometer-scale
particles.
Thomas and Bloomfield [83] explained the distinct condensate morphologies with a
complex dependence on the concentration of condensing agents. Arscott et al. [84]
monitored condensation induced by cobalt (III) hexammine [Co(NH3)6]3+ under
different conditions that favor the formation of A-DNA in random sequences, and
observed a gradual loss of regular condensate morphology.
Sun et al. [85] studied the atomic force microscopy (AFM) studies in an aqueous
solution for two ruthenium (II) complexes [Ru(bpy)2(PIPSH)]2+ and
[Ru(bpy)2(PIPNH)]2+, (50 mM Tris-HCl, 18 mM NaCl, pH 7.2) to gain detailed
structural information about the condensates, on an unmodified mica surface. Figure
17 shows typical AFM images of supercoiled pBR322 DNA in the absence and
presence of ruthenium (II) complexes. Without ruthenium (II) complexes, the free
DNA existed as loose clews or relaxed circles, with little twisting of the strands
(Figure 17a). This structure was characteristic of uncondensed DNA morphology.
Upon interaction with [Ru(bpy)2(PIPSH)]2+ (40 µM), DNA was induced to form small
nanoparticles with an average diameter of ca. 109 nm (Figure 17b) with the increase
in concentration of [Ru(bpy)2(PIPSH)]2+ (80 µM), larger nanoparticles with about 224
nm diameter were obtained (Figure 17c). Similar DNA condensation behaviors in the
presence of complex [Ru(bpy)2(PIPNH)]2+, were also observed (about 95 nm
diameter in figure 17d. This phenomenon clearly demonstrated the good DNA
condensation ability of complexes [Ru(bpy)2(PIPSH)]2+ and [Ru(bpy)2(PIPNH)]2+.
17
Figure 17. Tapping mode AFM height topographs of uncomplexed pBR322 (A) and linear DNA (C) alongside with complexes of these formed when mixed with the chitosan C (0.01, 162) (B and D, respectively). [DNA] 4 µg/mL and k1. The driving force of the DNA condensation induced by two ruthenium (II) complexes
was not only due to the electrostatic interactions between the divalent cations and the
negatively charged phosphates in DNA but also the high DNA binding affinities of
complexes as verified by their spectroscopic studies.
Moreno et al. [86] have also suggested that counter ion-induced DNA condensation is
a well-known electrostatic phenomenon attributed to the neutralization (~90%) of the
negatively charged phosphate backbones. The DNA base pairs to complex ratio used
for the linear DNA fragments and multivalent [Co(ambi)3]3+ (1:1) enables one to
compensate more than 90% of the negative charge for condensing. However,
decreased ionic strength caused by transient to afford unipositive does not lead to
condensation of the DNA structure [76,87].
The numerous side effects of cisplatin as a chemotherapeutic agent leave room for the
selection of other metals for the synthesis of bioactive molecules. Among these, Cu
18
(II) ion is especially attractive due to its occurrence in biological systems and
participation as an integral part of the active site of different types of metalloproteins,
which recognize its coordination with the human body functions. The exploration of
copper complexes as chemical nucleases is well documented [88-94] because they
possess biological accessible redox potential and relatively high nucleobases affinity
[95,96].
Seng et al. [97] evaluated the effect of methyl substituent on DNA binding and
nucleatic activity of Cu (II) complexes of glycine and methylated glycine derivatives
Scheme 1 . Structure of amino acids, (aa).
Cu(aa)2 consisting of C-dimethylglycine, L-alanine and sarcosine as given in scheme
1. These complexes were investigated by means of EPR, UV-visible spectroscopy and
gel electrophoresis. The copper (II) complexes of the C-methylated glycine
derivatives, i.e. Cu(C-dmg)2 and Cu(L-ala)2 (Figure 18, lanes 11 and 14, respectively)
were better nucleolytic agents as they were able to convert all the supercoiled DNA to
nicked and linear DNA. However, the copper (II) complexes of the N-methylated
glycine derivatives, i.e. Cu(N-dmg)2 and Cu(sar)2, were the least efficient as some
supercoiled DNA still remained uncleaved (Figure 18, lanes 12 and 15, respectively).
The order of nucleolytic efficiency was Cu(C-dmg)2 > Cu(L-ala)2, cis-Cu(gly)2 >
Cu(N-dmg)2, Cu(sar)2. Interestingly, the CuCl2(aq) could only nick some supercoiled
pBR322 (Figure 18, lane 5) while the Cu(aa)2 complexes can induce a greater amount
19
of DNA cleavage. This suggests chelation of each of the investigated amino acid to
the Cu (II) ion enhances its nucleolytic efficiency. This is unlike the tetradentate
amino acid, N,N-ethylenediaminediacetic acid, which upon chelation to copper (II)
ion diminished its nucleolytic efficiency [98]. This suggests bidentate amino acid
chelated to copper (II) ion can cause more damage to DNA than Cu (II) ion in the
borate buffer solution.
The cause of DNA cleavage by Cu (II) complexes in the presence of hydrogen
peroxide has been attributed to the formation of hydroxyl radicals [99,100]. Nakajima
et al. previously proposed that the hydroxyl radical was produced by a two step
reactions in a copper (II)-hydrogen peroxide system [101].
(i) Cu (II) + H2O2 → Cu (I) +HOO· + H+
(ii) Cu (I) + H2O2→ Cu (II) + OH− + OH·
The H2O2 has a dual function, i.e. acting initially as reducing agent and then as
oxidizing agent. The difference in production of OH· radicals by the Cu(aa)2
Figure 18. Electrophoresis results of incubating pBR322 (0.5 µg/lL) with 500µM of each Cu(aa)2 complexes at 37 °C in borate buffer pH 8.5 in an oven incubator for 2h in the absence and presence of 10 µM H2O2. Lanes 1 are gene ruler 1 Kb DNA ladder; lane 2, DNA alone; lane 3, DNA + 10 µM H2O2; lane 4, DNA + 500µM CuCl2; lane 5, DNA + 500 µM CuCl2 +10 µM H2O2. Lanes 6, DNA + Cu(aa)2 alone: lane 7, DNA + Cu(N-dmg)2; lane 8, DNA + cis-Cu(gly)2; lane 9, DNA + Cu(L-ala)2; lane 10, DNA + Cu(sar)2. Lanes 11-15 DNA + Cu(aa)2 +10 µMH2O2: 11, Cu(C-dmg)2; 12, Cu(N-dmg)2; 13, cis-Cu(gly)2; 14, Cu(L-ala)2; 15, Cu(sar)2. complexes may be the result of a number of factors, such as steric hindrance to
binding of H2O2 or −OOH due to methyl groups at the amino nitrogen, electronic
20
effect of electron donating methyl substituent, and difference in redox property of the
Cu(aa)2.
The ROS scavenging experiment for the reaction of DNA with Cu(aa)2 in the
presence of sodium ascorbate was carried out to determine the active species
responsible for DNA cleavage.
Recently, Alzuet et al. in 2009 [102], reported Cu (II) complex of ligand [Cu(N9-
ABS)(phen)2].3.6 H2O, H2N9-ABS = N-(9H-purin-6-yl)benzenesulfonamide and
phen = 1,10-phenanthroline, and the complex was thoroughly characterized by using
spectroscopic techniques and X-ray crystallography (Figure 19). The geometry of Cu
(II) was found to be distorted square pyramidal with the equatorial positions occupied
by three N atoms from two phenanthroline molecules and one N atom from the
adenine ring of the sulfonamide ligand.
Figure 19. ORTEP drawing of [Cu(N9-ABS)(phen)2].3.6 H2O.
Fluorescence spectroscopic studies were performed to confirm the mode of DNA
interaction. These studies were carried out with ethidium bromide (EB). EthBr a
planar aromatic heterocyclic dye, with strong affinity for double stranded DNA is a
fluorescence probe with high sensitivity and selectivity. The fluorescence of this
21
compound is very weak itself and the enhancement of fluorescence occurs when
special intercalation with DNA takes place [103,104].
EB (weak fluorescent) + DNA (non-fluorescent) → EB-DNA (strong fluorescent)
Addition of increasing concentrations (10, 20, 30, 40, and 50 µM) of the compound to
DNA previously treated with EB caused a reduction in emission intensity of ca. 25%,
Figure 20. Emission spectra of EB bound to DNA in the absence and presence of 10, 20, 30, 40, 50 µM [Cu(N9-ABS)(phen)2].3.6 H2O. The arrow shows the change in intensity at increasing concentrations of the complex. The inset is the Stern-Volmer plot.
indicating that [Cu(N9-ABS)(phen)2].3.6 H2O binds to DNA (Figure 20) and
efficiently competes with EB for intercalative binding sites. The complex [Cu(N9-
ABS)(phen)2].3.6 H2O was found to cleave DNA in a concentration-dependent
fashion as shown in figure 21. The complex was a potent chemical nuclease at low
concentrations in presence low concentration of activators. Indeed, at 3 µM, the
compound was able to mediate the complete conversion of supercoiled DNA to its
open circular and linear forms (lane 8). At 6 and 9 µM, the compound induced
degradation of the supercoiled form to produce the open circular and linear forms and
small linear fragments (lanes 9 and 10). At 12 µM, the plasmid was fully converted
into small linear fragments, as indicated by a smear on the gel (lane 11).
22
Figure 21. Agarose gel electrophoresis of pUC18 plasmid DNA treated with CuSO4, [Cu(N9-ABS)(phen)2].3.6 H2O or the bis(o-phenantroline) copper (II) and 2.5-fold-excess of ascorbate. Incubation time 60 min (37 °C). 1. kDNA/EcoR1 + Hind III Marker; 2. pUC18 control + ascorbate 30 µM; 3. Lineal pUC18; 4. CuSO4 3 µM; 5. CuSO4 6 µM; 6. CuSO4 9 µM; 7. CuSO4 12 µM; 8. complex 3 µM; 9. complex 6 µM; 10. complex 9 µM; 11. complex 12 µM; 12. [Cu(phen)2]2+ 3µM; 13. [Cu(phen)2]2+ 6µM; 14. [Cu(phen)2] 2+ 9µM; 15. [Cu(phen)2]2+ 12 µM; 16. [Cu(phen)2]2+ 24 µM. On the basis of the above cleavage activity, the following pathway or sequential event
for the DNA cleavage performed by the complex [Cu(N9-ABS)(phen)2].3.6 H2O in
the presence of ascorbate and other oxidizing and reducing agents was suggested that
the mechanism of the DNA cleavage could be explained on the basis of the ROS
species [105-108].
Pivetta et al. [109], synthesized the Cu (II) complexes of the formulation
[Cu(phen)2(L)](ClO4)2 , where phen is 1,10-ortho-phenanthroline and L is a series of
substituted imidazolidine-2-thione (Figure 22). The complexes have been
characterized by single crystal X-ray diffraction which revealed distorted trigonal
bipyramidal geometry for all the molecules.
23
Figure 22. Ortep views of the [Cu(phen)2(L)](ClO4)2 derived complexes.
The marked antiproliferative effects shown by these complexes were in agreement
with the cytotoxic activity reported [110] for phenanthroline derivatives complexed
with copper (II) metal ion, although mechanistic experiments are needed to identify
their mode of action, the cytotoxicity found in the N2a cell line might be due to
oxidatively induced effects on DNA. Indeed, oxidative damages have been described
in cells treated with phenanthroline copper (II) complexes, in which the increment of
reactive oxygen species (ROS) was associated with DNA degradation, DNA
oxidation, depletion of reduced-glutathione and/or cell death by apoptotic and non-
apoptotic dose-dependent mechanisms [111]. Moreover, copper complexes may
24
intensify the redox imbalance by activation of H2O2, leading to cell damage and,
consequently, to cell death by apoptosis or other mechanisms.
Transition metal ions have been employed to drive efficient artificial metallonucleases
which act through an oxidative pathway, for example, Cu (II) [112-118], or through a
hydrolytic mechanism, as in the case of Co (III) [119], Fe (III) [120]. Transition metal
complexes containing Zn (II) are of particular interest, as Zn (II) based artificial
nucleases would be highly valuable as Zn (II) is a good Lewis acid, redox inert,
exchanges ligands rapidly, is non-toxic and does not show ligand field stabilization
energy and, as a consequence it can easily adopt its coordination geometry to best fit
structural requirement of a reaction [121]. Besides this, Zn complexes form the most
propitious forms of Zn-metalloelement for the delivery to required cellular sites
enabling Zn-dependent enzyme syntheses and facilitation of Zn-dependent
biochemical processes [122]. Following the pioneering work of Zn-based artificial
nuclease by J.K. Barton and coworkers [123], many Zn complexes are known to be
synthetic hydrolases towards the hydrolytic cleavage of phosphate diesters [124-126].
Moreover, it has been shown that di- and trinuclear complexes generally display
higher hydrolytic activity, due to the cooperative role played by the metals in the
cleavage process [127].
Li J.H. et al. [128], reported the artificial nuclease activity of Zn (II) complexes
[Zn(L1)3](ClO4)8.3H2O and [Zn(L2)3](ClO4)8 with disubstituted 2,2-bipyridine with
ammonium groups (Figure 23), where (L1= [4,4-(Me2NHCH2)2-bpy]2+, L2= [5,5-
(Me2NHCH2)2-bpy]2+, bpy = 2,2-bipyridiyl). DNA binding studies of the complexes
[Zn(L1)3](ClO4)8.3H2O and [Zn(L2)3](ClO4)8 was studied by UV-visible and CD
studies.
25
Figure 23. Synthetic scheme for Zn (II) complexes.
The complexes [Zn(L1)3](ClO4)8.3H2O and [Zn(L2)3](ClO4)8 exhibited the
hypochromism on addition of DNA. Hypochromism results from the intercalation as a
Figure 24. Absorption spectra of complexes [Zn(L1)3](ClO4)8.3H2O and [Zn(L2)3](ClO4)8 (3.0 X 10-5 M) in the absence (dot line) and presence (solid line) of increasing amounts of CT-DNA (0-1.25 X 10-4 M) in 5 mM Tris-50 mM NaCl buffer (pH 7.5) at 25 ± 0.1 °C.
result of stacking effect of π electrons [129], which leads to the decrease of transition
probability of π electrons, and ultimately results in the decrease of absorption as
shown in figure 24.
The hydrolytic cleavage of plasmid DNA (pBR322) catalyzed by these complexes
was studied by agarose gel electrophoresis. In order to investigate the effect of pH
conditions, DNA cleavages were first performed at different pH values (pH 5.5-9.5).
Figure 25 shows the cleavage of DNA with 200 µM complexes
26
[Zn(L1)3](ClO4)8.3H2O and [Zn(L2)3](ClO4)8 at 37 °C in different pH conditions (20
mM MES, HEPES, TAPS or CHES according to pH) for defined incubation time (48
h and 36 h for [Zn(L1)3](ClO4)8.3H2O and [Zn(L2)3](ClO4)8, respectively). It can be
seen that DNA was converted from supercoiled Form (SC, Form I) to nicked Form
(NC, Form II).
Figure 25. The pH-dependence of the cleavage of pBR322 DNA (38 µM bp) by 200 µM of [Zn(L1)3](ClO4)8.3H2O and [Zn(L2)3](ClO4)8 in 20 mM buffer (MES, HEPES, TAPS or CHES according to pH) at 37 °C. Incubation time for [Zn(L1)3](ClO4)8.3H2O and [Zn(L2)3](ClO4)8 were 48 and 36 h, respectively. Lane C: DNA control. When the concentration of complexes [Zn(L1)3](ClO4)8.3H2O and [Zn(L2)3](ClO4)8 in
20 mM HEPES (pH 7.5) at 37 °C for 36h, DNA was converted from SC form to NC
Figure 26. Complex concentration-dependence of the cleavage of pBR322 DNA (38 µM bp) by [Zn(L1)3](ClO4)8.3H2O and [Zn(L2)3](ClO4)8 at the indicated concentrations in 20 mM HEPES buffer (pH 7.5) after incubation for 36 h at 37 °C.
Form. With the increase of complex concentration, the percent of NC DNA increased.
When the concentration of complex [Zn(L2)3](ClO4)8 increased to 300 µM, most of
the SC DNA (>90%) was converted to NC DNA (Figure 26).
Recently, Jiang et al. [130,131] synthesized four transition metal complexes Co (II),
Ni (II), Cu (II) and Zn (II) of the ligand 2-((2-((benzo[d]oxazol-2-yl)methoxy)
27
phenoxy)methyl)benzoxazole, and were characterized by single X-ray crystallography
(Figure 27).
Figure 27. ORTEP representation of the complexes showing the coordination environment of M (II) ions, Co (II),Ni (II), Cu (II) and Zn (II) with thermal ellipsoids at 30% probability level. Comparative study of the interactions of the ligand and the complexes with CT DNA
was studied by means of UV-visible, fluorescence, and circular dichroism. The
complexes were tested against four different (A549, HepG2, K562, K562/ADM)
cancer cell lines.
Upon the addition of CT DNA, notable hypochromic effect was observed. The
absorption bands of the ligand and the complexes (Co-L, Ni-L, Cu-L, Zn-L) at about
270 nm exhibited hypochromism of about 43.05 %, 53.22 %, 54.89 %, 62.26 %,
49.10 %, respectively; while the absorption bands at ca. 234 nm exhibited
hypochromism of about 34.59 %, 64.46 %, 65.16 %, 63.30 %, 61.84 %, respectively
(Figure 28).
28
Figure 28. Electronic spectra of the ligand (a), Co-L (b), Ni-L (c), Cu-L (d), Zn-L (e) upon addition of CT DNA. [Compound] = 10 µM, [DNA] = 0-10 µM. Arrows show the absorbance changes upon increasing DNA concentration.
Upon excitation at π-π* transitions either in CH3CN or in the presence of CT DNA,
Cu-L and the metal complexes cannot emit strong luminescence. Therefore, steady
state competitive binding studies of the compounds were monitored by a fluorescent
EB displacement assay, which could provide rich information regarding DNA-binding
nature and relative DNA-binding affinity.
The emission spectra of EB bound to CT DNA in the absence and presence of the
compounds with different concentrations are given in figure 29. With the addition of
29
Figure 29. Fluorescence spectra of EB bound to CT DNA in the presence of (a) ligand L, (b )Co-L, (c) Ni-L, (d) Cu-L, (e) Zn-L with different concentrations.
the samples into DNA, pretreated with EB, an appreciable decrease in the emission
intensity (λ = 594 nm) and an isosbestic point at 540-560 nm were observed. These
changes showed that all the complexes could replace EB from the DNA-EB system,
and a complex-DNA system was formed. The decreased emission of the DNA-EB
system was caused by EB being expelled from the hydrophobic environment into the
water solution [132]. To compare binding affinities of these samples to DNA
quantitatively, the apparent binding constants were calculated. The binding affinity
followed the order Cu-L > Ni-L > Co-L > Zn-L > L. The binding affinity of the
complexes may be attributed to the chelation and the planar structure of the ligand.
30
CD spectra of CT DNA in presence and in the absence of the complexes are shown in
figure 30.
Figure 30. CD spectra of CT DNA (1 X 10-4M) in the absence and presence of the compounds (0.5 X 10-4M).
Extensive evaluation of the design of medicinal organometallics has taken place
during the past few years, with the aid of structure activity relationship allowing the
design of novel, non-conventional platinum compounds as well as innovative non-
platinum metal-based antitumor agents. The structure activity relationships (SARs)
led to the discovery of novel anticancer drugs containing titanium [133], ruthenium
[134], rhodium [135], copper [136] and gold [137] complexes which are reported to
have promising chemotherapeutic potentials and different mechanisms of action than
those of platinum based drugs. Most of the metal-based chemotherapeutics agents are
DNA targeted and metal complexes covalently attach the metal atom to DNA
resulting in various metal-DNA adducts. The formation of metal DNA adducts
inhibits DNA replication and transcription of various important genes and leads to cell
death.
31
The antitumor potential of tin complexes have been established since 1929 [138-143]
and among them organotin (IV) complexes were regarded as effective candidates in
organometallic oncology due to their novel apoptosis inducing property. Promising
success of different organotin derivatives which has shown acceptable in vitro and in
vivo antiproliferative activity were developed as new lead chemotherapeutic agents
[144,145]. Recently, Li M.X. et al. (Figure 31) has developed a new series of
semithiocarbazone and its organotin (IV) complexes exhibiting the anticancer activity
[146] against selected bacteria and K562 leukaemia cells.
Figure 31. The reaction schemes for the synthesis of the complexes [(Me)2Sn(L1)(CH3COO)].CH3CH2OH, [(Ph)2Sn(L1)(CH3COO)].CH3CH2OH, [(Me)2Sn(L2)Cl] and [(Ph)2Sn(L2)(CH3COO)], the solvent molecules are omitted for clarity. The complexes [(Me)2Sn(L1)(CH3COO)].CH3CH2OH, [(Ph)2Sn(L1)(CH3COO)].
CH3H2OH, [(Me)2Sn(L2)Cl] and [(Ph)2Sn(L2)(CH3COO)] where HL1 = 2-
benzoylpyridine N(4)-phenylthio-semicarbazone and HL2 = 2-acetylpyrazine N(4)-
phenylthiosemicarbazone, to inhibit tumor cell growth against K562 leukaemia cells.
The comparison of cytoxicity of the two free ligands and their four diorganotin (IV)
32
complexes indicates that HL1shows much lower IC50 value (1.43 mM) [17g] than HL2
(12.3 mM), indicating importance of the substituent group of the parent ketone in
thiosemicarbazones on antitumor activities [147,148]. Complexes
[(Ph)2Sn(L1)(CH3COO)].CH3CH2OH and [(Ph)2Sn(L2)(CH3COO)] show higher
antitumor activities than complexes [(Me)2Sn(L1)(CH3COO)].CH3CH2OH and
[(Me)2Sn(L2)Cl], respectively, indicating that phenyl diorganotin complex of the same
ligand shows enhanced antitumor activity than that of their corresponding methyl
diorganotin derivatives [149]. It is worth noting that the remarkable antitumor activity
is observed for complex [(Ph)2Sn(L2)(CH3COO)] with 2-benzoyl-pyridine N-(4)-
phenyl thiosemicarbazone and (Ph)2Sn(IV) group with the lowest IC50 value 5.8 mM
among the studied four organotin (IV) complexes. In addition, it should be
emphasized that despite the fact that the complexes in this study resulted in lower
antitumor activity than their respective ligands, in general their antitumor activity is
still very gratifying likely attributable to striking antitumor activity of their
corresponding ligands, especially HL1.
Organotin (IV) complexes also exhibited other attractive properties such as
increased water solubility, lower general toxicity than platinum drugs [150-152],
better body clearance, fewer side effects and no emetogenesis. Most importantly,
cancer cells do not develop resistance against organotin complexes that is well
established for cisplatin and its analogs [153]. Additionally, it has been well
established that organotin (IV) compounds are involved in cancer chemotherapy
because of their apoptotic inducing property [154].
Gielen et al. [155-157] published a series of papers describing various biologically
active organotin complexes which exhibited potent in vitro and in vivo cytotoxicities
greater than the classical drug, cisplatin. In a review, Blower P.J. [158] also described
33
thirty interesting inorganic pharmaceuticals, four of which were tin complexes which
further attenuate the importance of these tin complexes.
The binding ability of organotin compounds towards DNA largely depends upon both
the nature and number of organic groups directly attached to the tin (IV) cation
[150,160]. The phosphate group of DNA sugar back bone usually acts as an anchoring
site. Nitrogen of DNA base binding is extremely effective, stabilizing the tin (IV)
centre as an octahedral stable species. However, researches indicate that there is
negligible interaction of tin complexes with nucleotide bases, but rather strong and
irreversible binding to the vicinal phosphate groups of phosphoribose residues
(Scheme 2) [161]. The presence of cyclic groups (aromatic or heterocyclic) in the tin-
containing molecules was found to be important for anticancer activity as well [162].
Scheme 2: Irreversible binding to the peripheral phosphate groups of phosphoribose residues by Sn(IV).
34
Present work
The development of metal-based chemotherapeutic drugs has gained much emphasis
owing to their superior binding ability and specific recognition to the molecular target
DNA. The interaction of metal complexes with nucleic acids and their constituents is
of central importance to many aspects of their structure and function. These
complexes offer opportunity to explore the effects of central metal atom, the ligand
and the coordination geometries on the binding events. Inorganic architecture utilizes a
unique building block strategy which involves combination of two or more active metal
centers exhibiting differential behavior towards the cellular target DNA. Furthermore, the
metal properties are fine-tuned by introducing appropriate ligands which are themselves
active pharmacophores. Tailored multifunctional ligands for metal-based medicinal
drugs play an integral role in modulating the potential toxicity of metallo-drug. Such
an approach complements the molecular diversity in the quest for the discovery of
therapeutic compounds with superior biological activity.
In this context, new metal-based antitumor agents have been developed that show
promise to overcome inherent resistance and exhibit fewer side effects, In the first
series, new heterobimetallic complexes [C16H22ON5Cl4CuSn]Cl and
[C16H22ON5Cl4NiSn]Cl derived from R(+)-phenylglycine chloride hydrochloride and
dichlorodimethyl bis(4-pyrazole N2) tin (IV) were synthesized. These complexes were
thoroughly characterized by spectroscopic (IR, 1H, 13C, 119 Sn NMR, XRD, EPR, UV-
vis, ESI-MS) analytical methods and TEM and AFM visualization techniques. In the
complexes, the geometry of copper and nickel ions were square pyramidal while tin
ions were present in hexacoordinate environment. Various biophysical methods viz.
electronic absorption titrations, fluorescence and cyclic voltammetric and DNA
condensation studies of free complex and complex in presence of DNA were carried
35
out to validate the DNA binding propensity of the complexes. Cleaving activity of the
complexes employing agarose gel electrophoresis with pBR322 plasmid DNA was
also carried out to examine their scission activity. These studies revealed that the
heterobimetallic complex [C16H22ON5Cl4CuSn]Cl was efficient cleaving agent of
pBR322 plasmid DNA.
In an another attempt, new modulated nano-sized heterobimetallic Co-Sn complex
derived from R(+)-phenylglycine chloride hydrochloride and dichlorodimethyl
bis(imidazole) tin (IV) was synthesized and thoroughly characterized by
spectroscopic, analytical and visualization techniques. The electronic absorption and
spectroscopic data reveal that the Co (II) ion exhibits a square pyramidal geometry.
119Sn NMR spectral data and powdered XRD measurements were carried out which
support the hexacoordinated geometry of the Sn (IV) ion. To explore the possibility of
using these chemotherapeutic compounds as novel cationic synthetic vectors for
DNA, DNA condensation properties of the complex [C16H22ON5Cl4CoSn]Cl were
probed and illustrated by employing the visualization techniques viz. TEM and AFM.
The present study contributes to better understanding of the factors which affect DNA
delivery at the molecular level and may lay a design paradigm for more efficient and
safer vectors for gene therapy.
The interaction of the heterobimetallic complex [C16H22ON5Cl4CoSn]Cl with CT
DNA was studied by using UV-vis absorption, emission spectroscopy and cyclic
voltammetric measurements. A concentration dependent cleavage activity experiment
of [C16H22ON5Cl4CoSn]Cl with pBR322 DNA was carried out by employing agarose
gel electrophoresis experiments. The rigid molecular docking study of complex
[C16H22ON5Cl4CoSn]Cl was performed by using HEX 6.1 software which is an
interactive molecular graphics program for calculating and displaying feasible
36
docking modes of a pairs of protein and DNA molecule. Structure of the complex was
sketched by CHEMSKETCH and converts it into pdb format from mol format by
molecular format converter by online OPENBABEL. The crystal structure of the B-
DNA dodecamer d(CGCGAATTCGCG)2 (PDB ID: 1BNA) was downloaded from
the protein data bank. Visualization of the docked pose has been done by using
CHIMERA molecular graphics programme.
Among the various factors governing the binding mode of complexes molecular
shape, size and stereo chemical orientation of the molecule are regarded as most
significant. Those complexes that best fit against the helical structure of DNA display
the highest binding affinity for DNA. Thus, chirality plays a profound role in the
different pharmacological effects exhibited by enantiomeric drug molecules revealing
a preferential binding of one conformation over another, which is termed as
enantioselectivity. The chiral discrimination of DNA has been crucial for the
determination of the binding mode of the complexes with DNA. Keeping this in mind
a new series of chiral complexes R-/S- [C18H26N2O3Cu]Cl2, R-/S- [C18H26N2O3Ni]Cl2
and R-/S- [C18H24N2O2Zn]Cl2, derived from R-/S-phenyl glycinol and dibromoethane
as linker were synthesized and thoroughly characterized by spectroscopic (IR, 1H, 13C
and 119Sn NMR, EPR, UV-vis, ESI-MS) and analytical methods. In the complexes,
the geometry of copper and nickel ions was square pyramidal while zinc metal was
present in the tetrahedral environment. Interaction studies of R-/S-
[C18H26N2O3Cu]Cl2, R-/S-[C18H26N2O3Ni]Cl2 and R-/S-[C18H24N2O2Zn]Cl2 with CT
DNA in Tris buffer were studied by electronic absorption titration, luminescence
titration, cyclic voltammetry and circular dichroism to evaluate the extent of DNA
binding for both the enantiomers. The DNA cleavage activity of R-/S-
[C18H26N2O3Cu]Cl2, (both concentration dependent and mechanistic investigations,
37
in presence of EtOH, NaN3, DMSO, MPA, ASc, H2O2,SOD,GSH and groove binders
DAPI and methyl green) were carried out by agarose gel electrophoresis with
pBR322 DNA. Additionally, complex R-[C18H26N2O3Cu]Cl2 was explored to
examine inhibitory effects on topo II catalytic enzyme.
In an another series, monometallic complexes [C18H22N2O5Cu]Cl2,
[C18H22N2O5Ni]Cl2 and heterobimetallic complexes [C18H22N2O5CuSnCl4]Cl2 and
[C18H22N2O5NiSnCl4]Cl2 were synthesized from a new chiral ligand [C18H20N2O4]
which was derived from (R)-2-amino-2- phenyl ethanol and diethyl oxalate as linker.
The proposed structure of the complexes was formulated on the basis of elemental
analysis, and other spectroscopic data including 1H, 13C and 119Sn NMR in case of
[C18H22N2O5Ni]Cl2 and [C18H22N2O5NiSnCl4]Cl2. In vitro DNA binding studies was
carried out to examine their DNA binding propensity as quantified by Kb and Ksv
values. DNA binding propensity of [C18H22N2O5CuSnCl4]Cl2 was also validated by
its nuclease activity with pBR322 DNA.
Organotin compounds may yield new leads for the development of anti-tumor drugs
as they display another spectrum of anti-tumor activity, may show non-cross-resis-
tance , and may possess less or different toxicity as compared to platinum compounds.
New tin (IV) complex [C26H28N2O6SnCl2] of the ligand [C13H15NO3] derived from
phenylglycine chloride hydrochloride and sodium salt of acetyl acetonate, and its
heterobimetallic complexes [C26H30N2O7SnCuCl2]Cl2 and [C26H28N2O6SnZnCl2]Cl2
were synthesized and thoroughly characterized. The proposed structures of the
complexes were formulated on the basis of elemental analysis, and other
spectroscopic data (IR, 1H, 13C and 119Sn NMR, EPR, UV-vis, ESI-MS) and analytical
methods. The 1H, 13C NMR in case of the ligand [C13H15NO3] and the complex
[C26H28N2O6SnCl2] and 1H, 13C NMR and 119Sn NMR in case of the complex
38
[C26H28N2O6SnZnCl2]Cl2. In vitro DNA binding studies of the complexes were
employed to determine the DNA binding propensity as quantified by Kb values. A
concentration dependent DNA cleavage activity of the complex
[C26H30N2O7SnCuCl2]Cl2 with pBR322 DNA and also in presence of different
activators was employed to examine the cleaving ability of the complex.