Evaluation of DNA Binding, Cleavage, and Cytotoxic Activity of Cu(II), Co(II), and Ni(II) Schiff Base Complexes of 1-Phenylindoline-2,3-dione with Isonicotinohydrazide

One new series of Cu(II), Co(II), and Ni(II) Schiff base complexes was prepared through the condensation reaction between 1-phenylindoline-2,3-dione with isonicotinohydrazide followed by metalation, respectively. The Schiff base ligand(L), (E)-N′-(2-oxo-1-phenylindolin-3-lidene)isonicotinohydrazide, and its complexes were found soluble in DMF and DMSO solvents and characterized by using the modern analytical and spectral techniques such as elemental analysis, conductivity, magnetic moments, IR, NMR, UV-visible, Mass, CV, and EPR. The elemental analysis data of ligand and their complexes were well agreed with their calculated values in which metal and ligand stoichiometry ratio 1 : 2 was noted. Molar conductance values indicated that all the complexes were found to be nonelectrolytes. All the complexes showed octahedral geometry around the central metal ions. Herein, we better characterized DNA binding with the complexes by UV-visible and CD spectroscopy and cyclic voltammetry techniques. The DNA cleavage experiments were carried out by Agarose gel electrophoresis method and the cytotoxicity experiments by MTT assay method. Based on the DNA binding, cleavage, and cytotoxicity studies, Cu and Ni complexes were found to be good anticancer agents against AGS-human gastric cancer cell line.


Introduction
Isatin(1-H indole-2,3-dione) Schiff bases are significant in therapeutic and pharmaceutical compounds in the field [1]. These complexes also exhibited their wide antibacterial [2], antifungal [3], and antitumor activity [4,5]. Nitrogen containing heterocyclic compounds are identified as indispensable structural units for both the chemists and biochemists [6,7]. Among the various classes of benzene fused five-membered nitrogen containing heterocyclic compounds, isatin derivatives could be used pharmacologically as an important class of active components [8][9][10]. The interactions of Schiff base metal complexes containing O and N coordination with DNA have been thoroughly considered [11,12]. Recently, there has been remarkable interest in studies related to the interaction of transition metal ions with nucleic acid because of their relevance in the development of new reagents for biotechnology and medicine [13]. These studies are also essential to understand the toxicity of drugs containing metal ions [14]. Transition metal complexes have gained significance for their applicability in the biological field [15,16]. Some of the transition metal complexes, such as Cu(II), were known to function as specific probe for DNA bulges due its ability to cleave DNA [17]. Recently, we have reported our results on the interaction of bidentate Schiff base complexes with DNA [18,19]. In this background, this study highlights the binding, cleavage, and cytotoxicity with the new series of transition metals N-phenylisatinisonicotinohydrazide Schiff base complexes with calf thymus DNA and AGS cell line, respectively.

Physical
Measurements. C, H, and N analyses of free Schiff base ligands and their metal complexes were performed in C, H, and N analyzer Elementar Vario EL III. Metal contents were analyzed by the standard procedures. Hand-Held Meter LF330 was used to measure the molar conductance of free Schiff base ligands and metal complexes in DMSO (1 × 10 −3 M). The electronic spectra were recorded in DMSO solutions using Shimatzu Model 160 UV-visible spectrophotometer. The IR spectra of the complexes were recorded on a JASCO V-550 UV-Vis spectrophotometer in KBr pellets. NMR spectra were recorded on BRUKER DPX-300 High performance Digital FT-NMR spectrometer in DMSO-d 6 using TMS as internal standard. Electrospray ionisation mass spectrometry (ESI-MS) analysis was performed in the positive ion mode on a liquid chromatography-ion trap mass spectrometer (LCQ Fleet), Thermo Fisher Instruments Limited, US. Magnetic susceptibility measurement of the powdered samples was carried out by the Gouy balance. EPR measurements were carried out by using a Varian E4 Xband spectrometer equipped with 100 Hz modulation. Cyclic Voltammetric measurements were carried out in a Bio-Analytical System (BAS) model CV-50W electrochemical analyzer.

DNA Binding and Cleavage
2.3.1. Electronic Absorption Studies. DNA-binding experiments were performed by UV-visible spectroscopy in Tris-HCl/NaCl buffer (5 mmol L −1 Tris-HCl/50 mmol L −1 NaCl buffer, pH 7.2) and used DMSO (10%) solution of metal complexes. The concentration of CT-DNA was determined from the absorption intensity at 260 nm with a value of 6600 (mol L −1 ) −1 cm −1 . Absorption titration experiments were made using different concentrations of CT-DNA, while keeping the complex concentration constant. Correction was made for the absorbance of the CT-DNA itself. Samples were equilibrated before recording each spectrum. For metal complexes, the intrinsic binding constant ( ) was determined from the spectral titration data using the following equation [21]: where @ a , @ b , and @ f are the molar extinction coefficients of the free complexes in solution, complex in the fully bound from with CT-DNA, and complex bound to DNA at a definite concentration, respectively.  added and the resulting solution was refluxed for 5 h. The results compounds were precipitated upon cooling to room temperature, isolated by filtration, and recrystallized from EtOH. Yellow colored crystalline compounds were obtained (Scheme 1). These ligands were confirmed by Elemental analyzer, IR, NMR, and Mass spectra. Yield: 95%, m.p. 180 ∘ C, elemental analysis: found (calculated) (%) for L: C, 70.01 (70.14); H, 3.97

Synthesis of Cu(II), Co(II), and Ni(II) Complexes.
The metal(II) complexes in this study were prepared by mixing of 1 mMol of corresponding metal(II) chloride in ethanol with 2 mM of the Schiff base in the molar ratio 1 : 2. The reaction mixture was refluxed at 60 ∘ C for 4 hrs [25]. Then it was allowed to cool at room temperature. Powdered solid obtained was filtered, washed with ethanol, and dried under vacuum (Scheme 2). The Schiff base complexes were characterized by elemental analysis, UV-visible, infrared (IR), electron paramagnetic resonance (EPR) spectroscopy, and magnetic moment.

Results and Discussion
The Schiff base ligand (L) and their complexes with Cu(II), Co(II), and Ni(II) were found to be air stable, amorphous, moisture free, and soluble only in DMF and DMSO solvents and kept in vacuum desiccators under nitrogen atmosphere and used for chemical and biological studies. The experimental results are discussed under various subheadings as detailed below.

Elemental Analysis and Conductivity Measurements.
Physicochemical characteristics such as melting point (m.p.), color, yield, elemental analysis, and conductivity of the ligand(L) and complexes were determined and the data shown in Table 1. The observed low conductivity values (22.0-38.40 Ω −1 cm 2 mol −1 ) were accounted for the dissociation and hence the complexes are found as nonelectrolytes [26].

NMR Spectra.
The 1 H-NMR (300 MHz, CDCl 3 , /ppm) spectrum of the Schiff base which exhibited a signal at 14.21(s, 1H) was assigned to the NH proton of isonicotinohydrazide and the signals at 7.93-6.86 (m, 13H) were assigned to aromatic protons (Figure 1(a)). The 13 C NMR spectra provide further support for the structure evidence of the ligand. The signals at 162.15 and 161.81 confirm the carbonyl carbon of isatin and isonicotinohydrazide. The signals appeared at 134.55; it confirms the formation of imine carbon and signals from 150.91 to 110.12, the aromatic rings (Figure 1(b)).

Infrared Spectra.
In order to study the bonding mode of ligand moiety to metal ion in the complexes, IR spectra of the free ligand were compared with those of the metal complexes. The FT-IR spectral data are summarized in Table 2. The IR spectrum of the free ligand (L) showed broadband at 3269 cm −1 , which can be attributed to NH stretching vibration of the isoniazid structural unit. The position of these bands remained at nearly the same frequency in the spectra of the metal complexes which suggests the noncoordination of this group to central metal ion in the metal complexes [27].
A sharp peak at 1602 cm −1 was assigned to ](C=N), which is characteristic of Schiff bases. In the spectra of the complexes, this peak is slightly shifted to higher frequency around 1610-1612 cm −1 . This suggested that one point of attachment of the metal is through the azomethine nitrogen atom [28,29]. The strong intensity bands of ligand were observed at the region 1685 cm −1 of the spectra indicating carbonyl group.  and 1691-1693 cm −1 in the spectrum of the free ligand and complexes, respectively, were assigned to ](C=O) of isatin moiety. The positions of these bands were found at nearly the same frequency in spectra of the metal complexes, suggesting the uncoordination of this group. New bands observed in the 447-453 and 572-601 cm −1 for the complexes were assigned to stretching frequencies of M-N and M-O, respectively [30,31]. Thus, the IR spectral results provide evidence for bidentate complex formation of Schiff bases with metals.

Electronic Spectra and Magnetic Moment Values.
The electronic spectra of the ligand and its Cu(II), Co(II), and Ni(II) complexes were recorded in DMSO and their probable assignments are given in Table 3. The    absorption bands at 36764 cm −1 and 29239 cm −1 attributed to → * and → * transitions in the ligand(L). The Cu(II) complex showed d-d band at 12345 cm −1 . These bands may be assigned to 2 B 1g → 2 B 2g , 2 E g transitions. The position of these bands is consistent with octahedral geometry around the Cu(II) ion. The electronic spectra of Co(II) complex exhibited the absorption d-d bands at 16339 and 14836 cm −1 . These bands may be assigned to 4 The position of these bands is consistent with octahedral geometry around the Co(II) ion. The Cu(II) and Co(II) complexes showed paramagnetism, 4.52 and 4.13 BM respectively [32,33]. Similarly, the electronic spectra of Ni(II) complex exhibited absorption band at 22132 cm −1 and assigned to be LMCT band and the d-d band suppressed by LMCT band. The position of these bands is consistent with octahedral geometry around the Ni(II) ion. The Ni(II) also showed paramagnetism, 3.91 BM [34].   3.6. EPR Spectra. The X-band EPR spectrum of the copper(II) complex was recorded in the solid state at room temperature and in DMSO solvents at liquid nitrogen temperature using the DPPH radical as the marker (Figure 3). The complex has a well-resolved ‖ and broadened regions and various Hamiltonian parameters have been calculated as ‖ = 2.339; ⊥ = 2.05; ‖ = 120 × 10 4 . The trend ‖ > ⊥ observed in this complex indicates that the unpaired electron is most likely to be in the 2 -2 orbital [35]. Table 4. Voltammogram (Figure 4) displays a reduction peak at Epc = −103.60 mV, with an associated oxidation peak at Epa = −214.77 mV at a scan rate of 50 mV/s. The peak separation of this couple (ΔEp) is 0.77V and increases with scan rate, I /I = 1.03. Thus, the analyses of cyclic voltammetric responses at different scan rate gave the evidence for quasireversible one electron reduction. The most significant feature of the Cu(II) complex is the Cu(II)/Cu(I) couple. The ratio of cathodic-to-anodic peak height was less than one. However, the peak current increases with the increase of the square root of the scan rates. This establishes the electrode process as diffusion controlled [29]. In Co(II) complex shows a redox process corresponding to the Co(II)/Co(I) couple at Epa = 790 mV and the associated cathodic peak at Epc = 610 mV and the Ni(II) complex showed redox process corresponding to the Ni(II)/Ni(I) couple at Epa = 521 mV and the associated cathodic peak at Epc = 384 mV. These couples are also found to be quasi-reversible as the peak separation between the anodic and cathodic potentials. But the ratio between the anodic and cathodic currents suggests that the process is simple one-electron transfer, quasi-reversible process [36]. DNA X ( M)

DNA Interaction Studies.
DNA is a molecule of great biological significance and controls the structure and function of cells [36]. These important biological activities will be started via receiving a signal to DNA, which is often in the form of a regulatory protein binding to a particular region of the DNA molecule. The binding specificity and strength of this regulatory protein may be imitated by a small molecule; consequently DNA function can be artificially modulated, inhibited, or activated by binding this molecule instead of the protein [37]. Some studies show that binding can occur between the DNA base pairs (intercalation) [38,39], while some results are indicative of their groove binding nature [40,41]. DNA interaction studies have been carried out with the prepared complexes by using UV-visible, CD, and CV spectral techniques and DNA cleavage activity also has been studied by gel-electrophoresis which showed significant results.

DNA Binding-Absorption Spectra.
The change of the UV spectra of complexes in the presence of different concentrations of DNA was studied. Hypochromism and red shift in the UV absorption spectra were observed upon addition of DNA increasing concentrations to the complexes solution in the absorption intensity region 275-280 nm and 344-346 nm. These effects are particularly pronounced for intercalators. In the case of groove binders wavelength shift is usually correlated with a conformational change on binding or complex formation [42]. In general, the extent of the hypochromism indicates the interaction binding strength and intrinsic binding constant, for complexes with CT-DNA was determined according to (1) [43], where [DNA] is the concentration of DNA in base pairs. @ a is the extinction coefficient for APM absorption band at a given DNA concentration, @ f is extinction coefficient of free complexes, and @ b is the extinction coefficient of complexes when fully bound to DNA (it is assumed when further addition of DNA does not change the absorbance).
In particular, @ f was determined by a calibration curve of the isolated complexes in DMSO solution, following Beer's law. @ a was determined as the ratio between the measured absorbance and the complex concentration. Plot of [DNA]/(@ a − @ f ) versus [DNA] gives a slope of 1/(@ b − @ f ) and a -intercept equal to 1/ (@ b − @ f ); is the ratio of the slope of the -intercept ( Figure 5 insert). The value was calculated to be 10.50 × 10 4 , 5.88 × 10 4 , and 6.81 × 10 4 M −1 ( Table 5). The value obtained here is less than that of reported for classical intercalator (for ethidium bromide whose binding constants have been found to be in the order of 10 6 -10 7 M −1 ) [44]. In comparing the intrinsic binding constant ( ) of Cu(II), Co(II), and Ni(II) complexes with DNA groove binders, as observed in the literature, we can deduce that this complex binds to CT-DNA via groove binding [45,46].

DNA Binding-Cd Spectra.
CD spectra is a useful technique in diagnosing changes in DNA morphology during drug-DNA interactions, since CD signals are quite sensitive to the mode of DNA interactions with small molecules [47]. In the case of CT-DNA interacting with metal complexes, the characteristic CD spectra showed two bands as a positive one Bioinorganic Chemistry and Applications at 275 nm due to the base stacking between the compounds and DNA bases and a negative band at 245 nm due to the right-handed helicity B form of DNA [48]. Observed changes in these CD signals of DNA are usually assigned to corresponding changes in its structure ( Figure 6). The simple groove binding or electrostatic interaction between small molecules and DNA causes less or no perturbation on the base stacking and helicity bands, whereas a classical intercalation enhances both CD bands, stabilizing the CT-DNA form B conformation, as observed for intercalative ligands [49]. Complexes Cu, Co, and Ni exhibited different binding constant values, determined in UV-visible experiments. Generally the DNA of A and B forms structures which have right-handed helix; however, the helical parameters are different in helix pitch, base pair tilt, and twist angle in degrees as 28, 20, and 33 (A form) and 24, −6, and 36 (B form), respectively. B form is the major form that is found in the cell (Watson and Crick 1953). However, after the complex addition to CT-DNA it was only verified as small perturbations in negative and positive bands of CD spectra for three complexes, as shown in Figure 6. Macías et al. [50] observed an increase in both positive and negative bands after incubating complexes with DNA, attributed to a typical intercalative mode, involving → * stacking and stabilization of the right-handed form of CT-DNA. Incubation of DNA with complexes shows little perturbation of the two bands, which is indicative of a nonintercalative interaction between complexes and DNA and offers another support to its groove binding nature [51]. electron transfer couple of Cu(II)/Cu(I) at the scan rate of 50 mVs (curve). The ratio of (Ipa/Ipc) value of 0.5 and the peak to peak separation (ΔEp) of 0.44 V suggested the characteristic of the of the electrotransfer process and this was fairly common for Cu(II)/Cu(I) couple because of the reorganization of the coordination sphere. After interaction with CT-DNA, the value of ΔEp was decreased to 0.23 V suggesting that the reversibility of the electron-transfer process of the copper complex was changed better. Moreover, both the oxidation and the reduction peak potentials underwent positive shifts accompanied by the decreases of the redox peak currents. It has pointed out that the electrochemical potential of the small molecules would shift positively when it interacted into DNA double helix, and if it bounds to DNA by groove binding only takes place. So we thought that the greater affinity of Cu with CT-DNA is most likely caused by a specific binding mode [52]. The two quasireversible redox couple for Cu(II) complex and other complexes are irreversible redox couple for Co(II) and Ni(II).

DNA Cleavage Studies By Agarose Gel Electrophoresis.
The ability of Cu(II), Co(II), and Ni(II) complexes to perform DNA cleavage was monitored by agarose gel electrophoresis with the pUC19 plasmid DNA. The experimental results were shown in Figure 8. Two clear bands were observed for the controls in which the three complexes were absent (lane 1). The relatively fast migration is the intact super coil form (Form II) and the slower moving migration is the open circular form (Form I), which was generated from super coiled when scission occurred on its one strand [53,54]. The amount of Form II diminished gradually and partly converted to Form I and it is obvious that the complexes have more ability to cleave the super coiled plasmid DNA.
3.9. Cytotoxicity. The cytotoxicity assay for the new complexes was assessed using the method of MTT reduction.  The market reference Mitomycin-C was used as a positive control. All the ligands and complexes were found to be cytotoxic to liver cancer cell line (HepG2). The IC 50 values (50% inhibition of cell growth for 48 h) for complexes Cu, Co, and Ni are 5 M, 10 M, 20 M, 25 M, and 30 M, respectively ( Figure 9). The complexes exhibited higher cytotoxic effects on liver cancer cells with lower IC 50 values indicating their efficiency in killing the cancer cells even at low concentrations. The ligand did not show any significant activity up to 100 M. However, cytotoxic effectiveness of the compounds with an IC 50 of Cu and Ni complexes was higher than that of control. There are reports in the literature on the cytotoxic effects of the complexes with longer incubation time periods [55][56][57]. The longer incubation period may result in the development of cellular resistance for that particular complex. The complexes Cu and Ni is showed better activity than Co complex because the reduction potential of Cu(II) and Ni(II), almost in same order of magnitude; however the Co(II) is widely varied and mostly biocompatible in living system. Moreover, the IC 50 values of our complexes are comparable with the reported IC 50 values of standard anticancer drugs such as Mitomycin-C.

Conclusion
One of the most important goals of pharmacological research is the search for new molecular structures which exhibit effective antitumor activities. This has driven inorganic and organometallic chemists to look for new metal compounds with good activities, preferably against tumors that are responsible for high cancer mortality. In this study, new series of Schiff base (L) and its complexes Cu(II), Co(II), and Ni(II) showed octahedral geometry. The binding behaviors of the complexes toward CT-DNA were investigated by absorption spectroscopy, CD, and CV techniques. In conclusion competitive binding of complexes for DNA indicated that complexes could interact as a groove binder. It should be noted that the observed intrinsic binding constant (5.88-10.50 × 10 4 M −1 ) is comparable to other groove binders as well and complexes Cu and Ni have stronger binding affinity than Co. The complexes bind to super coiled plasmid pUC19 DNA and display efficient hydrolytic cleavage and are a specific groove binder. The cytotoxic studies showed that the complexes Cu and Ni exhibit good cytotoxic activity against AGS cell line. Furthermore, these complexes have potential practical applications to formulate into an efficient drug against cancer.