SYNTHESIS, CHARACTERIZATION AND BIOLOGICAL ACTIVITY OF POLYMER NICKEL (II) COMPLEX

4-vinyl pyridine nickel complex containing polymerizable vinyl group, prepared by condensing (4:1 molar ratio) of 4-vinyl pyridine with Nickel chloride, then polymerized with methyl methacrylate at 70 °C using AIBN as initiator. Metal complex and polymer metal complex have been characterized by elemental analyses, molar conductance, IR, 1 H-NMR , Mass spectra and thermal analyses (DTA and TGA). Conductivity measurement reveals the nonelectrolytic nature of the complex. This confirms that, the anion is coordinated to the metal ion. The IR reveal the metal ion is coordinated via the nitrogen atom of 4-VP. Nickel complex and polymer nickel complex have been tested invitro against number of tumor and number of microorganisms in order to assess their anti tumor and antimicrobial properties. The antimicrobial activity was observed by compounds VP-Ni and MMA-VP-Ni under the screening conditions. The activity against HCT-116 cells was detected for compound VP-Ni (with IC50 value 9.8±0.6 μg/ml), compared with reference standard (24.6±0.3 μg/ml) followed by MMA-VP-Ni (48.3±1.5). In conclusion, this study highlighted the synthesis of polymer nickel complex, and proved the promising biological activity of the synthesized compounds.


Synthesis of VP-Ni complex.
A filtered ethanolic (100 ml) of NiCl2.6H2O (0.5 gm, 0.0021 mol) was added to an ethanolic (100 ml) of the 4-VP, (0.884 gm, 0.0084 mol), [1M:4L] Complex (VP-Ni). The mixture was refluxed with stirring for one hour at 60 o C, and then the mixture is cooled to room temperature. The green complex formed was filtered off, washed several times with ethanol and dried under vacuum over P4O10. The possible mechanism of the produced complex was represented in Fig 1. The reaction yield and physicochemical characteristics of the prepared VP-Ni complex was determined (Table1).

Synthesis of polymer Ni complex.
Metal complex (1.0 g) was added to Methyl methacrylate (4.0 g) in dimethylsulfoxide (DMSO; 50 mL). The mixture was refluxed with stirring for 30 min. Azobisisobutyronitrile (AIBN; 100 mg, 0.6 mmol) initiator was added to the mixture. The system was placed in a water bath at 70±1 • C for about 3 h and stirred at an appropriate speed. At the end of the reaction, the system was cooled to room temperature, and the products was filtered, washed with methanol and DMSO. then dried at 60 • C for 2 h. The product will be referred here as MMA-VP-Ni. The possible mechanism of the produced polymer complex is represented in Fig 2. The yield of the polymer complex is defined in Eq. (1): . Where M is the weight of the reagent (g), and M polymer complex is the total weight obtained after washing. The yield was determined gravimetrically.

Solubility studies
Solubility of the VP-Ni complex and MMA-VP-Ni were tested in a different polar and non-polar solvents. About 5-10 mg of the sample was added to about 2 ml of the respective solvent in a test tube and kept overnight with the tube tightly closed. The solubility of the compounds were noted after 24 hours.

Physical and spectroscopic measurements
C, H and N analyses were determined at the Analytical Unit of Cairo University, Egypt. The IR spectra were measured in KBr using a Perkin-Elmer 683 spectrophotometer (4000-200cm _1 ). The conductance of (10 _3 M) solutions of the complex in DMSO was measured at 25 o C with a Bibby conductimeter type MCl. 1 H NMR spectra were obtained with Perkin-Elmer R32-90-MHz spectrophotometer using TMS as internal standard. Mass spectra of metal complex was recorded using JEULJMS-AX-500 mass spectrometer provided with data system. The thermal analyses (DTA and TGA) were carried out in air on a Shimadzu DT-30 thermal analyzer from 27 to 1000 o C at a heating rate of 10 o C per minute. DPPH was used as a standard material. The T.L.C of the metal complex confirmed their purity.

Assessment of antibacterial and antifungal activity
Antimicrobial activity of tested metal complex and polymer metal complex were assessed against four bacterial strains (Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis and Staphylococcus pneumoniae) and two fungal strains (Aspergillus fumigatus and Candida albicans). All microorganisms were kindly obtained from the culture collection of the Regional center for mycology and biotechnology (RCMB), Al-Azhar University, Cairo, Egypt. Antimicrobial assays were carried out using an agar well diffusion method by using approved standard method (M38-A & M44-P, respectively) developed by the National Committee for Clinical Laboratory Standards [29]. The agar plates were prepared and the homogenous inocula of bacteria and fungus (in log phase) was made, then spreaded by swabbing on the solidified agar media (nutrient agar for tested bacteria and malt extract agar for tested fungi), in Petri-dishes (150 mm x 20 mm). Under aseptic condition, the plates were then punched with a 5 mm diameter cork-borer to create wells. The tested samples were suspended in DMSO at 10 mg/mL, and 100 µl of each suspended sample was added into each well. A well loaded with the solvent was used as negative control. The agar plates were incubated for 48 h at 28±2°C and 24 h at 37±2°C for fungi and bacteria, respectively. All experiments were carried out in triplicate. The antimicrobial activity of the tested substances was determined by measuring the sizes of inhibitory zones in millimeter (including the diameter of wells) on the agar surface around the wells. The results are reported as the mean of zones of inhibition ± standard deviation calculated from the triplicate samples for each test. The prepared materials were tested against two tumor cell lines i.e., Cervical cancer cell line (HeLa) and Colon carcinoma cell line (HCT-116) using crystal violet viability assay [30]. All the experiments concerning the cytotoxicity evaluation were performed and analyzed by tissue culture unit at the Regional Center for Mycology and Biotechnology, Al-Azhar University, Cairo, Egypt. In brief, the tumor cell lines were seeded in 96-well plate in 100µl of growth medium at a cell concentration of 1×10 4 cells per well. After 24 h of seeding, the cell monolayers were treated with different concentration from the tested compound. The microtiter plates were incubated at 37ºC in a humidified incubator with 5% CO2 for a period of 24 h. Three independent experiments were performed each containing six replicates for each concentration of the tested samples. Vinblastine sulfate was used as positive control. Untreated cells were served as controls. Because the stock solutions to prepare the different concentrations from the tested compounds were solubilized in DMSO, controls with DMSO alone were performed in parallel for each concentration.

Assessment of Antitumor activity
The cytotoxic effects of the tested compounds were then measured using crystal violet staining viability assay. Briefly, after 24-h of treatment, the medium was removed, 100 μL of 0.5% of crystal violet in 50% methanol was added to each well and incubated for 20 minutes at room temperature and subsequently excess dye was washed out gently by distilled water. The plate was allowed to dry then the viable crystal violet-stained cells were lysed using 33% glacial acetic acid solution. Absorbance at 570 nm was then measured in each well using microplate reader (SunRise, TECAN, Inc, USA). The percentage cell viability was calculated as follows: % cell viability = [1 − (ODt/ODc)] × 100%, where ODt is the mean optical density of wells treated with the tested compound and ODc is the mean optical density of untreated cells. The test compounds were also compared using the IC50 value, i.e., the concentration of an individual compound leading to 50% cell death that was estimated from graphical plots of surviving cells vs compound concentrations.

Conductivity
The molar conductance value of VP-Ni complex in DMSO (10 -3 M) is 11.6 Ω mol -1 cm 2 (Table 1), reveals nonelectrolytic nature of the complex. This confirms that, the anion is coordinated to the metal ion.

IR spectra
The IR spectrumof the 4-VP-Ni and MMA-VP-Ni compounds are presented in figure 3 and 4, respectively. In the spectra of 4-vinyl pyridine, the peaks at 1599,1548 and 1496 cm _1 are attributed to C=N stretching and ring vibration [32]. In the spectra of 4-VP-Ni these frequencies are shifted to 1625, 1492 and 1425 [33], clearly indicating that the ligand is bonded to the central metal atom through the hetero N-atom. This is also confirmed by the appearance of new bands at 259 and 261 cm _1 this have been assigned to the M-N in 4-VP-Ni and MMA-VP-Ni, respectively [34]. In the spectra of the MMA-VP-Ni the intensity of the beaks decrease. The bands of the C=N stretching of the pyridine ring displaced to 1622,1490 and 1421 cm _1 [35]. The absorption band at 1710 cm _1 is assigned to the C=O in MMA-VP-Ni and this band not found in 4-VP/Ni. The absorption band at 1125 cm _1 is assigned to the C-O in MMA-VP-Ni [36].The absorption band at 996 cm _1 is assigned to the -C-H bending in 4-VP-Ni and 1019 cm _1 in MMA-VP-Ni [33]. We have found that the absorption band at 733 cm _1 is due to the stretching vibration of Ni-Cl bond in 4-VP-Ni and it shifted to 628 cm _1 due to polymerization in MMA-VP-Ni [37]

1 H -NMR spectra
The 1 H-NMR spectra of 4-VP-Ni, show peak at 9.1 ppm is assigned to proton of CH=N . The aromatic proton observed at 8.079 ppm [37]. The peak at 5.162 ppm is assigned to proton of HC=C which attach to pyridine ring . The peaks at 5.10 and 5.088 ppm are assigned to proton of C=CH2. However, for MMA-VP-Ni , the peak at 8.5 ppm is assigned to proton of CH=N. Aromatic proton observed at 7.4 ppm. The peak at 3.303 ppm is assigned to proton of C-CH-C which attached to pyridine ring. The peaks at 2.532 and 2.528 ppm are assigned to proton of C -CH2-C-C=O and The peak at 2.498 ppm isassigned to proton of C-CH-C=O . The peak at 1.733 ppm is assigned to proton of C-CH2-C [36].

Thermal analysis
The thermal curves in the temperature 27-1000°C range for metal complex and polymer complex are thermally stable up to 60°C. Dehydration is characterized by endothermic peak within the temperature 65 -75°C range. The decomposition step for 4-VP-Ni shows endothermic peak at 75.2°C is due to loss of six hydrated water molecules with 8.22 % weight loss [38]. However, the endothermic peak observed at 279°C with 26.59% weight loss may be assigned to the loss of VP unit [34,13]. Oxidative thermal decomposition occurred at 359°C with exothermic peaks, leaving NiO. MMA-VP-Ni shows endothermic peak at 65.2°C with 5.75% weight loss, are assigned to elimination of six hydrated water molecules. The endothermic peak observed at 259°C with 24.88 % weight loss may be assigned to the loss of VP unit [39]. Oxidative thermal decomposition occured at 350°C with exothermic peaks, leaving NiO. The thermal data are shown in table 3

Antimicrobial activity
The antimicrobial activity of the compounds was performed using a modified well diffusion agar technique on four pathogenic bacterial strains (Escherichia coli, Pseudomonas aeruginosa, Bacillus subtilis and Streptococcus pneumoniae) and two pathogenic fungal strains (Aspergillus fumigatus and Candida albicans). The results of the antimicrobial activities showed variations in activities among the tested compounds. Interestingly, some of the tested compounds also exhibited highest tendency to inhibit Gram positive bacteria than Gram negative bacteria. This activity can be explained on the basis of chelation theory [40]. All compounds were found with no antibacterial activity against the Psedomonas aeruginosa under the screening conditions. Additionally, none of the tested compounds exhibited antifungal activity against the Candida albicans under the screening conditions (Table 4 and 5). The antibacterial activity was observed by compound VP-Ni, followed by MMA-VP-Ni under the screening conditions. Moreover, the antifungal activities against Aspergillus fumigatus were observed for VP-Ni, followed by MMA-VP-Ni. The mode of action of the compounds may involve the formation of a coordinate bond groups with the active centers of the cell constituents resulting in an interference with the cell process (Table4 and 5).

Evaluation of the antitumor activities
The antitumor activity of the compounds was performed using viability assay on two carcinoma cells (i.e. cervical cancer cell line (HeLa cells) and human colon carcinoma cell line (HCT-116 cells). The in vitro growth inhibitory rates (%) and inhibitory growth activity (as measured by IC50) of the synthesized compounds were investigated in comparison with the well-known anticancer standard drug Imatinib (2-substituted aminopyrimidine derivative; Gleevec®). Data generated were used to plot dose response curves and presented in Fig. (7 & 8).However, the results revealed that the tested compounds showed high variation in the inhibitory growth rates and activities against the tested tumor cell lines in a concentration dependent manner (Fig. 7& 8). The difference between inhibitory activities of all compounds with different concentrations was statistically significant P < 0.001. Furthermore, the highest activity against HCT-116 cells was detected for compound VP-Ni (with IC50 value 9.8±0.6 µg/ml), compared with reference standard (24.6±0.3 µg/ml) followed by MMA-VP-Ni (48.3±1.5) (Fig. 7). Lower sensitivity was detected for HeLa cell line (Fig. 8) showing higher IC50 values in the same trend of activity measured for HCT-116. This can be explained as Metal binds to DNA. It seems that, change the anion and the nature of the metal ion has effect on the biological behavior, due to alter binding ability of the DNA binding. Antitumor effect of the compounds may be attributed to the central metal atom which was explained by Tweedy's chelation theory [41]. Also, the positive charge of the metal increases the acidity of coordinated ligand that bears protons, leading to stronger hydrogen bonds which enhance the biological activity. Also, metal could act as a double-edged, sword by inducing DNA damage and also by inhibiting their repair. The OH radical reacts with DNA sugars and bases and the most significant and well-characterized of the OH reactions is hydrogen atom abstraction from the C4 on the deoxyribo unit to yield sugar radicals with subsequent β-elimination. Figure 9, by this mechanism strand breakage occurs as well as the release of the free bases. Another form of attack on the DNA bases is by solvated electrons, probably via a similar reaction to those discussed below for the direct effects of radiation on DNA [41].     S t r a n d b r e a k