Transition Metal (II) Complexes with Nitrogen, Oxygen and Sulphur (NOS) Donor Schiff Base hydrazone: Synthesis, Spectroscopic and Antimicrobial Studies

 

Jai Devi*, Nisha Batra

Department of Chemistry, Guru Jambheshwar University of Science and Technology, Hisar-125001 Haryana, India

*Corresponding Author E-mail: jaidevi_gju@yahoo.com

Monobasic tridentate Schiff base hydrazone (pyrazine-2-carboxylic acid (phenyl-thiophen-2-yl-methylene)-hydrazide, HL) derived from condensation of pyrazine carboxylic acid hydrazide and 2-benzoyl thiophene reacted with transition metal (II) nitrates to form complexes of ML₂ type [where M = Mn(II), Co(II), Ni(II), Cu(II) and Zn(II)]. Schiff base hydrazone and metal complexes were characterized by spectroscopic techniques (IR, NMR, mass, electronic and ESR), molar conductance and room temperature magnetic successptibility measurements. On the basis of various physicochemical studies, octahedral geometry was proposed for Mn(II), Co(II), Ni(II), Zn(II) complexes and distorted octahedral geometry for Cu(II) complex. Schiff base hydrazone coordinated to metal ion through carbonyl oxygen, azomethine nitrogen and sulfur of thiophene ring. Schiff base and metal complexes have been evaluated for antimicrobial activities against various bacteria (Bacillus subtilis, Micrococcus luteus, Pseudomonas aeruginosa, Pseudomonas mendocina) and various fungi (Verticillum dahliae, Cladosporium herbarium, Trichophyton soudanense) at different concentrations i.e. 25, 50, 100, 200  μg/mL. The hydrazone ligand was found to be biologically active and a marked enrichment was observed in antimicrobial activity on coordination with metal ions.

 

 

INTRODUCTION:

Schiff bases containing nitrogen, oxygen and sulphur donor atoms act as effective chelating agents and have developed rapidly due to their structural varieties, applications in catalysis ^[1], pharmacology and biological systems^[2]. Schiff base hydrazones are of particular interest due to presence of –NH–N=CH– group in their molecule with an additional donor site like C=O, which determine the versatility and flexibility of these compounds^[3]. Schiff base hydrazones exhibit keto-enol tautomerism and can coordinate in netural, monoanionic, dianionic or trianionic forms to the metal ions which have coordination number of six and seven. Schiff base hydrazones show anticonvulsant^[4-5], anti-inflammatory^[6], antimicrobial^[7-9], antimalarial^[10], antituberculosis^[11] and anti-HIV properties. Aroylhydrazone Schiff bases also have in vitro bacteriostatic properties against various microorganisms^[12].

 

Such hydrazide derived compounds are potent inhibitors of DNA synthesis in variety of cultured human and rodent cells^[13,14] and their metal complexes are known to provide useful models for the elucidation of mechanism of enzyme inhibition and  also produce significant inhibition of tumor growth. Recently, we have synthesized and characterized some transition metal (II) complexes of Schiff base hydrazones and their in vitro antibacterial activity was also investigated^[15]. So in continuation of our earlier research on isolation of antimicrobial compounds^[16], in this article we report the synthesis and characterization of Mn(II), Co(II), Ni(II) , Cu(II) and Zn(II) complexes from Pyrazine-2-carboxylic acid (phenyl-thiophen-2-yl-methylene)-hydrazide. These compounds have also evaluated for their antimicrobial activity against various bacteria and fungi for their use as potential biocides. 

 

EXPERIMENTAL:

All the chemicals used for the synthesis were of analytical grade. IR spectra were recorded on Shimadzu IR affinity-I 8000 FT-IR spectrometer using KBr disc having wavelength range 4000-400 cm^-1.

Table-I: Analytical  data of ligand and its transition metal (II) complexes

Compounds	Mol. Formula of complex (mol. wt.)	Yield (%)	M.Pt. (˚C)	Color	m/z	(ΩM) Molar conductance x10-3
HL	C16H12N4OS (308.36)	65	155	white	308.2	--
Mn(L)2	C32H22N8O2S2Mn (669.64)	60	> 287 d	orange	669.1	8.2
Co(L)2	C32H22N8O2S2Co (673.63)	55	> 295 d	green	673.3	7.4
Ni(L)2	C32H22N8O2S2Ni (673.39)	63	> 290 d	Brown red	673.9	6.3
Cu(L)2	C32H22N8O2S2Cu (678.25)	61	> 284 d	Dark green	678.6	5.7
Zn(L)2	C32H22N8O2S2Zn (680.09)	59	> 288d	Color-less	680.4	--

 

¹H NMR and ¹³C NMR spectra were recorded on Bruker Avance II 300 MHz NMR spectrometer and the chemical shifts were reported in parts per million relative to TMS as internal standard in CDCl₃. Electronic spectra were recorded in DMF on UV-VIS-NIR Varian Cary-5000 spectrometer. Magnetic susceptibilities of complexes were measured on vibrating sample magnetometer by Gouy’s method, using Hg [Co(SCN)₄] as the calibrant at room temperature. An ESR spectrum of the copper complex was recorded in DMSO at room temperature on a Varian E112 X-band spectrometer using tetracyanoethylene (TCNE) as the internal standard. Molar conductance measurements of a 10^-3 M solution of metal complexes in DMF at room temperature were carried out using a model-306 Systronics conductivity bridge.

 

Synthesis of Schiff base HL (Pyrazine-2-carboxylic acid (phenyl-thiophen-2-yl-methylene)-hydrazide)

A methanolic solution of pyrazine carboxylic acid hydrazide (1.38g, 0.01 mol) was added to methanolic solution of 2- benzoyl thiophene (1.88g, 0.01 mol) and the solution was refluxed for 5 hr. The solvent was evaporated to half of its volume and cooled to room temperature. The solid obtained was filtered and washed with methanol [7] (yield 65%, mpt. 155^°C)

 

Synthesis of metal complexes ML₂

A solution of metal salts (5 mmol) in dry methanol (10 mL) was added dropwise to a solution of pyrazine-2-carboxylicacid (phenyl-pyridin-2-yl-methylene)-hydrazide (HL) (3.08g, 10 mmol) in 20 mL of dry methanol at room temperature in 1:2 molar ratio with constant stirring. The reaction mixture was refluxed for 2 hr. The precipitate separate out during refluxing was filtered and washed with hot methanol, finally with petroleum ether and then dried.

 

Methodology of antimicrobial activity

The in vitro antibacterial activities of compounds were evaluated against two Gram positive bacteria Bacillus subtilis (MTCC No. 1790), Micrococcus luteus (MTCC No. 4821, two Gram negative bacteria Pseudomonas aeruginosa (MTCC No. 9126), Pseudomonas mendocina (MTCC No. 7094) and antifungal activities of compounds were evaluated against three fungi Verticillum dahliae (MTCC No. 2063), Cladosporium herbarium (MTCC No. 351), Trichophyton soudanense (MTCC No.7859) by agar plate disc method using nutrient agar / Sabouraud dextrose as medium. Streptomycin and Fluconazole were used as standard for antibacterial and antifungal activity. All the microbial strains were selected on the basis of their clinical importance in causing diseases.

In vitro antibacterial activity

The test solution was prepared by dissolving compound in minimum amount of DMSO. The media was made by dissolving Nutrient agar (15g) in 1 L distilled water.  The mixture was autoclaved for 15 min at 120 °C. Microorganisms cultures were prepared in 15 ml of liquid Nutrient agar. Two wells having diameter of 10 mm were made on the agar medium inoculated with microorganisms. The well was filled with the 100 μl of test solution using a micropipette and incubated at 37 °C for 48 hrs. Activity was determined by measuring the diameter of zone showing complete inhibition and has been expressed in mm.

 

In vitro antifungal activity

The test solution was prepared by dissolving compound in minimum amount of DMSO. The media was made by dissolving Sabouraud dextrose agar (15g) in 1 L distilled water. The mixture was autoclaved for 15 min at 120 °C. Microorganisms’ cultures were prepared in 15 ml of liquid Nutrient agar. Two wells having diameter of 10 mm were made on the agar medium inoculated with microorganisms. The well was filled with 100 μl of the test solution using a micropipette and incubated at 25 °C for 7 days. All these experiments were performed in triplicate. Diameter of the fungal colonies was measured and expressed as percent mycelial inhibition determined by applying the following formula.

 

Inhibition of mycelial growth % = (d_c-d_t)/d_c × 100

d_c, average diameter of fungal colony in negative control; d_t average diameter of fungal colony in experimental plates.

 

RESULTS AND DISCUSSION:

The transition metal (II) complexes were obtained by the reaction of metal nitrates M(NO₃)₂. xH₂O [M = Mn(II), Co(II), Ni(II), Cu(II) or Zn(II)] with Schiff base hydrazone derived from pyrazine carboxylic acid hydrazide and 2-benzoylthiophene. The reaction of ligand with metal salts in 1:2 molar ratio in dry methanol afford metal complexes of type ML₂. All these metal complexes have been obtained as colored solid, stable on prolonged exposure to air and insoluble in most of the common organic solvents expect in DMF and DMSO. The molar conductivity of metal complexes in DMSO has low value 5.7-8.2 ohm^-1 cm² mol^-1 indicating non electrolytic nature. The geometry of these metal complexes has been determined using spectroscopic techniques (IR, NMR, Electronic, ESR and Mass). The ligand was chelated to metal ions in enolic form with the replacement of one hydrogen atom as shown in Scheme-1.

Scheme-1

 

IR spectra

To study the bonding mode of Schiff base hydrazone to the metal ion the IR spectrum of free ligand was compared with that of transition metal (II) complexes. In IR spectra of Schiff base hydrazone HL, the characteristic absorption bands of υ (C=O), υ (N-H), υ (C=N) and υ (N-N) were observed respectively at  1651 cm^-1, 3300 cm^-1, 1575 cm^-1 and 1070 cm^-1. These bands showed that Schiff base hydrazone exist in keto-form in solid state. In transition metal (II) complexes, the characteristic absorption frequencies of υ (C=O), υ (N-H) disappeared and new bands were observed at  1056-1092 cm^-1 due to υ (C-O) The bands due to  υ (C=N) and υ (N-N) were shifted to lower frequency i.e.1540-1560 cm^-1 and 1042-1069 cm^-1 respectively on complexation^[17].  These results showed that ligand enolized in solution and coordinated with metal ion through nitrogen atom of azomethine group, oxygen of carbonyl group and sulfur of thiophene ring which was confirmed by shifting of in plane and out of plane vibrations of thiophene ring from 671 cm^-1 to 675-701 cm^-1. Bonding was further substantiated by apperance of new stretching vibrations at 430-444 cm^-1, 498-515 cm^-1 and 485-499 cm^-1 due to υ(M-O), υ(M-N) and  υ(M-S) respectively^[18]. (Table-II) 

 

NMR Spectra of ligand and Zinc (II) complex

The ¹H and ¹³C NMR spectra of ligand and zinc (II) complex were recorded in CDCl₃ containing small amount of DMSO-d₆ with TMS as the internal reference. The chemical shift (ppm) and coupling constant (Hz) values are summarized in Table-III. In ligand HL, the chemical shift due to azomethine proton appeared as singlet around δ 14.08 ppm.

 

Table-II: IR spectra of transition metal (II) complexes

Complexes	υ (C-O)	υ (C=N)	υ (N-N)	υ (M-O)	υ (M-N)	υ (M-S)	υ (th)
Mn(L)2	1056	1540	1042	430	505	495	680
Co(L)2	1075	1560	1048	444	498	496	694
Ni(L)2	1087	1550	1056	432	508	499	675
Cu(L)2	1092	1556	1069	434	515	492	699
Zn(L)2	1080	1543	1043	442	500	485	701

 

Table-III: ¹H and ¹³CNMR (ppm) of ligand HL and its Zn(II) complex

Ligands	1H NMR(CDCl3) δ in ppm	13CNMR(CDCl3) δ in ppm
HL	14.08 (s, 1H, NH proton), 9.25 (s, IH, C3-H), 8.65 (d, J= 3.3Hz, 1H, C5-H),  8.76 (d, J=3.3 Hz, 1H, C6-H),  7.25 (d, J= 7.2 Hz, 1H, C3׳-H), 7.05 (dd, J= 7.8 Hz, J= 2.1 Hz, 1H, C4׳-H), 7.34 (d, J= 7.8 Hz, 1H, C5׳-H), 7.48 -7.28 (m, 5H, Ph-H)	161.32 (C=O), 152.05 (C=N), 147.91 (C2), 137.24(C3), 147.34(C5), 136.84(C6), 149.52(C2׳), 126.90(C3׳), 136.73(C4׳), 148.85(C5׳), , 129.32-128.24(Ph-C),
Zn(L)2	9.10 (s, 1H, C3-H),  8.54 (d, J= 4.4 Hz, 1H, C5-H), 8.42 (d, J=4.7 Hz, 1H, C6-H), 7.43 (d, J= 8.4 Hz, 1H, C3׳-H), 7.18 (dd, J= 9.4 Hz, J= 4.2 Hz, 1H, C4׳-H), 7.41 (d, J= 9.4 Hz, 1H, C5׳-H), 7.46 -7.28 (m, 5H, Ph-H)	162.47 (C=O), 153.08 (C=N), 147.12 (C2), 137. 43(C3), 148.14(C5), 136.53(C6), 150.12(C2׳), 126.34(C3׳), 136.24(C4׳), 148.53(C5׳), 129.81-128.02(Ph-C),

Table-IV: Electronic Spectra and magnetic moment of transition metal (II) complexes

Complexes	Absorption (cm-1)	Band assignment	Geometry	Magnetic moment µeff(BM)
Mn(L)2	25450	n→π*	Octahedral	5.8
Co(L)2	23550 15335 9850	4T1g(F)→ 4T1g(P) 4T1g(F)→ 4A2g(F) 3A2g(F)→ 3T1g(F)	Octahedral	4.4
Ni(L)2	24550 15890 10540	3A2g(F)→ 3T1g(P) 3A1g(F)→ 3T1g(F) 3A2g(F)→ 3T2g(F)	Octahedral	2.9
Cu(L)2	25320 16390	π N→ Cu* 2Eg(D)→ 2T2g(D)	Distorted Octahedral	1.9
Zn(L)2	24330	LMCT	Octahedral	--

^bINCT-intranuclear charge transfer, LMCT-Ligand to metal charge transition

 

Disappearance of this –NH proton signal in zinc (II) complex showed its deprotonation and indicated the involvement of azomethine nitrogen in coordination to zinc metal ion. A singlet was observed due to proton attached to C₃ at δ 9.25 ppm was shifted to δ 9.10 ppm on complexation. Aromatic ring protons which appeared as doublet and multiplet in the range δ 7.05-8.76 ppm showed not much variation. In ¹³C NMR of ligand, carbonyl carbon, azomethine carbon were observed at δ 161.32 ppm, δ 152.05 ppm and shifted to δ 162.47 ppm, δ 153.08 ppm respectively due to deshielding thereby confirmed the complexation with zinc metal ion and not much variation were observed for aromatic carbons.

 

Mass Spectra

The LC-MS of Schiff base hydrazone and its complexes showed molecular ion peaks which were in agreement with their molecular formula. The molecular ion peak for the ligand HL (C₁₆H₁₂N₄OS) and its cobalt (II) complex Co(L)₂ (C₃₂H₂₂N₈O₂S₂Co) at m/z 308.2 and 673.3 respectively. 

 

 

Electronic spectra and magnetic susceptibility measurements

The electronic spectral measurements were used for assigning the stereochemistry of the metal ions in the complexes based on position and number of d-d transition peaks. Electronic spectra of complexes were recorded in DMF. In the electronic spectra of Mn (II) complexes band due to n→π*  transition was observed in the range of 25,450 cm^-1 confirming their octahedral geometry whereas d-d transition was not observed probably for low intensities for these complex with magnetic moment value 5.8 BM which was expected for high spin d⁵ system having octahedral geometry.  In the electronic spectra of Co(II) complex three bands were observed at 9,850 cm^-1, 15,335 cm^-1 and 23,550 cm^-1 have been assigned due to ⁴T_1g(F)→ ⁴T_2g(F), ⁴T_1g(F)→ ⁴A_2g(F) and ⁴T_1g(F)→ ⁴T_1g(P) transition with magnetic moment 4.4 BM corresponded to the presence of three unpaired electrons indicating a quartet ground state, which was orbitally triply degenerate and would cause an angular moment contribution to magnetic moment expected for octahedral geometry of Co(II) complex^[19]. Ni(II) complex showed three bands  at  10,540 cm^-1, 15,890 cm^-1 and 24,550 cm^-1 due to ³A_2g(F)→ ³T_2g(F),  3A_2g(F)→ 3T_1g(F) and ³A_2g(F)→ ³T_1g(P) respectively, which was in accordance with octahedral geometry around nickel ion^[20] with magnetic moment 2.9 BM corresponding to two unpaired electrons indicating a triplet ground state having octahedral geometry around metal ion.  In the electronic spectra of Cu (II) complex,  bands at  25,320 cm^-1  and 16,390 cm^-1 were due to charge transfer and ²E_g→ ²T_2g transition, respectively. In the electronic spectra of Cu (II) complex broadness of band occur due to Jahn-Teller distortion, indicating the distortion from octahedral geometry^[21] with magnetic moment of 1.9 BM which was due to one unpaired electron and suggested octahedral geometry of the complex.  In the electronic spectrum of Zn (II) complex only one band at 24,330 was observed due to LMCT transition and found to be diamagnetic as expected for d¹⁰ configuration with octahedral geometry (Table-IV).

 

 

Table-V: in vitro antibacterial activity of ligands and their complexes

Compounds	Zone of Inhibition(mm)
	Gram +ve								Gram -ve
	B. subtilis				M. luteus				P.aeruginosa				P. mendocina
	25      50     100      200				25        50        100          200				25         50          100         200				25         50       100        200
HL	11	13	14	17	13	14	16	19	10	12	14	16	12	13	15	19
Mn(L)2	18	19	19	22	17	17	20	22	16	17	18	20	16	16	18	20
Co(L)2	15	16	18	20	14	16	18	19	13	16	16	19	14	14	16	20
Ni(L)2	16	16	19	21	16	18	18	20	14	17	17	21	15	17	17	21
Cu(L)2	20	21	23	24	18	20	22	23	16	18	20	23	18	20	24	26
Zn(L)2	14	16	16	18	14	16	17	19	13	15	15	17	13	15	15	19
Streptomycin	19	21	25	26	19	20	23	25	20	23	25	28	22	25	27	30

ESR spectra of copper complex

The ESR spectra of copper complex provide information in studying the metal ion environment. ESR spectra of copper complexes were recorded in DMSO at room temperature. The observed value for Cu (II) complex with ligand HL was g_┴ =2.07 and g_║=  2.30. In addition to this, the value of    g_║ > g_┴ > 2.0023 it is evident that complex have axially elongated octahedral geometry and indicated that unpaired electron present in  d_x²_-y² ground state of Cu (II)^[22]. The anisotropic G value was calculated using G = (g_║-2.0023) / (g_┴-2.0023) gave idea for exchange interactions between Cu(II) centres. Since in this complex G= 4.39 indicated that there was no exchange in the copper complex and hence distorted octahedral geometry proposed for Cu (II) complex. The spin orbital coupling constant λ value (-573 cm^-1) was calculated using the relation g_av= 2 (1-2 λ/10Dq), g_av=1/3(g_║ + 2g_┴) which is less than free ion Cu (II) λ (-832cm^-1), supported the covalent character of M-L bond in complex. The covalency parameter α² and β² value calculated by relations:

   α² = -(A_║/ 0.036) + (g_║-2.0023) +3/7(g_┴-2.0023) +0.04

    β² = (g_║-2.0023)E/-8 λα²

 

From the value of α²= 0.65 and β²= 0.97, it was obvious that there is an interaction  in in-plane σ bonding (α²), where as in-plane π bonding (β²) is almost ionic^[23]. Lower value of α² shows in-plane σ bonding is more covalent than in-plane π bonding which is confirmed by orbital reduction parameters i.e. K_║(1.02) and K_┴ (0.92) calculated from these equations.

K_║= (g_║-2.0023) d-d transition/8 λ

K_┴= (g_┴-2.0023) d-d transition/2 λ

 

 

Figure –I: In vitro antibacterial activity of ligand and its transition metal (II) complexes (as in Table –V)

 

Table-VI: in vitro antifungal activity of ligand and their complexes

Compounds	Mycelial growth Inhibition (%)
	V. dahlia				C. herbarium				T. soudanense
	25          50                100               200				25              50            100                200				25        50                100              200
HL	54.6	55.3	56.5	57.4	50.1	52.3	53.4	54.3	53.2	54.1	55.5	56.4
Mn(L)2	61.3	62.4	62.6	64.3	57.4	59.3	60.4	61.5	58.3	59.4	60.8	62.3
Co(L)2	58.4	58.4	59.6	59.9	56.2	57.4	58.0	59.2	56.2	56.4	56.9	57.8
Ni(L)2	59.8	59.9	62.3	64.5	56.4	56.8	58.2	59.4	57.3	58.4	59.2	60.4
Cu(L)2	62.6	63.4	64.3	66.6	58.3	60.2	61.9	62.8	61.2	62.4	63.6	64.3
Zn(L)2	57.2	57.8	58.4	59.3	56.5	58.3	59.6	60.2	55.4	55.8	56.2	57.4
Fluconazole	76.4	78.2	80.5	81.2	76.3	77.4	78.3	79.7	77.2	79.2	79.6	80.2

 

Figure –II: In vitro antifungal activity of ligand and its transition metal (II) complexes (as in Table –VI)

 

 

For this copper complex the observed value of K_║ > K_┴, suggested a greater contribution from out of plane π bonding rather than in plane π bonding in metal ligand bonding^[24].

 

Antimicrobial activity

Schiff base hydrazone and transition metal (II) complexes were evaluated for antibacterial and antifungal activity against Gram positive bacteria Bacillus subtilis, Micrococcus luteus, Gram negative bacteria Pseudomonas aeruginosa, Pseudomonas mendocina and fungi Verticillum dahliae, Cladosporium herbarium, Trichophyton soudanense using different concentrations of ligand and their complexes (25, 50, 100, 200 µg/mL). Streptomycin and Fluconazole were used as a standard drug for antibacterial and antifungal activity respectively (Table-V, VI, Figure-1, 2).

 

(i)       Activity of ligand and complexes is simplified on the basis of the structure of hydrazone which possess azomethine (C=N) linkage which imports in illuminating the mechanism of transamination and resamination reactions in biological system ^[25]. Complexes were found to be more active than respective ligand. The enhancement in biocidal activity on coordination of ligand with metal ion was explained by chelation / overtone theory ^[16]. According to this theory, on chelation the polarity of metal ion is reduced due to overlap of ligand orbital and sharing of positive charge of metal ion with donor groups. Further it increases delocalization of chelate ring and increases the lipophilicity of complexes. This increased lipophilicity enhances penetration of complexes there by disturbing the respiration process of cell and blocking the synthesis of proteins, which further restricts growth of organisms.

 

(ii)     In antibacterial activity, hydrazone showed zone of inhibition from 10 to 19 mm and with complexation zone of inhibition increased from 14 to 24 mm for Gram positive bacteria and 13 to 26 mm for Gram negative bacteria.

 

(iii)    The antibacterial data showed that Cu(L)₂ was found to be best from all compounds in inhibiting the growth of bacteria and showed highest zone of inhibition against Gram negative bacteria P. mendocina as closed to standard drug Streptomycin.

 

(iv)    Copper (II) complex at lower concentration is more toxic towards gram positive strains as compared gram negative strains which may be due to the reason that cell walls of Gram negative strains have outer lipid membrane of lipopolysaccharides which have more antigenic properties.

 

(v)      In antifungal activity, ligand showed 50.1-57.4 % mycelial growth inhibition and on complexation it increased to 56.2-66.6 %.

 

It is assumed that increase in antimicrobial activity is also due to many other factors such as dipole moment, conductivity, solubility and cell permeability mechanism influenced by the presence of metal ion.

 

CONCLUSION:

The synthesized Schiff base pyarzine-2-carboxylic acid (phenyl-thiophen-2-yl-methylene)-hydrazide acted as tridentate ligand coordinating through carbonyl oxygen, azomethine nitrogen and sulfur of thiophene ring to metal ion as confirmed by IR, NMR, electronic, ESR and magnetic sucesspibility measurements. The antimicrobial activity of ligand and metal complexes showed that complexes are more potent than ligand. At lower concentration copper (II) complex was found to be most active against gram positive bacteria as close to standard drug.

 

ACKNOWLEDGEMENT:

One of author Ms. Nisha Batra is thankful to Department of Science and Technology, Panchkula Haryana for providing HSCST-SRF fellowship vide letter no. HSCST/4117 on dated 11/11/10.

 

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Received on 07.08.2013          Modified on 30.08.2013

Accepted on 07.09.2013         © AJRC All right reserved