Biokemistri Nigerian Society for Experimental Biology ISSN: 0795-8080 Vol. 23, Num. 1, 2011, pp. 21-27

Biokemistri, Vol. 23, No. 1, March, 2011, pp. 21-27

Antibacterial and antioxidant properties of macrocyclic Schiff bases with vanadium (V) complexes

Vatsala Pawar, Sunil Joshi & V.Uma*

Department of Chemistry, Government College, Ajmer, India *E-mail: drvuma@gmail.com,

(Received December 23, 2010; Accepted March 1, 2011)

Code Number: bk11004

ABSTRACT

Macrocyclic Schiff bases have been synthesized by the condensation of acetyl acetone with semicarbazide hydrochloride and thiosemicarbazide in presence of methanol. Further, their oxovanadium complexes have been synthesized by using vanadium acetylacetone. The structural assignment of these compounds has been made on the basis of MP, TLC, MW determination, conductivities, elemental analysis and UV, IR and ¹HNMR spectral datas. The synthesized compounds were screened for their in vitro growth inhibiting activity against different strains of bacteria viz., Staphylococcus aureus, Bacillus licheniformis, Escherichia coli and Micrococcus luteus (ATCC) and were compared with the standard antibiotic oflaxocin. Also in-vitro antioxidant activity of all compounds was determined by nitric acid free radical scavanging assay.

Keywords: Macrocyclic Schiff Bases; Vanadium complexes; Antibacterial; Antioxidant activities.

1. INTRODUCTION

The macrocyclic ligands are highly significant in bioinorganic chemistry, catalysis, extraction of metal ions from solution and many more.¹ Macrocyclic when complexes with transition metal ions show some interesting properties and biological functions, such as being models for metalloproteins and oxygen carrier systems.² Nowadays interest is focused on the synthesis of macrocyclic complexes with potential medicinal applications. Similarly macrocyclic complexes of vanadium are very useful, due to the fact that vanadium compounds are in clinical trials as a potential treatment for non-insulin dependent diabetes mellitus.³ Also, they are highly significant from the biological point of view.^4-5 Keeping the above facts in mind and in continuation of our research work, in the present paper we report the synthesis and characterization of V(V) complexes of Macrocyclic Schiff bases derived from the condensation of acetyl acetone with semicarbazide hydrochloride and thiosemicarbazide.

Antioxidant compounds reduce the action of reactive oxygen species (ROS) in damaged tissues during the recovery process. The search for new bioactive products with antioxidant activity has lead to the present study, whose aims were to investigate the antioxidant activity and also the effects of antibacterial activities.

2. EXPERIMENTAL

All chemicals were used of AR grade. All the solvents used were of high purity and distilled in the laboratory before use. The identification and purity of the products were checked by TLC with Ethanol: water (3:1) using iodine vapours for visualization of the spots. Melting points were measured by open capillaries using Sunsim electric melting point apparatus and are uncorrected. Molecular weights were determined by Rast Camphor. Conductivities measured on Equiptronics model no. EQ-660A of 10^-3 M solution in DMF. Electronic spectra of the compounds were recorded on a digital spectrophotometer.

IR Spectra were taken on a Perkin Elmer, FTIR Spectrophotometer in range 4000-500cm^-1 using potassium bromide pellets. ¹H NMR spectra were recorded in DMSO and MeOD on Bruker Advance 400 MHz FT NMR spectrometer using TMS as an internal standard. Elemental analysis was obtained on a Vario EL III Elementar Carlo Erba 1108. All done at CDRI, Lucknow.

2.1 Synthesis of Macrocyclic Schiff bases

Both macrocyclic ligands SCHA & TSCA were synthesized by taking equimolar ratios of Acetylacetone in ethanol with the solutions of Semicarbazide hydrochloride in hot water (neutralized by dil.NaoH) and Thiosemicarbazide in ethanol respectively and then both were added dropwise in 25ml of ethanol under constant stirring for atleast 3 hours. Precipitate was obtained which were filtered, collected and dried over CaCl2 in vaccum and were recrystallised by ethanol & Petroleum ether. The color of both ligands SCHA & TSCA was ivory.

2.2 Synthesis of Vanadium (V) complexes

The complexes of Vanadium (v) have been prepared by reacting an ethanolic solution of vanadium acetylacetone salt with ethanolic solution of prepared ligands (SCHA) & (TSCA) in 1:1 molar ratio. Resulting reaction mixture was refluxed on water bath for 5-6 hours. Dark colored precipitate was obtained which was recrystallised by petroleum ether. (60-80°C)

Fig. 1 , Fig. 2

Antibacterial activities of the compounds were tested against using Muller Hinton agar medium.^6-7 The sterilized (autoclaved at 121°C for 15 min) medium (40-50°C) was poured into the Petri dishes to give a depth of 3-4 mm and allowed to solidify. The suspension of the microorganism then streaked on plates. The paper discs impregnated with the test compounds was placed on the solidified medium. The plates were pre-incubated for 1 h at room temperature and incubated at 37°C for 24 h.⁸ Ofloxacin was used as standard.

2.4 Scavenging of nitric oxide

Sodium nitroprusside (5mM) in standard phosphate buffer solution was incubated with different concentration of (125,100,75,50 µg/ml) the test extracts dissolved in standard phosphate buffer (0.025M, pH 7.4) and the tubes were incubated at 25 °C for 5 hr. After 5 h, 2 ml of incubation solution was removed and diluted with 2 ml Griess reagent (prepared by mixing equal volume of 1% sulphanilamide in 2% phosphoric acid and 0.1% naphthylethylene diamine dihydrochloride in water). The absorbance of solution formed was read at 546 nm. The experiment was performed in triplicate and % scavenging activity was calculated using the formula (%) = Ao – A1 / Ao x100 where Ao is control absorbance and A1 is the absorbance of the sample. The activity was compared with ascorbic acid. Then % inhibitions were plotted against respective concentrations used and from the graph IC50 values were calculated.^9-11

3. RESULTS AND DISCUSSION

The synthesized macrocyclic ligands were having ivory colour while their complexes were intensely coloured. The solubility tests for the compounds in different solvents, established their solubility in methanol, ethanol, DMF and DMSO. The electrical conductivities of 10^-3 M solution of the complexes measured in DMF are low, with values less than 4.0 ohm^-1 cm² mol^-1 indicating non electrolytical nature of the compounds. Purity of compounds was confirmed as both ligands and complexes moves as a single spot indicating the presence of only one component. Molecular weights determined by Rast Camphor method were found in accordance with calculated value, confirming the monomeric nature of the compounds. Microanalytical datas are shown in table 1.

All the spectral data was consistent with the assigned structure of the compounds. Electronic spectra of ligand (SCHA) shows weak band at 300nm and 360nm attributable to п-п* and n-п* transition respectively, in its complex first remains unchanged while second shows blue shift and a band appear at 310nm due to donation of lone pair of C=N group to vanadium atom. Ligand (TSCA) shows the same weak band at 310nm and 365nm while second appears at 320nm.

The band in region1590-1640cm-1 due to C=N which is assignable to the macrocyclic Schiff bases, which appeared in both synthesized ligands. This band gets shifted to lower frequency in the complexes, indicating the coordination through azomethine nitrogen. It is found from the IR spectra of the complexes that there are wide and strong band at 990 – 996 cm^-1 , which are assigned to V=O stretching vibration. The ¹HNMR spectral data of ligand (SCHA) shows signal at δ2.18, δ2.48 and δ5.82-5.99 assigned to methyl, methylene and NH group respectively. Ligand (TSCA) shows signal at δ2.16-2.29, δ2.50-2.64 and δ9.13-9.55 assigned to methyl, methylene and NH group respectively. All the signals get shifted downfield in their vanadium complexes, thus confirming the coordination.

All the compounds were evaluated for their antibacterial activity in vitro by using zone inhibition technique against E.coli(-) S.aureus(+) M.luteus(+) and B.licheniformis (+) at different concentration (100,500 and 1000ppm). Experiments were repeated three times and the results were expressed as (Mean±SEM) values in table 2. The results obtained were compared with the standard drug Ofloxacin. The IC50 values are also shown in Table 3.

All compounds showed significant free radical scavenging action against nitric oxide (NO) induced release of free radicals at different concentration 125, 100, 75, 50 µg / ml.

Ascorbic acid was used as reference standard. The % inhibition as (Mean±SEM) is shown in Table 4.

All the compounds showed antimicrobial activity against all the types of bacteria tested. The IC50 values for all compounds was found to be in the range of 0.23-0.60mg/ml. Ligand (SCHA) was found to be most susceptible to S. aureus as its IC50 value was 0.42 mg/ml. Ligand (TSCA) was most susceptible to E. coli with having 0.50 mg/ml IC50 value. Complexes of these ligands showed the most potent activity against all the bacteria.

All the compounds also showed strong antioxidant activity as determined by nitric oxide scavenging method. The IC50 values for ligands (SCHA) & (TSCA) are144 ug/ml & 170ug/ml which were lowered down to 72ug/ml & 162ug/ml in their complexes respectively. It indicates that the compounds are effective antioxidants. However the antioxidant activity of ligand (SCHA) was more compared to ligand (TSCA). This could be due to the lack of Oxygen in the structure of ligand (TSCA).

4. CONCLUSION

From the results of antibacterial effect we can conclude that all compounds exhibited strong to moderate activity. Oxovanadium complexes have been found to be more effective then their precursor macrocyclic ligands as the process of chelation dominantly affects the overall biological behaviour of the compounds also the zone of inhibition increases with the concentration. All compounds showed varying antioxidant (free radical scavenging) activities when compared to ascorbic acid. The results suggest that the antioxidant activity of these compounds may contribute to their claimed antioxidant property and may lead to chemical entities with potential for clinical use.

ACKNOWLEDGEMENT

We are thankful to Head, Dept. of Chemistry, Government College, Ajmer for providing the research facilities, to the microbiological department of MDSU Ajmer for providing us bacteria and to CDRI, Lucknow for providing all spectral and elemental analysis.

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