SCHIFF BASES DERIVED FROM 4-AMINO-N-SUBSTITUTED BENZENESULFON- AMIDE: SYNTHESIS, SPECTRAL CHARACTERISATION AND MIC EVALUATION

The present study is aimed to synthesise Schiff bases from sulfathiazole/ sulfamethoxazole/ sulfadimidine with 2-hydroxybenzaldehyde. The synthesized Schiff bases were characterized by analytical data, IR, H-NMR, C-NMR, UV-Vis spectra, mass spectra and screened for antibacterial activity against gram positive bacteria Staphylococcus aureus and gram negative bacteria Salmonella typhiand antifungal activity against Candida albicans and Mucor by disc diffusion method. Zone of inhibition indicated that the Schiff base possessed highly potent antimicrobial activity when compared to sulpha drugs.


INTRODUCTION
Schiff bases are condensation products of primary amine and carbonyl compounds and they were discovered by a German chemist, Noble prize wined Hugo Schiff in 1864 [1]. Schiff base ligands are essential in the field of coordination chemistry, especially in the development of complexes of Schiff bases because these compounds are potentially capable of stable complexes with metal ions [2].
A large number of Schiff base complexes are characterized by an excellent catalytic activity in a variety of reactions at high temperature (>100 o C) and in the presence of moisture [3]. In recent years, there have been numerous reports of their use in homogeneous and heterogeneous catalysis [4]. They are increasingly being used as catalysts in various biological systems, polymers and dyes [5]. A large number of different Schiff bases ligands have been used as cation carriers in potentiometric sensors [6]. So much interest in imines can be explained by the fact that they are widely distributed in many biological systems and they are used in organic synthesis and chemical catalysis, medicine, pharmacy and chemical analysis, as well as new technologies [7].
Sulfonamides, also known as sulfa drugs were the first effective chemotherapeutics agents employed systematically for the prevention and cure of bacterial infections in humans. Many different families of organic and inorganic compounds are currently investigated today because of their applications. Sulfonamides and their N-derivatives are one of the outstanding groups. Sulfonamides represent are important compounds which are extensively used antibacterial agent. It interferes with p-aminobenzoic acid (PABA) in biosynthesis of tetrahydro folic acid which is a basic growth factor essential for the metabolic process of bacteria [8][9][10].

EXPERIMENTAL
All chemicals and reagents used were of AR grade except ethanol which was purified prior to use. Solvents were purified and dried according to the standard procedures. Elemental analysis of the ligand and complexes were obtained using ElementarVario EL CHN rapid analyzer. IR spectra of the complexes were recorded as KBr pellets on a SHIMADZU 8000 -FTIR spectrophotometer. The 13 C-NMR and 1 H-NMR spectra of the ligand was recorded with a Bruker Spectrospin Advance (DPX-400) using TMS as internal standard and DMSO-d 6 as solvent. Melting points were determined by open capillary method (silicon bath electric melting point apparatus) and uncorrected.

Analytical data
The Schiff base ligand L 1 was synthesized by condensing 2-hydroxy benzaldehyde and 4amino-N-(1,3-thiazol-2-yl)benzenesulfonamide. It is an orange colored solid with melting point =270 o C. The Schiff base ligand L 2 was synthesized by condensing 2-hydroxy benzaldehyde and 4-amino-N-(5-methylisoxazol-3-yl)benzenesulfonamide. It is a yellow colored solid with melting point = 271 o C. The Schiff base ligand L 3 was synthesized by condensing 2-hydroxy benzaldehyde and 4-amino-N-(4,6-dimethylpyrimidin-2-yl)benzenesulfonamide. It is an orange colored solid with melting point = 264 o C. The analytical data and physical characteristics of the Schiff base are (Table 1) indicates that the molecular formula as, L 1 : C 16

H-NMR spectra
The proton magnetic resonance spectrum of the Schiff base was taken in DMSO-d 6 solvent. The 1 H NMR spectrum of ligand (L 1 ) exhibits peak at δ 8.96 ppm (1H) (Figure 3) suggesting the appearance of -CH = N proton [16]. The peak at δ 12.61 ppm (1H) and peak in the range 6.58-7.88 ppm indicates hydroxyl and aromatic protons. The 1 H NMR spectrum of ligand (L 2 ) exhibits peak at δ 8.94 ppm (1H) suggesting the appearance of -CH=N proton. The peak at δ 12.47 ppm (1H) and peak in the range 6.57-7.91 ppm indicate hydroxyl and aromatic protons [17]. The 1 H NMR spectrum of ligand L 3 has a signal at 8.97 ppm (Figure 3) suggesting the presence of -CH=N-linkage. The multiplet in the proton NMR spectrum which extends from 6.5 to 7.6 ppm corresponds to the ten protons of the aromatic ring. The signal at 12.6 ppm indicates the presence of hydroxyl proton and 2.25 to 2.27 ppm corresponds to the presence of -CH 3 proton. The presence of NH proton is confirmed by the signal at 7.7 ppm. Other peaks from 1 H-NMR spectra for the ligands are assigned as [DMSO-d 6 ,  ppm]: 6.76-7.02 (4H, phenolic ring), 6.74 ppm (1H, pyrimidine) 6.98-7.70 ppm (4H, N-phenyl) [18], 7.47 ppm (d, 2H, oxazole moiety), 7.45 ppm (d, 2H, thiazole moiety).

Mass spectra
The mass spectral data of the ligand is consistent with the formulation corresponds to [M+3] and M peaks, respectively, as shown in Figure 5.

Anti microbial studies -minimum inhibitory concentration
Antibacterial, antifungal and MIC activity of sulpha drugs and their Schiff base ligands have been tested by disc diffusion technique [21,22]. The test was carried out in DMSO solution at a concentration of 50, 75, 100 ppm ( Table 2). All the sulpha drugs and their Schiff bases individually exhibit varying degree of inhibitory effects on the growth of tested bacterial and fungal species (Figures 6-8).

Antibacterial bioassay (in-vitro)
Antibacterial activity of sulfa drugs and their Schiff bases were tested against bacterial species like gram positive bacteria Staphylococcus aureus and gram negative bacteria Salmonella typhi. The activity is enhanced at the concentrations 50, 75, 100 ppm. Results were compared with standard drug ciprofloxacin at the same concentration. According to antibacterial studies, the efficacy against gram negative is higher than gram positive bacteria. The inhibition of growth of bacteria is found to be maximum for the Schiff bases than the corresponding sulpha drugs. However, the Schiff base L 2 is more active against Staphylococcus aureus and Salmonella typhi at lower concentration.

Antifungal bioassay (in-vitro)
The sulpha drugs and their Schiff bases were carried out against fungi Candida albicans and Mucor. The activity is greatly enhanced at the concentrations 50, 75, 100 ppm. Compared to the standard drug Nystatin, sulpha drugs show moderate activity against fungi, whereas the Schiff bases derived from sulpha drugs show high activity. However, the Schiff base L 1 are observed to be the more active (80-85%) against Mucor and Candida albicans.
The antimicrobial activity of the ligands are enhanced due to the mode of action of the ligands through the azomethine group with the active centers of cell constitutions, resulting in an interference with the normal cell process.
The enhancement of antimicrobial activity of the ligands as compared to sulpha drugs may be explained on the basis of Overtone's concept. According to Overtone's concept of cell permeability, the lipid membrane that surrounds the cell favours the passage of only lipid soluble materials due to which liposoluble is considered to be an important factor that controls the antimicrobial activity. The large ring size of ligand moiety makes the complexes more lipophilic. This increased lipophilicity enhances the penetration of the Schiff bases into lipid membranes of the microorganisms and block metal binding sites in the enzymes.
The Schiff bases also disturb the respiration process of the cell and thus block the synthesis of proteins, which restricts further growth of the organism. The variation in the activity of different compounds against different organisms depend either on the impermeability of the cells of the microbes or difference in ribosomes of microbial cells. Apart from this, other factors such as solubility, conductivity may also be the possible reasons for increasing this activity.

CONCLUSION
In conclusion, the bidentate coordination ability of the newly synthesized azo Schiff base was proved by IR, UV, NMR and mass spectra confirms two donor sites azomethine nitrogen and phenolic oxygen. The synthesized Schiff bases were subjected to antimicrobial activity at a concentration of 50 ppm. The order of activity is L 2 > L 1 > L 3 . The greater activity of L 2 may be due to the presence of oxazole moiety in the ligand.