INTRODUCTION:

In recent years, Pyridine derivatives display diverse pharmacological activities like anti bacterial, anti convulsant, antibacterial, anti-HIV, anti-inflammatory, anti fungal, anti cancer etc. and also imino derivatives of pyridine and triazole shows anti bacterial, anti fungal activity, anticancer, analgesic, antimicrobial, and antidepressant activities [1-4]. In view of these reports, the synthesis of a new series of pyridyl triazole derivatives is now reported. Several compounds were screened for their antifungal activity. Fungi have been identified as causative agents of human diseases earlier than bacteria. In spite of earlier beginnings, the study of pathogenic fungi received only scant attention in comparison with study of other pathogens. In various immuno comprised diseases like HIV.

Therefore, it was necessary to identify potent pharmacophores for antibacterial activity in order to develop new antibacterial agents. Many studies have been carried out on various rings such as triazoles, pyrazoles, oxadiazoles, and imidazoles to develop new antibacterial agents [5]. Hence, there is a need to analyze the correlation present in between antibacterial activity and physico-chemical parameters using the Quantitative Structure Activity Relationship (QSAR) methods. Quantitative structure-activity relationship (QSAR) enables the investigators to establish reliable quantitative structure-activity and structure-property relationships to derive an in silico QSAR model to predict the activity of novel molecules prior to their synthesis. In order to study and deduce a correlation between structure and biological activity of N-substituted [5-(1H-1,2,4-triazol-5-yl)pyridine-2-yl]methanimine derivatives as antibacterial agents, we have developed QSAR models. Together with these models derived it revealed the significance of some steric, electrostatic, hydrophobic parameters with biological activity. Structural variations in the molecular fields of particular regions in the space can be studied and QSAR models can be used to give an insight in the design of potent antibacterial agents.

MATERIAL AND METHODS:

Twenty different N-substituted [5-(1H-1,2,4-triazol-5-yl)pyridine-2-yl]methanimine derivatives were synthesized in laboratory employing various chemical reactions like Condensation, Hydrazinolysis, Cyclisation, Rearrangement & various substitution reactions etc. (scheme I) and evaluated for their antibacterial activity. Melting points of synthesized compounds were determined by an open capillary method and are uncorrected. Qualitative chemical analysis involves simple laboratory tests to identify the functional groups present in the synthesized compound. Analytical TLC was performed on Silica gel-G. Spot was detected by using iodine vapours or under UV light (254 nm). The IR spectra were recorded on a FT-IR spectrophotometer (Model-Agilent) instrument. The NMR spectra of the compounds were carried on 400 MHz Varian NMR. The solvent used was DMSO. The mass spectra of the compounds were carried on Q- T of micro (YA-105) and MDS Sciex (API 3000) LC-MSMS spectrometer.

PROCEDURE:

Synthesis of 6-aminopyridine-3-carbohydrazide:

6-aminopyridine-3-carbohydrazide were synthesized by refluxing 0.01M of 6-aminopyridine-3-carboxylic acid & 0.01M of hydrazine hydrate for 6 h. poured in cold water neutralized the mixture using 0.1N NaOH, filtered.

Synthesis of 5-(1H-1,2,4-triazol-5-yl)pyridin-2-amine:

To a solution of potassium hydroxide (0.37 g, 0.0067 mol) in absolute ethanol (30 mL), 4-pyrrol-1-yl benzoic acid hydrazide1 (0.578 g, 0.003 mol) and carbon disulphide (0.45 mL, 0.006 mol) were added and the mixture was agitated for16 h. To the resulting solution anhydrous ether was added and the precipitated potassium dithiocarbazinate was collected by filtration, washed with ether and dried under vacuum. The potassium salt was obtained in quantitative yield and was used in the next step without further purification.

A suspension of the potassium salt, hydrazine hydrate (1.5 mL) and water (1.0 mL) was heated under reflux for 5 h. Hydrogen sulphide evolved and homogenous solution resulted which was diluted with 50 mL water and subsequent acidification with dilute acetic acid gave a white precipitate which was filtered, washed with water and recrystallized from aqueous DMF and obtained as pale yellow crystals in81% yield. M.p. 250-252⁰C.

Synthesis of 1-phenyl-N-[5-(1H-1,2,4-triazol-5-yl)pyridin-2-yl] methanimine:

0.01M of 5-(1H-1,2,4-triazol-5-yl)pyridin-2-amine, 0.01M of Aromatic aldehyde, 0.001M of Glacial acetic acid, reflux for 3 hr. pour in cold water neutralize using 0.01M of NaOH. Filter and recrystallized in methanol [6, 7].

Antibacterial Activity:

The synthesized compounds were screened for antibacterial activity against Escherichia coli (NCIM-27350), Staphylococcus aureus (NCIM-2197). Pseudomonas aeruginosa (NCIM-501) Bacillus subtilis (NCIM1156). The cup plate agar diffusion method was used for antibacterial activity; MIC was calculated using serial dilution method. The tested compounds were dissolved in N, N-dimethylformamide (DMF) to get a solution of 1000, 750, 500, 250, 125, 62, 31.5 mg/ml. The inhibition zones were measured in millimeters at the end of an incubation period of 48 h at 28 ºC. DMF was used as control. Commercial antibacterial Ciprofloxacin was also tested under similar conditions for comparison.

QSAR Analysis

Data Set

The builder module of the Vlife MDS was used to generate molecular models of series of N-substituted [5-(1H-1,2,4-triazol-5-yl)pyridine-2-yl]methanimine derivatives. They were then energy-minimized using the Merck Molecular Force Field (MMFF). The charge equilibration method was used to assign atomic partial charges to each of the compounds. Activity values for the QSAR equation were obtained using the negative logarithm of Minimum Inhibitory concentration. The physicochemical properties of each compound were specified using 252 descriptors, which delineate liphophillic, conformational, electronic, spatial, structural, thermodynamic and quantum mechanical information [8].

Full Search Multiple Linear Regression Method

Activity prediction

To systematically assess a QSAR model, a reliable validation is required. Usually, a QSAR model is evaluated by the predicted results for the given dataset

RESULT AND DISCUSSION:

Synthesis of compounds:

Add all the synthesis scheme details about the steps involved in the synthesis of all final compounds were confirmed from the results of chemical, physicochemical, chromatographic and spectral analysis as shown in table no.1

Antimicrobial Activity:

All the 15 synthesized compounds were evaluated for the antimicrobial activity by using serial dilution method [6]. Using Escherichia coli (NCIM-27350), Staphylococcus aureus (NCIM-2197), Pseudomonas aeruginosa (NCIM-501), Bacillus subtilis (NCIM1156). All compounds showed very good activity against all microorganisms shown in table no.2. The halogen substituted (Compound 4 and 15) and nitro substituted compound are more active than other compounds in the series. The activities of all the derivatives are found to compare to the standard.

Interpretation of the 2D QSAR Models:

In the present study 10 molecules were used in the training set (Table 1) to derive QSAR models with the number of descriptors not more than 2 per model. To evaluate the predictive ability of g-QSAR model which are generated internal validation is carried out using 5 molecules in the test set which are selected on the random basis. (Table No. 1). A prerequisite for QSAR study is all molecules are belonging to the same series with similar basis structural skeletal. The best generated four QSAR models are presented in the table No. 3.

Interpretation of 2D QSAR Model A

Model A best describes antimicrobial activity of the synthesized derivatives against Staphylococcus aureus as confirmed by internal validation and external prediction. The other statistical terms like F test and pred_r2 signifies the importance of the selected model. SaaNE-index and SA Most hydrophobic hydrophilic distance are two important descriptors contributing towards antimicrobial activity of synthesized derivatives. SaaNE-index is a electro topological state indices for number of nitrogen atom connected with two aromatic bonds, the positive contribution of this indicates the importance of nitrogen systems in [5-(1H-1,2,4-triazol-5-yl)pyridine-2-yl]methanimine derivatives towards antimicrobial activity. SA Most Hydrophobic Hydrophilic Distance is a lipophilic descriptor signifies the distance between most hydrophobic and hydrophilic point on the vdW surface. The negative contribution of this descriptor indicates the substitutions of groups imparting lipophilic characters will lead increase in the biological activity of the molecules.

Interpretation of 2D QSAR Model B

Model B best describes antimicrobial activity of the synthesized derivatives against Bacillus subtilis as confirmed by internal validation and external prediction. This model was selected on the basis of the statistical terms like F test and pred_r²signifies the importance of the selected model. SaaNE-index and chiV4 are two important descriptors contributing towards antimicrobial activity of synthesized derivatives against Bacillus subtilis. The SaaNE-index is a topological descriptor, the positive contribution of this shows the importance of nitrogen ring systems towards antimicrobial activity. chiV4 is another physicochemical descriptor contributing towards biological activity and this descriptor signifies atomic valence connectivity index. The negative contribution of this descriptor indicates aromatic substitution bearing nitrogen ring systems are essential for the antimicrobial activity of the synthesized derivatives.

Interpretation of 2D QSAR Model C

Model C describes antimicrobial activity of the synthesized derivatives against E Coli as confirmed by internal validation and external prediction. This model was selected on the basis of the statistical terms like F test and pred_r²signifies the importance of the selected model. XA Most Hydrophobic Hydrophilic Distance and Hydrogens Count are two important descriptors contributing towards antimicrobial activity of synthesized derivatives against E coli XA Most Hydrophobic Hydrophilic Distance is a lipophilic descriptor and it This descriptor signifies distance between most hydrophobic and hydrophilic point on the vdW surface, positive contribution of this shows the importance of hydrophobic surface systems towards antimicrobial activity. Hydrogens Count is a physicochemical descriptor contributing negatively towards biological activity. The negative contribution of the hydrogen count signifies the importance of unsaturation in antimicrobial activity of synthesized derivatives.

Interpretation of 2D QSAR Model D

Model D describes antimicrobial activity of the synthesized derivatives against Pseudomonas aeruginosa. This model was selected on the basis of the statistical terms like F test and pred_r²signifies the importance of the selected model Nitrogens Count and Balaban Index J are two important descriptors contributing towards antimicrobial activity of synthesised derivatives. Balaban Index J is a distance based topological descriptor. Both the descriptors are contributing positively towards antimicrobial activity of synthesized derivatives. Substitution of the bulkier groups bearing nitrogen and lead to potent compounds.

ACKNOWLEDGEMENTS:

The author acknowledges Dr. John I. Disouza, Principal, Tatyasaheb Kore College of Pharmacy, Warananagar, for providing the necessary facilities to carry out this work.

The authors are grateful for the contributions and technical assistance offered by Dr. P. B. Choudhary, Assistant Professor, Department of Pharmaceutical Chemistry, Bharati Vidyapeeth College of Pharmacy, Kolhapur. We also extend our thanks to Shivaji University, Kolhapur, for awarding Teachers Research Grant to this work.

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Received on 26.05.2015 Modified on 17.06.2015

Asian J. Research Chem. 8(9): September 2015; Page 561-565

DOI: 10.5958/0974-4150.2015.00089.9