synthesis and molecular modeling of novel …ejmanager.com/mnstemps/36/36-1422345789.pdf · novel...

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Indo American Journal of Pharmaceutical Research, 2014 ISSN NO: 2231-6876

SYNTHESIS AND MOLECULAR MODELING OF NOVEL PYRROLYL OXADIAZOLE

DERIVATIVES AS ANTIMYCOBACTERIAL AGENTS

Shrinivas D. Joshi*, Manoj S. Kulkarni, Devendra Kumar, Uttam A. More Novel Drug Design and Discovery Laboratory, Department of Pharmaceutical Chemistry, S.E.T’s College of Pharmacy, Sangolli

Rayanna Nagar, Dharwad 580 002, Karnataka, India.

Corresponding author

Shrinivas D. Joshi,

Novel Drug Design and Discovery Laboratory,

Department of Pharmaceutical Chemistry,

S.E.T’s College of Pharmacy, Sangolli Rayanna Nagar,

Dharwad 580 002, Karnataka, India.

[email protected].

+91 9986151953;

91 836 2467190

Copy right © 2014 This is an Open Access article distributed under the terms of the Indo American journal of Pharmaceutical

Research, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

ARTICLE INFO ABSTRACT

Article history

Received 24/11/2014

Available online

17/12/2014

Keywords

1,3,4-Oxadiazole,

Surflex-Dock,

Enoyl-Acp Reductase,

Antitubercular Activity.

In the present study we report synthesis, molecular modeling and antitubercular evaluation of

pyrrolyl oxadiazole derivatives. The result indicated that, the synthesized compounds

exhibited moderate antitubercular activity. In docking analysis it is cleared that compound 5a

and 5b bounds tightly to the Enoyl ACP-reductase enzyme (CScore 6.80 and 6.91). Key

interaction observed in all the compounds, but the ranking of derivatives (CScore) helped to

find out potent molecule in the series in comparison with in-vitro assay (MIC value). From

our study it was found that, oxadiazole ring attached to pyrrole are attractive candidate for

antitubercular activity and if we further modified the aromatic ring attached to oxadiazole

ring and changes electronegative substitution within a fixed parameter of van der Waals

radius, the activity of such derivative may increase. Conclusion: If there is p-chloro

substitution on the benzyl group, the MIC value of such compounds found improved. All

compounds have the same orientation and bind to same cavity of protein as that of ligand that

is 4TZK. Analysis of the docking study provided details on the fine relationship linking

structure and activity, and offer clues for structural modifications that can improve the

activity.

Please cite this article in press as Shrinivas D. Joshi et al. Synthesis and Molecular Modeling of Novel Pyrrolyl Oxadiazole

Derivatives As Antimycobacterial Agents. Indo American Journal of Pharm Research.2014:4(12).

mailto:[email protected]

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Vol 4, Issue 12, 2014. Shrinivas D. Joshi et al. ISSN NO: 2231-6876

INTRODUCTION

Tuberculosis is a major threat, high mortality rate was found approximately 2 million people were killed every year. It

was estimated by World Health Organization 1 billion people will be newly infected in the period 2000-2020, follow-on 35 million

more deaths. Infection caused by Mycobacterium species is currently the leading killer of youths, women, and AIDS patients in the

world [1]. It is clear that there is a threatening synergy between Mycobacteria (e.g. M. tuberculosis, M. avium intercellular) and the

human immunodeficiency virus (HIV). It has been predictable that TB accounts for 32% deaths in HIV-infected individuals, and only

in Africa, about 15% of HIV-associated motility are make happened by tuberculosis [2]. The entire scenario become more serious

because of the presence of some complicating factors like, emergence of multi-drug resistant tuberculosis [3], HIV co-infection [4],

lack of patient compliance with chemotherapy, and variable efficacy of Bacilli-Calmette Guerin (BCG) vaccine.

Isoniazid (INH), a frontline antitubercular agent, is a pro-drug that requires activation to form the active metabolite (INH-

NAD adduct), which exerts its lethal effect on intracellular target [5, 6, 7]. The INH-NAD adduct inhibits mycolic acid biosynthesis in

M. tuberculosis by affecting the InhA, an enoyl-ACP reductase enzyme of the type II fatty acid synthesis (FAS-II) system, which

catalyzes the last step of fatty acid elongation cycle [8]. Among these, fatty acid biosynthesis-I (FabI) constitutes a single isoform in

the majority of pathogens such as Staphylococcus aureus [9], Escherichia coli [10] and M. tuberculosis [11]. The clinical success of

InhA inhibitor INH [9] and numerous reports of FabI inhibitors [12] involving the diazaborines [13], 4-pyridones [14],

naphthyridinones [15], triclosan and analogues [16-21] have validated this target as one of the most attractive of the FAS-II pathway.

This enzyme is recognized and validated as an important drug target in M. tuberculosis, since its homolog in the humans is absent.

Previously we have reported some enoyl ACP reductase inhibitors [22-25] and also 2D-, 3D-QSAR study [26-30] on pyrrole

derivatives, along with this we did careful literature survey for functional groups which could be considered as pharmacophores for

the antitubercular activities revealed that the most of the compounds contains 1,3,4-oxadiazole moiety [31, 32]. Lipophilicity is

governing feature that effects the ability of a drug to reach the target by means of transmembrane diffusion and to have a major effect

on the biological activity. The azole antituberculars are detected as the new group and oxadiazoles known as lipophilic analogs that is

very much similar to imidazoles are projected to increase log P. 1,3,4-Oxadiazoles are considered as due to their favorable metabolic

profile and ability to engage in hydrogen bonding. Biological activities of oxadiazole are well documented [33, 34]. Therefore, in view

of the above facts and in continuation of our search for biologically active pyrrole with various heterocylces, it was contemplated to

synthesize some novel pyrrolyl 1,3,4-oxadiazole derivatives and study their antitubercular activity. In present research work we tried

to design, develop and synthesized pyrrolyl oxadiazole derivatives and we focused on such type of modification which lead to develop

the hit molecule enriched with drug likeness property.

Molecular modeling/docking studies

Molecular docking and scoring

The 3D structures were generated using the Chem draw 3D software. By using the standard bond lengths and bond angles,

the geometry optimization was carried out with the help of standard Tripos force field [35] with a distance dependent-dielectric

function, energy gradient of 0.001 kcal/mol and Gasteiger-Huckel as the electrostatics. Conformational analyses of all compounds

were performed using repeated molecular dynamics-based simulated annealing approach as implemented in Sybyl-X 2.0 [36]. All the

conformations were minimized with Gasteiger-Huckel charges.

The protein coordinates of enoyl-ACP-reductase from M. tuberculosis (4TZK) bound to 4TZK ligand were downloaded from

the Protein Data Bank [37]. The ligand was separated from the protein and hydrogens were added to free templates for protein

residues. All the compounds were docked using Sybyl-X 2.0 software [36]. Compounds that were docked in this study contain

Gasteiger-Huckel charge, while the biopolymer contains Amber7FF99 charges; the geometry of the enzyme was optimized using the

Tripos molecular mechanics force field. Docking was carried on 4TZK (A chain) protein using the default setting of Sybyl-X 2.0

program, in which all the water molecules were removed and essential H atoms were added randomly and then the Dock scores were

evaluated by the CScore (Consensus Score) [35]. The CScore integrates a number of popular scoring functions for ranking the affinity

of ligand bound to the active site of a receptor. The strengths of individual scoring functions combine to produce a consensus that is

more robust and accurate than any single function for evaluating the ligand-receptor interactions. Moreover, the D score, PMF Score

(Potential of Mean Force) [38], G Score [39] and CHEM Score [40] were also calculated to get better insights.

RESULTS AND DISCUSSION

Chemical synthesis

Compounds are synthesized by scheme 1, benzocaine (1) and 2,5-dimethoxy tetrahydrofuran was reacted in acetic acid to

yield a ethyl 4-pyrrole-1H-benzoate (2). Compound 2 was treated with hydrazine hydrate, using alcohol as solvent gave 4-(1H-pyrrol-

1-yl)benzohydrazide (3). When compound 3 was heated with KOH/CS2 in alcohol, 5-(4-(1H-pyrrol-1-yl) phenyl)-1,3,4-oxadiazole-2-

thiol (4) was obtained which on further reaction with substituted benzyl chlorides in ethanol, sodium acetate furnished 5(a-c), 5d was

synthesized by reacting piperidine in dioxane with compound 4. Compounds 5e and 5f were synthesized by reacting same

intermediate (4) with 2chloroacetic/3-chloropropionic acid in presence of few drops of pyridine and ethanol.

Antitubercular activity

The MIC values of the compounds vs selected M. tuberculosis H37Rv are given in Table 1. Antitubercular activity was

performed by Alamar Blue Dye method. Activity of each compounds was compared with Pyrazinamide (MIC 3.125 µg/ml) as

standard. Drug design study revealed that hit molecule having structural similarity with 5a may become promising candidate for

tuberculosis. When we modified the hit molecule 5a to synthesized novel analogs, halogen containing derivatives (especially chlorine

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containing) showed promising antimyobactrial activity. In this study we found that p-chloro derivative (5b) has the lowest MIC value

(12.5 µg/ml). Due to this type of finding, further we prepared the halogen derivatives but none have fair antitubercular activity.

Some aliphatic side chain containing derivatives are synthesized because in we thought that aliphatic chain containing polar

substation may increase the activity. On above hypothesis compound 5(e, f) are synthesized, but none of these have an impressive

MIC (Table 1).

Table 1 MIC, Docking score, CScore, crash value, polar area and different scores of all studied compounds.

Compound No. MIC (µg/ml) CScore Crash Polar D_score PMF_score G_score Chem score

4TZK --- 8.73 -1.39 1.18 -168.11 -49.19 -285.29 -37.478

5b 12.5 6.91 -2.62 0.00 -236.39 -22.89 -273.94 -34.48

5a 25.0 6.80 -2.28 1.35 -126.65 -24.28 -222.66 -33.33

5c 25.0 5.87 -2.57 1.05 -232.01 -40.24 -144.15 -24.43

5f 25.0 5.58 -1.98 0.00 -198.46 -32.66 -167.38 -32.10

5d 50.0 4.69 -2.62 0.01 -136.51 -22.89 -217.51 -34.96

5e 25.0 4.10 0.59 2.27 -218.39 -47.09 -140.37 -37.45

Molecular modeling

Surflex-docking was employed to understand the interaction between pyrrole derivatives and InhA and to ultimately

elucidate the interaction mechanism. In present study, we have used Amber7FF99 for the biopolymer and Gateiger-Huckel charges for

small molecules to carry out the docking study; with this charge, 4TZK showed 8.73 CScore, and showed interaction with TYR158

and co-factor NAD+ was used as a standard drug to compare other molecules in the series (Fig. 1 A and B). In the synthesized

compounds, each molecule have retained the key interactions similar to that of standard ligand 4TZK. In most potent compound (5b),

the 2nd

nitrogen of the 1,3,4-oxadiazole ring is involved in the interaction with co-factor NAD+ and TYR158 of enoyl ACP-reductase

(Fig. 2A). Compound 5b has also occupied the same cavity as in the case of ligand molecule (4TZK) (Fig. 2B). From Fig. 3A it is

cleared that compound 5b and ligand 4TZK have the same binding cavity, both the ligand 4TZK and newly synthesized compound

have the approximately same orientation and same pattern of binding with in receptor cavity. Except compound 5d, all compounds lie

in same orientation as that of ligand molecule and have same pattern of interactions with protein (Fig. 3B). From Fig. 3B it is also

cleared that all synthesized compounds (except 5d) have the same kind of orientation as that of the ligand molecule 4TZK. Compound

5d has same pattern of binding pattern with protein but its orientation in the binding cavity is reversed as compared to other

compounds (Fig. 4).

Compounds 5(a-d) showed better Crash score as compared to 4TZK and 5e that means our molecule have better penetration

power. Compound 5b, 5c, 5e, and 5f showed better D score than 4TZK and other molecules in the series. In case of PMF, G and Chem

score 4TZK showed better score than synthesized molecule. In conclusion, all molecules make key interaction with TYR158 and

NAD+. Hence, in-vitro assay and docking score suggest that compound 5b can be a lead molecule to design a new chemical entity.

Fig. 1 Ligand 4TZK docked on at active site of InhA (A) with van der waal surface (B).

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Fig. 2 Compound 5b docked on protein 4TZK (A) with channel (B).

Fig. 3 Ligand 4TZK and 5b docked protein 4TZK (A), Ligand 4TZK, 5a, 5b, 5c, 5e and 5f docked on 4TZK protein.

Fig. 4 Compound 5d docked on protein 4TZK.

Experimental section

Chemical synthesis

Chemicals used in the synthesis of the titled compounds were purchased from Sigma-Aldrich Pvt Ltd, HiMedia laboratories,

S.D. Fine Chem. Pvt Ltd and Spectrochem Pvt. Ltd. All the solvents and chemicals were purified before use by

distillation/recrystallization. Melting points of synthesized compounds were determined by Shital digital melting point apparatus. FT-

IR spectra were recorded on Bruker spectrophotometer by using KBr pellets and values are expressed in cm-1

. The 1H and

13C NMR

spectra were recorded on Bruker AVANCE-II 400/100 MHz instrument using chloroform (CDCl3) and dimethylsulfoxide (DMSO-d6)

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as a solvent and TMS as an internal standard. The chemical shifts are expressed as δ values (ppm) and the splitting of the NMR

spectra are termed as singlet (s), doublet (d), doublet of doublet (dd), triplet (t), quartet (q) and multiplet (m). Mass spectra (MS) were

recorded on JEOL GCMATE II GC-mass spectrometer, Analytical thin-layer chromatography (TLC) was performed on the precoated

TLC sheets of silica gel 60F254 (Merck, Darmstadt, Germany) visualized by long- and short-wavelength ultraviolet lamps.

Compounds 2-4 were synthesized by previously described procedure [41].

General procedure for synthesis of compound 2-(4-(1H-pyrrol-1-yl) phenyl)-5-(substituted benzylthio)-1,3,4-oxadiazoles [5(a-c)]: A mixture of 0.08 mol of compound 4, 0.08 mol of substituted benzyl chlorides and 0.12 mol of sodium acetate in 50 mL of

ethanol was heated under reflux for 8-10 h, then allowed to cool, and poured into 100 mL of cold water containing ice. The solid

product was collected and recrystallized from acetone.

2-(4-(1H-Pyrrol-1-yl)phenyl)-5-(benzylthio)-1,3,4-oxadiazole (5a)

Yield 65%; mp 162-165°C; FTIR (KBr): 3108, 2926 (Ar-H), 1608 (C=N) cm-1

; 1H NMR (400 MHz, CDCl3) 4.55 (s, 2H, CH2), 6.31

(t, 2H, pyrrole-C3 and C4-H), 7.29-7.35 (m, 3H, benzyl-C2-H, C3-H and C4-H), 7.42 (t, 2H, pyrrole-C2 and C5-H), 7.48-7.62 (m, 2H,

benzyl-C5 and C6-H), 7.75-7.90 (m, 2H, phenyl-C3 and C5-H), 8.00-8.24 (m, 2H, phenyl-C2 and C6-H). 13

C NMR (75 MHz, CDCl3) δ

ppm: 36.89 (-CH2-), 111.58 (pyrrole-C3 and C4), 118.98 (benzyl-C4), 120.45 (pyrrole-C2 and C5), 128.21 (benzyl-C3 and C5), 129.19

(phenyl-C6), 134.81 (phenyl-C1), 135.60 (phenyl-C3), 142.95 (phenyl-C2), 143.45 (phenyl-C4), 144.01 (benzyl-C1), 163.88

(oxadiazole-C5), 166.26 (oxadiazole-C2). MS (EI): m/z Calculated 333.41, found 334.10 (M+1). Anal. Calcd. For C19H15N3OS: C,

68.45; H, 4.53; N, 12.60. Found: C, 68.45; H, 4.53; N, 12.59.

2-(4-(1H-Pyrrol-1-yl)phenyl)-5-((4-chlorobenzyl)thio)-1,3,4-oxadiazole (5b)



(t, 2H, pyrrole-C3 and C4-H), 7.32 (d, 2H, benzyl-C2-H and C6-H), 7.40 (t, 2H, pyrrole-C2 and C5-H), 7.50 (d, 2H, benzyl-C3 and C5-

H), 7.76-7.90 (m, 2H, phenyl-C3 and C5-H), 8.12-8.29 (m, 2H, phenyl-C2 and C6-H). 13

C NMR (75 MHz, CDCl3) δ ppm: 36.92 (-CH2-

), 111.62 (pyrrole-C3 and C4), 118.92 (benzyl-C4), 120.55 (pyrrole-C2 and C5), 128.28 (benzyl-C3 and C5), 129.25 (phenyl-C6), 134.87

(phenyl-C1), 135.65 (phenyl-C3), 142.94 (phenyl-C2), 143.42 (phenyl-C4), 144.24 (benzyl-C1), 163.84 (oxadiazole-C5), 166.22

(oxadiazole-C2). MS (EI): m/z Calculated 367.85, found 367.01 (M+). Anal. Calcd. For C19H14ClN3OS: C, 62.04; H, 3.84; N, 11.42.

Found: C, 62.00; H, 3.82; N, 11.41.

2-(4-(1H-Pyrrol-1-yl)phenyl)-5-((4-bromobenzyl)thio)-1,3,4-oxadiazole (5c)



(t, 2H, pyrrole-C3 and C4-H), 7.31 (d, 2H, benzyl-C2-H and C6-H), 7.35 (t, 2H, pyrrole-C2 and C5-H), 7.48 (d, 2H, benzyl-C3 and C5-

H), 7.74-7.95 (m, 2H, phenyl-C3 and C5-H), 8.10-8.34 (m, 2H, phenyl-C2 and C6-H). 13

C NMR (75 MHz, CDCl3) δ ppm: 36.90 (-CH2-

), 111.61 (pyrrole-C3 and C4), 118.90 (benzyl-C4), 120.55 (pyrrole-C2 and C5), 128.25 (benzyl-C3 and C5), 129.22 (phenyl-C6), 134.87

(phenyl-C1), 135.62 (phenyl-C3), 142.94 (phenyl-C2), 143.40 (phenyl-C4), 144.24 (benzyl-C1), 163.81 (oxadiazole-C5), 166.21

(oxadiazole-C2). MS (EI): m/z Calculated 412.30, found 411.10 (M-1).Anal. Calcd. For C19H14BrN3OS: C, 55.35; H, 3.42; N, 10.19.

Found: C, 55.33; H, 3.41, N, 10.17.

Synthesis of compound 2-(4-(1H-pyrrol-1-yl)phenyl)-5-(piperidin-1-ylthio)-1,3,4-oxadiazole (5d):

To a stirred solution of compound 4 (0.003 mol) in dry 1,4-dioxane (10 mL) was added a solution of the piperidine (0.003

mol) in dry dioxane (5 mL). The mixture was refluxed for 20 h. After cooling, the precipitate was filtered and recrystallized from

acetone.


; 1H NMR (400 MHz, CDCl3) 1.58 (s, 1H, piperidine-

C4-H), 2.48 (s, 2H, piperidine-C3 and C5-H), 2.98 (s, 2H, piperidine-C2 and C6-H), 6. 28 (s, 2H, pyrrole-C3 and C4-H), 7.44 (s, 2H,

pyrrole-C2 and C5-H), 7.73 (d, 2H, phenyl-C2 and C6-H), 7.91 (d, 2H, phenyl-C3 and C4-H).13

C NMR (75 MHz, CDCl3) δ ppm: 23.79

(piperidine-C4), 24.94 (piperidine-C3 and C5), 47.15 (piperidine-C2 and C6), 111.27 (pyrrole-C3 and C4), 118.98 (phenyl-C4), 120.45

(pyrrole-C2 and C5), 121.19 phenyl-C6), 141.93 (phenyl-C1), 135.60 (phenyl-C3), 158.40 (oxadiazole-C5), 164.33 (oxadiazole-C2). MS

(EI): m/z Calculated: 294.35 found 295.14 (M+1). Anal. Calcd. For C17H18N4OS: C, 62.55; H, 5.56; N, 17.16. Found: C, 62.54; H,

5.55; N, 17.01.

General procedure for synthesis of compound 3-((5-(4-(1H-pyrrol-1-yl) phenyl)-1,3,4-oxadiazol-2-yl)thio) propanoic acid/acetic

acid [5(e, f)]: Equimolar (0.002 mol) amount of oxadiaxole-2-thiol (4) and equimolar amount of acid (2-chloroacetic acid/3-

chloropropionic acid) was mixed with 10 mL of ethanol. To this 2-3 drops of pyridine was added, and then refluxed for 4-7 h

(monitored by TLC). Then the reaction mixture was poured in to the ice cold water, the precipitated solid was filtered, dried and

recrystallized from ethanol.

2-((5-(4-(1H-Pyrrol-1-yl)phenyl)-1,3,4-oxadiazol-2-yl)thio)acetic acid (5e)

Yield 68%; mp 187-190°C; FTIR (KBr): 3529 (OH), 3094, 2942 (Ar-H), 1566 (C=N) cm-1

; 1H NMR (400 MHz, CDCl3) 4.22 (t, 2H,

CH2), 6.35 (t, 2H, pyrrole-C3 and C4-H), 7.31 (t, 2H, pyrrole-C2 and C5-H), 7.66 (d, 2H, phenyl-C2 and C6-H), 7.99 (d, 2H, phenyl-C3

and C5-H), 14.48 (s, 1H, OH).13

C NMR (75 MHz, CDCl3) δ ppm: 40.18 (-CH2-), 111.33 (pyrrole-C3 and C4), 118.81 (phenyl-C2 and

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C6), 119.34 (pyrrole-C2 and C5), 127.50 (phenyl-C3 and C5), 142.37 (phenyl-C1), 159.91 (oxadiazole-C2 and C5), 177.36 (C=O). MS

(EI): m/z Calculated 301.32 found 301.35. Anal. Calcd. For C14H11N3O3S: C, 55.80; H, 3.68; N, 13.95. Found: C, 55.78; H, 3.66; N.

3-((5-(4-(1H-Pyrrol-1-yl)phenyl)-1,3,4-oxadiazol-2-yl)thio)propanoic acid (5f)

Yield 64%; mp 175-180°C; FTIR (KBr): 3425 (OH), 3092, 2927 (Ar-H), 1567 (C=N) cm-1

; 1H NMR (400 MHz, CDCl3) 2.55 (t, 2H, -

CH2-), 2.85 (t, 2H, -CH2-), 6.33 (t, 2H, pyrrole-C3 and C4-H), 7.34 (t, 2H, pyrrole-C2 and C5-H), 7.69 (d, 2H, phenyl-C2 and C6-H),

8.05 (d, 2H, phenyl-C3 and C5-H), 14.54 (s, 1H, -OH-). 13

C NMR (75 MHz, CDCl3) δ ppm: 27.45 (-CH2-), 32.12 (-CH2-), 111.22

(pyrrole-C3 and C4), 118.29 (phenyl-C2 and C6), 119.35 (pyrrole-C2 and C5), 127.77 (phenyl-C3 and C5), 142.38 (phenyl-C1), 159.87

(oxadiazole-C2 and C5), 177.42 (C=O). MS (EI): m/z Calculated 315.35, found 314.12 (M+). Anal. Calcd. For C15H13N3O3S: C, 57.13;

H, 4.16; N, 13.33. Found: C, 57.11; H, 4.14; N, 13.31.

Scheme

Biological activities

Antitubercular activity

The MIC values were determined for synthesized compounds against M. tuberculosis strain H37Rv using the

MicroplateAlamar Blue Assay (MABA) [42, 43]. For MIC measurement 200 µl of sterile deionzed water was added to all outer

perimeter wells of sterile 96 wells plate to minimize the evaporation losses of the medium during incubation. The 96 wells plate

received 100 µl of the Middlebrook 7H9 broth and serial dilution of compounds were made directly on the plate. The final drug

concentrations tested were 100 to 0.2 µg/ml. Plates were covered and sealed with parafilm and incubated at 37 0C up to five days.

Then, 25µl of freshly prepared 1:1 mixture of Almar Blue reagent and 10% Tween 80 was added to the plate and incubated for 24 h. A

blue color in the well was interpreted as no bacterial growth and pink color was scored as growth. Table 1 reveals antitubercular

activity in terms of MIC data.

CONCLUSIONS

1,3,4-oxadiazole derivatives were synthesized and screened for antitubercular activity against M. tuberculosis H37RV stain.

Among synthesized compounds, 5b is the most potent one. Docking study revealed that compounds 5a and 5b bounds more tightly to

the enoyl ACP-reductase enzyme (CScore 6.80 and 6.91). On the basis of drug design we synthesized the different halogen analogs. If

there is p-chloro substitution on the benzyl group, the MIC of such compound decreased. During drug design study it was found that

compound 5a has fair binding affinity with enzyme. When this compound was modified to different aromatic and aliphatic derivatives,

chloro (5b) (MIC 12.5 µg/ml) derivative, evolved as potent analog. In this study all the synthesized compounds retained their key

interaction with protein 4TZK, compound 5d was found with different orientation. All these molecules retain the interaction with co-

factor NAD+ and amino acid TYR158. Compound 5d has same pattern of binding with protein but its orientation in the binding cavity

is reversed as compared to other compounds.

Aromatic group at oxadiazole ring is essential for activity and when we replace aromatic ring to non-aromatic or aliphatic

chain the activity of compounds decreased (5d-f). Electronegative groups on the aromatic ring are desirable for activity (5d) but the

size of the electronegative group is key feature. If the van der Waals radius of electronegative group is more than radius of Chlorine,

MIC value of such compound decreased (5c). If we further modified the aromatic ring attached to oxadiazole ring and change

electronegative substitution within fixed van der Waals radius, the activity of such derivative may increase.

ACKNOWLEDGMENTS

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Authors immensely thank research support from the Board of Research in Nuclear Sciences (BRNS), Bhabha Atomic

Research Centre (BARC), Mumbai (File No. 2013/37B/17/BRNS/0417 dated-14/05/2013). We also thank Dr. V.H. Kulkarni,

Principal and Mr. H.V. Dambal, President, SET’s College of Pharmacy, Dharwad, India for providing the facilities. The authors are

grateful to Mr. Ravindra N. Nadagir for his technical assistance.

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