Design, Synthesis, Antimicrobial Evaluation and Molecular Docking Studies of Some Novel Monoterpenoids Substituted Quinoxaline Thiosemicarbazide Derivatives

Abstract

Owing to the increasingly serious problems caused by multidrug resistance in acquired infection pathogens, it has become an urgent need to develop new classes of antibiotics for overcoming the resistance. The present study aims to increase the antimicrobial activity of quinoxaline thiosemicarbazide derivatives by introducing a hydrophobic alkyl chain, an electron-releasing group in the ring, and substitution by some acyclic, cyclic and bicyclic monoterpenes and their antimicrobial evaluation against various strains with molecular docking studies. The lead molecule (1E, 4E)-1-(7-chloro-3-isopropyl- quinoxalin-2(1H)-ylidene) thiosemicarbazide was synthesized and condensed with various monoterpenes to synthesize different derivatives. The structures of compounds were confirmed through IR., NMR & mass spectroscopy. The synthesized derivatives were evaluated in vitro for antibacterial and antifungal activities against various strains using the agar dilution method. Molecular docking studies of the derivatives (Va– Vg) were performed to find out essential binding sites against target protein (PDB: 3 FAP, receptor: FKBP 12) using Autodock 4.2. The compounds Va, Vd, Vf & Vg exhibited potent antibacterial and antifungal activity. Among all these compounds Vd was found to exhibit more potent activity against gram +Ve, gram –Ve bacterial and fungal strains at MIC 0.19 μg/ml, 0.39μg/ml, and 1.56 μg/ml respectively. The docking studies of all the compounds exhibit potent binding energy, but the compound Vd exhibit interactive binding energy (-9.98 kcal/mol) to the active pockets of the receptor FKBP12. The compound Vd interacting with various active sites of amino acids of receptors like PHE128, TRP190, TYR26, VAL55, ILE56, PHE99, and TRP59. In terms of structure- activity relationship study it is revealed that the activity profile against bacterial and fungal strains was altered by the formation of monoterpenoid substituted (1E, 4E)-1-(7- chloro-3-isopropylquinoxalin-2(1H)-ylidene) thiosemicar- bazide derivatives. The study reveals that bicyclic monoterpenes substituted compounds exhibit greater activity than cyclic and acyclic. The molecular docking studies also showed that all the compounds exhibit good docking energy to bind and inhibit the FKBP12 receptor.