Synthesis, Spectral Characterization, Antibacterial Evaluation and Pharmacophore Analysis of Novel 1,3,4-Thiadiazole-Indole Schiff Bases

Abstract

A series of novel Schiff bases (1A–1I) was synthesized from 5-(1H-indol-2-yl)-1,3,4-thiadiazol-2-amine (ITA) through condensation with substituted aromatic aldehydes and acetophenones under reflux conditions. The synthesized compounds were characterized by melting point, FT-IR, 1H NMR, and EI-MS analyses. Formation of the Schiff bases was confirmed by the characteristic C=N stretching absorption at 1611–1636 cm?¹. Among the synthesized derivatives, the 2-chlorophenyl analogue 1H exhibited the most promising antibacterial activity, particularly against E. coli and S. typhi, whereas compounds 1B and 1D showed comparatively better activity against Gram-positive bacteria. Compound 1E also demonstrated notable activity against Gram-negative strains. Pharmacophore modelling following MMFF94 minimization revealed important hydrophobic/lipophilic, hydrogen-bond donor, and hydrogen-bond acceptor features. Overall, structural modification of the Schiff-base aryl moiety influenced both antibacterial activity and pharmacophoric characteristics, with compound 1H identified as the most promising candidate for further optimization.

Introduction

This study focuses on the design, synthesis, characterization, antibacterial evaluation, and pharmacophore modelling of indole-containing Schiff bases based on a 1,3,4-thiadiazole scaffold. The 1,3,4-thiadiazole ring is an important pharmacophore in medicinal chemistry because of its electronic properties, hydrogen-bonding capability, structural flexibility, and reported antibacterial, antifungal, antiviral, and anti-inflammatory activities. A series of nine Schiff bases (1A–1I) was developed to investigate how different substituents influence antibacterial activity and pharmacophoric features.

The key intermediate, 5-(1H-indol-2-yl)-1,3,4-thiadiazol-2-amine (ITA), was synthesized by reacting indole-2-carboxylic acid with thiosemicarbazide and POCl?, giving a 64% yield. ITA was subsequently condensed with substituted aromatic aldehydes and acetophenones in glacial acetic acid to produce Schiff bases 1A–1I, with yields ranging from 57% to 70%. Structural characterization was performed using FT-IR, ¹H NMR, GC-MS, melting-point analysis, and TLC. The characteristic imine C=N bands and N=CH proton signals confirmed Schiff-base formation, while additional spectral signals verified hydroxyl, methoxy, nitro, chloro, and methyl substituents.

Preliminary antibacterial activity was evaluated using the disc diffusion method at 40 μg/disc against E. coli, S. typhi, S. aureus, and P. acnes, with tetracycline as the reference drug. Among the synthesized compounds, compound 1H showed the strongest overall antibacterial activity, producing inhibition zones of approximately 24–25 mm against the Gram-negative bacteria and 22–23 mm against the Gram-positive bacteria. Compound 1E also showed notable activity against E. coli and S. typhi, while phenolic derivatives such as 1B and 1D demonstrated comparatively stronger activity against Gram-positive organisms. These results suggest that substituent type and hydrogen-bonding characteristics influence antibacterial performance; however, inhibition-zone measurements alone cannot establish MIC values or the precise mechanism of action.

Pharmacophore models were developed using ChemSketch, Avogadro, Open3Dalign, and PyMOL with the LIQUID plugin. The models identified hydrophobic/lipophilic, hydrogen-bond donor (HBD), and hydrogen-bond acceptor (HBA) regions and were used to relate molecular structure to antibacterial activity. The analysis indicated that phenolic and nitro substituents increase hydrogen-bond donor/acceptor characteristics, whereas the 2-chlorophenyl substituent in compound 1H provides a favorable combination of hydrophobicity, molecular shape, and retained HBD/HBA functionality.

Conclusion

A series of indole -linked 1,3,4-thiadiazole Schiff base of aromatic aldehyde/ketone was synthesized and characterized by FT-IR, 1HNMR and mass spectrometry. The presence of imine, indole NH, and substituent -specific functional group were identified successfully. Disc-diffusion screening showed that derivative 2H was the most active against both gram positive and negative bacteria. Compound 2E showed good antibacterial activity against gram negative bacteria, whereas 2B and 2D showed promising activity against Gram-positive bacteria. Pharmacophore analysis indicated a common hydrophobic aromatic–thiadiazole framework with substitution-dependent donor and acceptor features. Overall, Schiff base derivative 2H showed the highest biological activity and can be consider a promising lead compound for further study. Further research, including MIC determination, evaluation of additional compounds, and molecular modelling, is useful to better understand the structure-activity relationship

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Copyright © 2026 Rahul P. Rahate. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.