In silico and Experimental Study of Two Hydrazones: The Impact of Positional Isomerism and Metal Complexation on Antibacterial and Antioxidant Activities

Abstract

The geometric disposition of pharmacophoric groups is a critical determinant of drug efficacy 
and target recognition. This study investigated the influence of structural isomerism on the 
pharmacological performance of two isomeric hydrazone-based drug candidates, differing in 
the position of the pyridine nitrogen atom and an ethoxy substituent located at the ortho and 
para positions of an adjacent benzene ring, together with their Fe³⁺ and La³⁺ coordination 
complexes. The ligands and their corresponding metal complexes were synthesized and 
structurally characterized using Fourier-transform infrared (FTIR) and nuclear magnetic 
resonance (NMR) spectroscopy. Density Functional Theory (DFT) calculations at the 
B3LYP/6-311G(d,p) level, molecular docking, and ADMET modelling were employed to evaluate 
molecular geometry, pharmacokinetic properties, and binding interactions with Escherichia 
coli DNA Gyrase (PDB: 6F86) and human Keap1 (PDB: 4L7B). Computational predictions were 
validated through in vitro antibacterial and antioxidant assays. Spectroscopic 
characterization identified L1 as the linear isoniazid isomer, exhibiting a lower carbonyl 
stretching frequency (1645 cm⁻¹) than the kinked nicotinic hydrazide analogue (L2). 
Molecular docking demonstrated that the linear geometry of L1 enabled deeper penetration 
into the narrow DNA Gyrase binding pocket, forming a stabilizing dual-lock interaction with 
Glu50 and Asn46, whereas the bent geometry of L2 induced steric hindrance that reduced 
binding affinity, highlighting the significant influence of structural isomerism. Experimental 
antibacterial assays corroborated these findings, with L1 exhibiting superior activity relative 
to L2 and the reference drug. Although the free ligands displayed moderate antioxidant 
activity, the Fe³⁺ complex (C1) exhibited the strongest radical-scavenging capacity, 
significantly outperforming C2 and the uncoordinated ligands. This enhanced activity is 
attributed to the redox-active iron centre and paramagnetic enolic coordination, as supported 
by NMR analysis. Overall, the isoniazid scaffold proved pharmacologically superior to the 
nicotinic hydrazide analogue, while the para-oriented pyridine nitrogen and ethoxy 
substituent promoted a favourable molecular architecture that enhanced antibacterial 
efficacy. Furthermore, coordination to Fe³⁺ unlocked superior antioxidant activity, 
identifying C1 as a promising dual-action therapeutic lead candidate.