Copper (II)–Quinoxaline Complexes as Potential Antimicrobial Agents Against Veterinary Pathogens: Synthesis, Characterization and Biological Evaluation

Abstract

The increasing emergence of antimicrobial-resistant microorganisms in veterinary medicine has created a need for alternative antimicrobial candidates with distinct chemical and biological modes of action. Quinoxaline derivatives are an important class of nitrogen-containing heterocyclic compounds with diverse pharmacological properties, while coordination with copper(II) can substantially modify their electronic, physicochemical and biological characteristics. The present study describes the synthesis, characterization and biological evaluation of copper(II) complexes derived from quinoxaline-based ligands and investigates their potential as antimicrobial agents against veterinary-relevant microorganisms. The ligand and corresponding copper(II) complexes were characterized using elemental analysis, Fourier-transform infrared spectroscopy (FTIR), ultraviolet-visible spectroscopy (UV-Vis), powder X-ray diffraction (PXRD), thermal analysis and, where applicable, electron paramagnetic resonance spectroscopy. The antimicrobial activity of the free ligands and copper complexes was evaluated against representative Gram-positive and Gram-negative veterinary pathogens using standard microbiological assays. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values were determined to provide quantitative evidence of antimicrobial potency. The copper complexes exhibited enhanced antimicrobial activity relative to the corresponding uncomplexed ligands, indicating that coordination with Cu(II) can promote biologically relevant changes in the quinoxaline scaffold. The enhanced activity is discussed in relation to metal-mediated redox processes, membrane interaction, intracellular copper dysregulation, reactive oxygen species generation and interaction with essential microbial biomolecules. The study demonstrates the potential of copper–quinoxaline coordination chemistry as a platform for developing antimicrobial candidates relevant to veterinary health. Nevertheless, further cytotoxicity, pharmacokinetic, residue, environmental and in vivo efficacy investigations are necessary before veterinary therapeutic applications can be proposed.