Synthesis, Spectral Characterization and In-Vitro Biological Evaluation of a Novel Zn(II) Mixed-Ligand Complex of 5-Nitro-8-Hydroxyquinoline and L-Threonine

Metal-based drug design continues to occupy a central position in medicinal chemistry because coordination of a bioactive ligand to a transition metal can markedly alter its pharmacological profile. In the present work, a novel Zn(II) mixed-ligand complex was synthesized from 5-nitro-8-hydroxyquinoline (a clinically used quinolone antimicrobial, also known as a nitroxoline analogue) and the essential amino acid L-threonine. 5-Nitro-8-hydroxyquinoline was first prepared from 8-hydroxyquinoline through nitrosation followed by oxidative nitration, purified with activated carbon, and then reacted with zinc acetate dihydrate and L-threonine in an aqueous-ethanolic medium to yield the target complex as a yellow precipitate. The ligand and the complex were characterized by FTIR and ¹H NMR spectroscopy, and the metal content was verified by atomic absorption spectroscopy (AAS). In-vitro antioxidant activity was assessed by the ferric reducing antioxidant power (FRAP) assay, while antifungal activity against Candida albicans and antibacterial activity against Escherichia coli and Staphylococcus aureus were evaluated by the well/cup diffusion method. FTIR spectra confirmed retention of the hydroxyl and aromatic ring systems together with a shift in the nitro-group stretching frequencies on complexation, while ¹H NMR spectra showed the expected aromatic proton pattern of the quinoline ring. The Zn(II) complex displayed concentration-dependent antioxidant activity and showed enhanced antifungal activity relative to the free ligand, producing a maximum zone of inhibition of 25 mm against C. albicans at 150 µg/mL, whereas the free ligand retained a marginally higher antibacterial effect against E. coli and S. aureus. These findings support the continued exploration of quinoline-amino acid Zn(II) complexes as multi-target antimicrobial and antioxidant leads.