DNA targeting mononuclear Ni(II) and Cu(II) metallocarboxylates of 2,3,4,9-tetrahydro-β-carboline-3-carboxylic acid: a combined in silico to in vitro approach with antibacterial and anticancer assays against MDA-MB-231 cell lines

β -carboline alkaloid-based ligand, 2,3,4,9-tetrahydro- β -carboline-3-carboxylic acid (NZ1), a recognized active metabolite in plants, animals, and humans, was synthesized and subsequently complexed with Ni(II) and Cu(II) ions to yield the new mononuclear metallocarboxylate complexes NZ2 and NZ3 respectively. The structural characterization confirmed the successful coordination of the central metal atom to the ligand framework. The non-covalent interactions of these compounds with the chicken blood ds-DNA were investigated through UV–visible absorption, fluorescence emission spectroscopy, viscosity measurements and molecular docking studies. Photophysical changes observed under UV-Visible and Fluorescence spectroscopic studies clearly predicted the presence of dominant minor groove binding as all compounds exhibited hyperchomic effect without any remarkable shift in UV absorption maxima and progressive quenching of intrinsic fluorescence intensities upon addition of ds-DNA without notable shifts in emission maxima. While viscometric measurements and molecular docking studies for all compounds confirmed dominant minor groove binding interaction rather than classical intercalation. Notably, NZ3 demonstrated the highest DNA affinity as followed by NZ2 and NZ1. In antioxidant assays, metal complexation significantly enhanced the free-radical scavenging activity and NZ3 complex demonstrated superior efficacy. The anticancer potential against triple-negative breast cancer cell lines (MDA-MB-231) was assessed by MTT assay and revealed dose-dependent decline in cell viability. The IC₅₀ values were > 200 µg/mL for NZ1, 123.46 µg/mL for NZ2, and 10.50 µg/mL for NZ3, establishing NZ3 (Cu(II) complex) as the most potent candidate, causing a marked reduction in cell viability (7.46 ± 3.15% at 200 µg/mL). These findings demonstrated that Ni(II) and Cu(II) coordination significantly improved the DNA binding, antioxidant, and anticancer activities of the 2,3,4,9-tetrahydro-1 H- β -carboline-3-carboxylic acid ligand NZ3 as a promising scaffold for DNA-targeting and therapeutic applications.