Characterization and biological evaluation of a Rh(III) complex featuring Phosphonate-Modified bipyridine ligands.

A cyclopentadienyl rhodium(III) complex incorporating a phosphonate-functionalized 2,2′-bipyridine ligand was synthesized and comprehensively characterized to elucidate its biomolecular interactions and biological activity. The complex was structurally confirmed by spectroscopic and analytical techniques and further investigated using density functional theory (DFT) calculations to evaluate its electronic structure and frontier molecular orbitals. The interaction of the Rh(III) complex with calf thymus DNA was systematically examined using UV–visible absorption, fluorescence displacement assays with ethidium bromide, circular dichroism spectroscopy, and viscosity measurements. The results indicate a strong affinity toward DNA with an intrinsic binding constant of 2.85 × 10⁴ M⁻¹, consistent with a non-classical binding mode dominated by groove association rather than full intercalation. Molecular docking studies support this conclusion, revealing preferential minor-groove binding stabilized by hydrogen bonding and electrostatic interactions between the cationic metal center and the DNA phosphate backbone. Protein binding studies with bovine serum albumin (BSA) were carried out using absorption and fluorescence quenching experiments, demonstrating static quenching behavior and strong binding affinity, consistent with a single-site interaction. In vitro cytotoxicity assays revealed that the Rh(III) complex exhibits significant antiproliferative activity against MDA-MB-231 and A549 cancer cell lines, while displaying minimal toxicity toward normal VERO cells. Fluorescence microscopy using AO/EB staining confirmed apoptosis as the predominant mode of cell death. In addition, the complex showed pronounced antibacterial and antifungal activity against both Gram-positive and Gram-negative microorganisms. Overall, this study highlights the utility of phosphonate-modified Cp*Rh(III) complexes as multifunctional bioactive platforms, integrating detailed analytical characterization, theoretical insight, and biological evaluation, and provides a framework for the rational design of metal-based agents with tunable biomolecular interactions.