Dual HSA and DNA affinity of a free-base porphyrin nitro-ruthenium(II) complex: Spectroscopic evaluation and...

Porphyrins substituted with ruthenium(II) complexes have emerged as promising photosensitizers for photodynamic therapy (PDT) due to their tunable photophysical properties and potential for multimodal reactivity. Here, we investigate the interaction of a meso-substituted nitro‑ruthenium porphyrin complex (RuNO₂TPyP) with human serum albumin (HSA) and deoxyribonucleic acid (DNA), as well as its DNA photocleavage capability. Ultraviolet visible (UV-Vis) and fluorescence spectroscopy revealed that RuNO₂TPyP binds spontaneously to HSA with moderate affinity (Binding constant (K_b) ≈ 10³ M^-1), driven predominantly by hydrophobic forces. Fluorescence quenching analyses and Förster resonance energy transfer (FRET) suggested close proximity between the complex and the Tryptophan-214 (Trp-214) residue, with fluorescence quenching mainly governed by a static quenching mechanism. Interaction with DNA induced hypochromism and bathochromic shifts in the Soret band, consistent with groove-binding and partial intercalation modes (K_b ≈ 3 × 10⁴ M^-1). Competitive binding assays with ethidium bromide (EB) and 4',6-diamidine-2-phenylindole (DAPI) confirmed strong DNA association and preferential groove interaction. Importantly, RuNO₂TPyP promoted light-dependent plasmid DNA cleavage under 400 nm irradiation, generating strand breaks without dark toxicity. Together, these results demonstrate that RuNO₂TPyP displays moderate biomolecular affinity, effective photoreactivity, and oxygen-dependent DNA damage, supporting its potential as a phototherapeutic agent for PDT applications.