Interaction of Cyclometallated Ruthenium Nitrosyl Complexes with Human Serum Albumin: Spectroscopic, STD-NMR and Molecular Docking Studies

Human serum albumin (HSA) is an important transport protein capable of binding metal-based drugs and influencing their pharmacokinetic properties. In this work, the interaction of the cyclometallated nitrosyl ruthenium complexes [Ru(tpy)(C∩N)NO]²⁺ (C∩N = benzo[h]quinoline, 1; 2-phenylpyridine, 2) with HSA was investigated by steady-state and time-resolved fluorescence spectroscopy, ¹H STD-NMR, circular dichroism (CD), and molecular docking. Fluorescence lifetime measurements indicated a static quenching mechanism, while complex 2 exhibited a higher binding affinity (K_a = 1.5 × 10⁶ M^-1) than complex 1 (Ka = 1.0 × 10⁶ M^-1). Docking studies suggest that π-cation interactions involving Arg-117 contribute to the stronger association of complex 2. Site marker experiments indicated preferential binding at site III for 1 and site II for 2. CD measurements revealed only minor changes in HSA secondary structure, whereas STD-NMR and docking results consistently indicated the insertion of aromatic ligand moieties within the protein binding cavity.