Biophysicochemical studies of a ruthenium (II) nitrosyl thioether‐thiolate complex binding to BSA: Mechanistic information, molecular docking, and relationship to antibacterial and cytotoxic activities

The purpose of this study was to extensively explore in vitro protein binding to [Ru (NO)(Et₂NpyS₄)]Br (RuNOTSP) and its ligand (TSP), correlate the findings with antibacterial and cytotoxic effects, and try to answer the question: Is the metal center always important for binding? With the aid of stopped-flow and other spectroscopic and computational methods, the interaction between RuNOTSP and TSP and bovine serum albumin (BSA) was studied and a mechanism was proposed. From UV–Vis and fluorescence experiments, a static quenching mechanism and binding constants at a single site demonstrated the higher stability of the RuNOTSP-BSA system at 298 K compared with TSP-BSA. Stopped-flow experiments indicated that binding of RuNOTSP and TSP to BSA is accomplished mainly via a two-step mechanism: a fast second-order binding followed by a slow first-order isomerization process. The first reaction step is reversible with coordinate affinity K_a1 190 (RuNOTSP) and 50 M⁻¹ (TSP). The second step for RuNOTSP is a reversible reaction with K_a2 of 76.4 M⁻¹, whereas an irreversible step that was observed with a K₂ of 0.18 M⁻¹ for TSP indicates that TSP-BSA is kinetically more stable than RuNOTSP-BSA. The results revealed that the ruthenium center not only improves reaction rate through coordination affinity but also changes the binding process. Mode and strength of interaction of RuNOTSP and TSP with protein have also been supported by molecular docking and showed that RuNOTSP is located in IA pocket (Trp134) with a binding affinity (−7.27 kcal/mol), a slightly lower than TSP (−8.05 kcal/mol), and these results are in agreement with the binding constants. Furthermore, antibacterial testing revealed that RuNOTSP and TSP had high antibacterial activity against both Gram-positive and Gram-negative bacteria. They are also highly cytotoxic to HepG2 human liver cancer cells. Because the TSP-BSA complex is kinetically more stable than that of RuNOTSP, the former illustrated better antibacterial and cytotoxic activities.