Selective prolongation of NO donor activity of binuclear tetranitrosyl phenylmethanethiolate iron complexes upon binding to serum albumin

Decomposition of two promising nitrosyl iron complexes [Fe₂(R)₂(NO)₄] (R = 4-methoxyphenylmethanethiolyl (1a), 4-chlorophenylmethanethiolyl (1b)) in the model systems with bovine serum albumin (BSA) was studied. The complexes demonstrate prolonged release of NO in the presence of BSA: the curves reach a plateau after a few minutes in aqueous solutions and after 50–70 h in the protein solutions, depending on the type of substituent in the benzene ring. Binding of the complexes on the protein surface leads to efficient quenching of the intrinsic fluorescence of BSA. The calculated Förster radii are equal to 27.5 (1a) and 25.4 Å (1b), and the Stern—Volmer constants are 2.0 • 10⁵ (1a) and 1.8 • 10⁵ L mol⁻¹ (1b). Molecular docking of compounds 1a,b and the previously described reference compound [Fe₂(C₈H₈NOS)₂(NO)₄] with the protein showed that ligand binding occurs at two pockets at the interface between BSA domains. The free energies of the protein—ligand complexes correlate with the prolongation time of NO generation.