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In this work, transformations of two cationic dinitrosyl iron complexes (DNICs), namely, mononuclear with N-ethylthiourea ligands [Fe(SC(NH2)(NHC2H5))2(NO)2]+Cl−∙[Fe(SC(NH2)(NHC2H5))Cl(NO)2]0 (complex 1) and binuclear with penicillamine ligands [Fe2(S(C(CH3)2CH(NH3)COOH))2(NO)4]SO4·5H2O (complex 2) have been investigated in an aqueous solution in the presence of bovine serum albumin (BSA) under anaerobic and aerobic conditions. The spectroscopic measurements have shown that the complexes bind to the protein, which leads to their stabilization. The resulting products are much more stable in the absence of oxygen. The fluorescence measurements indicated that complexes 1 and 2 interact with BSA with equilibrium constants K = 4.9∙105 M−1 and 5.3∙105 M−1, respectively. Complex 1 can bind to the surface due to weak intermolecular interactions, resulting in prolonged generation of NO. Also, the decay product of complex 1, both under anaerobic conditions and in the presence of oxygen, is coordinated with Cys34 and His39 in the hydrophobic pocket of albumin, as evidenced by the appearance of a shoulder at 370–410 nm. As a result of these interactions, a high molecular weight albumin-bound complex is formed, which then slowly decomposes in more than 40 h. Complex 2 and its products are adsorbed on the surface of the protein. However, in the presence of oxygen, protein does not significantly affect the NO release rate.

In this work, transformations of the anionic binuclear tetranitrosyl iron complex with thiosulfato ligands (Na2[Fe2(S2O3)2(NO)4]·4H2O) (1) have been studied in aqueous anaerobic and aerobic solutions. Under anaerobic conditions, complex 1 was shown to undergo intramolecular isomerization, in addition to NO generation, due to embedding of the oxygen atom of the thiosulfato ligand in the Fe-S bonds. The complex becomes more stable, so its further transformation, including NO release, is unlikely. The model for the complex decomposition has been suggested, and reaction rate constants have been calculated using the kinetic modeling method. As follows from the experimental data, under aerobic conditions, complex 1 is oxidized and becomes a more effective NO donor. Due to the incorporation of the oxygen atom into the Fe-S bonds, mononuclear nitrosyl intermediates are formed, which have a typical EPR signal (g = 2.03).