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The tetranitrosyl iron complex with pyridin 2 yl [Fe 2 (SC 5 H 4 N) 2 (NO) 4 ] (1) has higher NO donating activity in 3% aqueous solutions of DMSO (pH 7.0, 25 °C) than the organic NO donor, viz., adduct of NO with diethylenetriamine (NO adduct). The NO concentration was determined by the spectrophotometric method based on the formation of an NO complex with hemoglobin (Hb). The apparent first order rate constants of the studied reactions are (6.15±0.6)•10 -1 s -1 and (0.8±0.08)•10 -1 s -1 for complex 1 and the NO adduct, respectively, at an Hb concentration of 2•10 -1 mol L -1 and the ratio [NO donor]/[Hb] = 10. The effect of Hb and [NO donor]/[Hb] ratio on the rate of NO generation from a solution of complex 1 was studied. For a fourfold decrease in the concentration of complex 1 the reaction rate constant decreases to 0.5•10 -4 s -1 , whereas the fourfold increase in the Hb concentration results in the stabilization of complex 1.

Structure of the binuclear tetranitrosyl iron complexes with a pyrimidin 2 ylligand of the μ μ μ μ μ 2 S type and the pH effect on its NO donor ability in aqueous solutions* New binuclear tetranitrosyl iron complex with pyrimidin 2 yl of the μ 2 S type [Fe 2 (SC 4 H 3 N 2 ) 2 (NO) 4 ] (1) was synthesized by the exchange reaction of thiosulfate ligands in the [Fe(S 2 O 3 ) 2 (NO) 2 ] 3anion for pyrimidin 2 yl ligands. The crystal structure of complex 1 was studied by single crystal X ray diffraction analysis. According to the X ray diffraction data, pyrimidin 2 yl is coordinated to the iron atom in the thiol form. According to the quantum chemical calculations, the low stability of complex 1 is related to a possibility of formation of the coordination bond of the iron atom with the atom of the pyrimidine cycle of the ligand after NO group detachment. The ability of complex 1 to donate NO and the kinetics of its hydrolysis in aqueous solutions were studied by electrochemical analysis using sensor electrodes amiNO 700, by spectrophotometry in the pH interval from 6.0 to 7.76, and in the reaction with hemoglobin. Complex 1 is most stable in a neutral medium and more vigorously evolves NO in acidic and alkaline media.

This paper describes a comparative study of the decomposition of two nitrosyl iron complexes (NICs) with penicillamine thiolic ligands [Fe2(SC5H11NO2)2(NO)4]SO4 ·5H2O (I) and glutathione- (GSH-) ligands [Fe2(SC10H17N3O6)2(NO)4]SO4 ·2H2O (II), which spontaneously evolve to NO in aqueous medium. NO formation was measured by a sensor electrode and by spectrophotometric methods by measuring the formation of a hemoglobin- (Hb-) NO complex. The NO evolution reaction rate from (I)  k 1 = (4.6 ± 0.1)·10−3 s−1 and the elimination rate constant of the penicillamine ligand k 2 = (1.8 ± 0.2)·10−3 s−1 at 25°C in 0.05 M phosphate buffer,  pH 7.0, was calculated using kinetic modeling based on the experimental data. Both reactions are reversible. Spectrophotometry and mass-spectrometry methods have firmly shown that the penicillamine ligand is exchanged for GS− during decomposition of 1.5·10−4 M (I) in the presence of 10−3 M GSH, with 76% yield in 24 h. As has been established, such behaviour is caused by the resistance of (II) to decomposition due to the higher affinity of iron to GSH in the complex. The discovered reaction may impede S-glutathionylation of the essential enzyme systems in the presence of (I) and is important for metabolism of NIC, connected with its antitumor activity.