The Nature of Thiol-Containing Ligands in Dinitrosyl Iron Complexes as a Factor Determining the Stability of These Complexes

Abstract

The stability of the binuclear form of dinitrosyl iron complexes with various thiol-containing ligands, such as glutathione, L-cysteine, N-acetyl-L-cysteine, mercaptoethanol, and mercaptosuccinate, has been compared based on the results of exposure of these complexes in an aqueous medium at pH 1.0–2.0, leading to their decomposition as a result of protonation of the thiol group of thiol-containing ligands with the release from the complexes of nitrogen oxide (NO) and nitrosonium (NO+) cations. Complexes with L-cysteine and mercaptosuccinate turned out to be the least stable, they decomposed in acidic solutions at room temperature, whereas the more stable complexes with N-acetyl-L-cysteine, mercaptoethanol, and glutathione decomposed when their solutions were heated at 70–80°C. The decomposition of all these complexes under these conditions occurred within a few min with a concentration ratio of free (not included in the complexes) thiols and the complexes themselves of no more than 1 : 2. With an increase in this ratio, the decomposition time of the complexes increased symbatically with the level of free thiols. The mononuclear form of the thiosulfate complexes decomposed at a thiosulfate/complex concentration ratio of 2 for several min in an aqueous solution at neutral pH. Based on the results obtained, it is assumed that binuclear dinitrosyl iron complexes with L-cysteine can act as the most effective exogenous NO and NO+ donors in experiments on living organisms.