Dinitrosyl iron complexes with natural thiol-containing ligands in aqueous solutions Synthesis and some physico-chemical characteristics (A methodological review)

Two approaches to the synthesis of dinitrosyl iron complexes (DNIC) with glutathione and l-cysteine in aqueous solutions based on the use of gaseous NO and appropriate S-nitrosothiols, viz., S-nitrosoglutathione (GS-NO) or S-nitrosocysteine (Cys-NO), respectively, are considered. A schematic representation of a vacuum unit for generation and accumulation of gaseous NO purified from the NO2 admixture and its application for obtaining aqueous solutions of DNIC in a Thunberg apparatus is given. To achieve this, a solution of bivalent iron in distilled water is loaded into the upper chamber of the Thunberg apparatus, while the thiol solution in an appropriate buffer (рН 7.4) is loaded into its lower chamber. Further steps, which include degassing, addition of gaseous NO, shaking of both solutions and formation of the Fe2+-thiol mixture, culminate in the synthesis of DNIC. The second approach consists in a stepwise addition of Fe2+ salts and nitrite to aqueous solutions of glutathione or cysteine. In the presence of Fe2+ and after the increase in рН to the physiological level, GS-NO or Cys-NO generated at acid media (pH < 4) are converted into DNIC with glutathione or cysteine. Noteworthy, irrespective of the procedure used for their synthesis DNIC with glutathione manifest much higher stability than DNIC with cysteine. The pattern of spin density distribution in iron-dinitrosyl fragments of DNIC characterized by the d7 electronic configuration of the iron atom and described by the formula Fe+(NO+)2 is unique in that it provides a plausible explanation for the ability of DNIC to generate NO and nitrosonium ions (NO+) and the peculiar characteristics of the EPR signal of their mononuclear form (M-DNIC).