How is Nitric Oxide (NO) Converted into Nitrosonium Cations (NO) in Living Organisms? (Based on the Results of Optical and EPR Analyses of Dinitrosyl Iron Complexes with Thiol-Containing Ligands).

The present work provides theoretical and experimental foundations for the ability of dinitrosyl iron complexes (DNICs) with thiolcontaining ligands to be not only the donors of neutral NO molecules, but also the donors of nitrosonium cations (NO) in living organisms ensuring S-nitrosation of various proteins and low-molecular-weight compounds. It is proposed that the emergence of those cations in DNICs is related to disproportionation reaction of NO molecules, initiated by their binding with Fe ions (two NO molecules per one ion). At the same time, possible hydrolysis of iron-bound nitrosonium cations is prevented by the electron density transition to nitrosonium cations from sulfur atoms of thiol-containing ligands, which are included in the coordination sphere of iron. It allows supposing that iron in iron-nitrosyl complexes of DNICs has a electronic configuration. This supposition is underpinned by experimental data revealing that a half of nitrosyl ligands are converted into S-nitrosothiols (RSNOs) when those complexes decompose, with the other half of those ligands released in the form of neutral NO molecules.