Interplay between iron complexes, nitric oxide and sulfur ligands: Structure, (photo)reactivity and biological importance

Nitric oxide is one of the most important signalling and regulatory molecule in all animal and plant organisms. Various iron complexes are involved in numerous steps of nitric oxide metabolism; most of these complexes contain diverse sulfur ligands. Nitric oxide is naturally generated from the amino acid arginine in the reaction catalyzed by an enzyme called nitric oxide synthase. The active centre of this enzyme contains an iron porphyrin complex with one thiolate axial ligand. Iron nitrosyls and nitrosothiols are the most relevant agents responsible for storage and transport of the NO and related compounds. Most of the target receptors of nitric oxide also contain iron centres and/or thiol groups. Nitrosothiols and metal nitrosylcomplexes belong to the most important external sources of nitric oxide (NO-donors). The most complex interactions are observed in ternary iron–sulfur–nitrosyl systems. Depending on the nature of the bond, the ternary iron–sulfur–nitrosyl species may be classified into two groups, (i) iron complexes with the S-nitrosothiol ligand, containing the {Fe–N(SR)O} moieties and (ii) RS–Fe–NO compounds, where both NO and sulfurs are coordinated to the Fe-centre. Irrespective of the structure distinctness, the mutual interactions of all components are very strong due to considerable bond delocalization within both ternary systems. Some differences appear, however, in their sensitivity to the R nature.