Nonheme iron nitrosyl complexes

Lancaster, J. R., Wemerfelmayer, G., and Wachter, H. (1994). Coinduction of nitric oxide synthesis and intracellular nonheme iron-nitrosyl complexes in murine cytokine-treated fibroblasts. Free Radical Biol. Med. 16, 869-870. 

Stadler, J., Bergonia, H. A., Di Silvio, M., Sweetland, M. A., Billiar, T. R., Simmons, R. L., and Lancaster, J. R., Jr. (1993). Nonheme iron-nitrosyl complex formation in rat hepatocytes Detection by electron paramagnetic resonance spectroscopy. Arch. Biochem. Biophys. 302, 4-11. 

The first detailed EPR studies of biological nonheme iron-nitrosyl complexes were carried out primarily by the laboratories of Commoner, Kayushin, and Vanin. Utilizing the results of a study by McDonald et al. on small inorganic complexes, the characteristic axial g = 2.04 signal resulting from the exposure of biological samples to -NO or -NO-generating systems, ... 

A large number of iron-containing proteins form nitrosyl complexes. Heme proteins, iron-sulfur proteins, and other iron proteins such as nonheme iron dioxygenases all form characteristic nitrosyl complexes. In enzymes in which the metal center has an open coordination position, NO often can be bound without severe disruption of the site. This introduces the possibility of reversibility of inhibition. 

Nonheme ferrous centers in some metalloproteins react reversibly with NO forming nitrosyl complexes with S = 312 characterized by the g values of about 4.0 and 2.0 [51]. The EPR spectrum of the nitrosylated NorR (abacterial NO-responsive transcription factor, the enhancer binding protein) is typical of a d high-spin Fe NO", where the S = 5/2 iron is antiferromagnetically coupled to the NO (Fig. 5, [52]). This is confirmed by the X-ray, resonance Raman, MCD, Mossbauer spectroscopies, and DFT calculations. Similar structures were proposed for the classical complexes, [Fe(NO)(l-isopropyl-4,7-(4-ferf-butyl-2mercaptobenzyl)-l,4, 7-triazacyclononane)], [53], Fe(EDTA)NO [54—56], the brown-ring compound, Fe(H20)5N0 [57], and for the Fe(N/V ,N -trimethyl-l,4,7-triazacyclononane) (N3)2N0 [54]. Interestingly, for the latter a spin equilibrium between the valence tautomers 5=1/2 and 3/2 in the solid state was observed. 

The EPR-silent [Fe(Papy3)(NO)] complex is very stable, allowing for the preparation of crystaUine solids. The NO-release represented by reversing reaction (10) is not available thermally, but it becomes possible upon illumination (21). In a similar way, the compoimd [Fe(bpb)(N0)(N02)] complex was synthesized upon bubbhng NO into an oxygen-free acetonitrile solution of the low-spin [Fe (bpb)(py)2] ion (a nonheme planar analog of the iron(III)-porphyrinate) (57). Notice that the high-spin complex [Fe (bpb)(Cl)2] did not afford any isolable nitrosyl, a fact that has been employed to propose that the spin state of the iron center of the precursor complex dictates its affinity toward NO. 

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