Non-heme ferrous centers and nitric oxide

The similarities and differences between iron-NO and iron-oxygen bonding have been the subjects of debate, and model compounds provide important correlations between structure, spectroscopy, and redox chemistry of iron nitrosyls. The nitrosyl groups in these complexes can be described as NO S = 0), NO S = 1/2), NO S = 0), NO (S = 1), and NO S = 1/2). The associated iron ions have charges and spins such that the overall numbers of iron d electrons plus NO Jt electrons are 6-8 in most experimentally accessible model compounds. The notation for these complexes follows the Enemark and Feltham conventions [51]. The EPR visible complexes are d , or (Fe-NO) , and d , or (Fe-(NO)2). Both S = 3/2 and S = 1/2 d complexes are found. Table 2 summarizes possible electronic states that might be considered for d iron-nitrosyl complexes. Representative references to those electronic states that are experimentally observed, or calculated, are given in the table footnotes. 

Although 5=1/2 nitrosyl complexes with heme iron are well known and described, low-spin (5 = 1/2) non-heme iron-NO examples are uncommon. Debate on the electronic descriptions of the low-spin d series has focused on whether the redox chemistry is ligand centered, as suggested by variation in infrared NO stretching frequencies, or metal centered, as implied by Mossbauer isomer shifts. What Serres et al. [52] describe as a unified picture of the electronic structure of low-spin non-heme iron nitrosyls in perhaps unprecedented detail has recently 

Several model iron-nitrosyl compounds with S = 3/2 group spins were examined by optical, infrared, resonance Raman, x-ray absorption, and calculations [53]. The data indicate a description of the complexes as antiferromagnetically coupled (Fe(lll) (S = 5/2) (NO ) (S = 1)) . A number of other studies of model compounds agree with this assignment of Su,t = 3/2 Fe-NO model compounds. In a separate study, the cis- and frans- isomers of the iron complex of 1,4,8,11-tetraazacyclo- 

Fe—NO as a Model of Iron-Reactive Oxygen Intermediates in Proteins 

In contrast, superoxide reacts with ferrous iron in SOR, and in this case, the fer-rous-NO - ferric-NO interaction must model formation of a ferric-peroxyl complex, but with one less electron [55]  

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