Reactive Species with Fe-oxo Motifs

1 Reactive Species from Monomeric Heme Iron-Dioxygen Complexes 

Studies on P450 have dominated this field for the last 25 years and, while much has been discovered about the intermediates formed during turnover, controversies StiU exist One of the central issues involves the hydroxylation step in the mechanism (Fig. 6.1). Mechanistic studies have led researchers to propose that the [Fe(lV)=0(Hgand radical)] complex (compound I) homolyticaUy cleaves the C-H bond on the substrate to give an Fe(IV)-OH species and an alkyl radical intermediate (Fig. 6.17 A). The next step, termed oxygen rebound , has the alkyl radical attacking the hydroxo Ugand to form the iron-bound producL which is subsequently displaced by water [42]. 

The important functional role of the axial thiolate ligand in P450 has been acknowledged for several years. Green recently proposed that the thiolate is directly involved in the formation of compound I [44]. A series of DFT studies indicated that the thiolate acquires radical character during compound I formation. 

This opposes the normal depiction of this species, which has the ligand-radical centered on the porphyrin. Compound I has not been cleanly isolated and thus it is difficult to discern these possibilities. Green argues that the [Fe(IV)=0(thio-late radical)] provides a reasonable explanation for the unusual electronic coupling observed in CPO-compound I, yet this issue will not be reconciled until better experimental data are obtained. 

2 Reactive Species from Monomeric Nonheme Iron-Dioxygen Complexes 

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