Model Studies of Catechol Dioxygenases. Important Role of Monodentate Catecholate-Iron(III) Intermediate

The catechol dioxygenase model reactions of three model systems have been investigated by EPR, optical, and electrochemical studies. In each model system, some kinds of intermediates could be detected by EPR and optical spectroscopies. The intermediate structures and the reaction times suggest that the monodentate catecholate complexes play an important role in the catalytic cycle. Based on the EPR spectra obtained aerobically and anaerobically, iron(III)-monodentate dianionic catecholate is the O2 reactive species for Fe(nta) (nta = nitrilotriacetato) and Fe(sal-l-aa)Cl (sal-l-aa = N-salicylidene l-amino acidato) systems and iron(II)-semiquinonate for Fe(salen)Cl (salen = N,N′-ethylenebis(salicylideneaminato) system. Electrochemical data suggest that this electron transfer in Fe(salen)Cl system is caused by the ligand distortion. The catecholate-(sal-l-val)iron(III) complex reacted with dioxygen to yield the ring cleavage products (ca. 80%). On the basis of the observations, the novel reaction mechanism of the Fe(sal-l-aa)Cl system having mainly monodentate catecholate intermediate, is proposed. Finally, the correlation between the coordination environments of non- heme iron(III) complexes and EPR parameters is discussed.