Reaction Intermediates in Oxygen Activation Reactions by Enzymes Containing Carboxylate-Bridged Binuclear Iron Clusters

The rapid freeze-quench method was used to trap intermediates in the reaction of O₂ with the dinuclear iron(II) clusters in the soluble methane monooxygenase from Methylococcus capsulatus (Bath) and in the R2 subunit of the ribonucleotide reductase from Escherichia coli. Mössbauer and EPR spectroscopies were used to characterize the trapped intermediates. For methane monooxygenase, the initial intermediate formed is a peroxodiiron(III) complex which undergoes further structural and electron reorganization to form a high-valent, formally diiron(IV), complex termed Q. In the case of R2, reaction of O₂ with the diiron(II) center in the presence of reducing agents generates a mixed valent, formally Fe(III), Fe(IV), complex termed X, which is capable of oxidizing the proximal Y122 to its radical form with formation of the resting diiron(III) cluster. In this chapter, spectroscopic characteristics of these intermediates are presented. Structural and mechanistic implications are also discussed.