Peroxo-Type Intermediates in Class I Ribonucleotide Reductase and Related Binuclear Non-Heme Iron Enzymes

We have performed a systematic study of chemically possible peroxo-type intermediates occurring in the non-heme di-iron enzyme class Ia ribonucleotide reductase, using spectroscopically calibrated computational chemistry. Density functional computations of equilibrium structures, Fe−O and O−O stretch frequencies, Mössbauer isomer shifts, absorption spectra, J-coupling constants, electron affinities, and free energies of O2 and proton or water binding are presented for a series of possible intermediates. The results enable structure−property correlations and a new rationale for the changes in carboxylate conformations occurring during the O2 reaction of this class of non-heme iron enzymes. Our procedure identifies and characterizes various possible candidates for peroxo intermediates experimentally observed along the ribonucleotide reductase dioxygen activation reaction. The study explores how water or a proton can bind to the di-iron site of ribonucleotide reductase and facilitate changes that affect the electronic structure of the iron sites and activate the site for further reaction. Two potential reaction pathways are presented: one where water adds to Fe1 of the cis-μ-1,2 peroxo intermediate P causing opening of a bridging carboxylate to form intermediate P′ that has an increased electron affinity and is activated for proton-coupled electron transfer to form the Fe(III)Fe(IV) intermediate X; and one that is more energetically favorable where the P to P′ conversion involves addition of a proton to a terminal carboxylate ligand in the site which increases the electron affinity and triggers electron transfer to form X. Both pathways provide a mechanism for the activation of peroxy intermediates in binuclear non-heme iron enzymes for reactivity. The studies further show that water coordination can induce the conformational changes observed in crystal structures of the met state.