Model Iron–Oxo Species and the Oxidation of Imidazole: Insights into the Mechanism of OvoA and EgtB?

A density functional theory cluster and first-principles quantum and statistical mechanics approach have been used to investigate the ability of iron–oxygen intermediates to oxidize a histidine cosubstrate, which may then allow for the possible formation of 2- and 5-histidylcysteine sulfoxide, respectively. Namely, the ability of ferric superoxo (Fe^IIIO₂^•–), Fe^IVO, and ferrous peroxysulfur (Fe^IIIOOS) complexes to oxidize the imidazole of histidine via an electron transfer (ET) or a proton-coupled electron transfer (PCET) was considered. While the high-valent mononuclear Fe^IVO species is generally considered the ultimate biooxidant, the free energies for its reduction (via ET or PCET) suggest that it is unable to directly oxidize histidine’s imidazole. Instead, only the ferrous peroxysulfur complexes are sufficiently powerful enough oxidants to generate a histidyl-derived radical via a PCET process. Furthermore, while this process preferably forms a HisN_δ(−H)^• radical, several such oxidants are also suggested to be capable of generating the higher-energy HisC_δ(−H)^• and HisC_ε(−H)^• radicals. Importantly, the present results suggest that formation of the sulfoxide-containing products (seen in both OvoA and EgtB) is a consequence of the reduction of a powerful Fe^IIIOOS oxidant via a PCET.