Proton-Shuffle Mechanism of O−O Activation for Formation of a High-Valent Oxo−Iron Species of Bleomycin

Bleomycins (BLMs) can utilize H<sub>2</sub>O<sub>2</sub> to cleave DNA in the presence of ferric ions. DFT calculations were used to study the mechanism of O−O bond cleavage in the low-spin Fe<sup>III</sup>−hydroperoxo complex of BLM. The following alternative hypotheses were investigated using realistic structural models:  (a) heterolytic cleavage of the O−O bond, generating a Compound I (Cpd I) like intermediate, formally BLM−Fe<sup>V</sup>O; (b) homolytic O−O cleavage, leading to a BLM−Fe<sup>IV</sup>O species and an OH<sup>•</sup> radical; and (c) a direct O−O cleavage/H-abstraction mechanism by ABLM. The calculations showed that (a) is a facile and viable mechanism; it involves acid−base proton reshuffle mediated by the side-chain linkers of BLM, causing thereby heterolytic cleavage of the O−O bond and generation of Cpd I. Formation of Cpd I is found to involve a barrier of 13.3 kcal/mol, which is lower than the barriers in the alternative mechanisms (b and c) that possess respective barriers of 31 and 17 kcal/mol. The so-formed Cpd I species with a radical on the side-chain linker, methylvalerate (<b>V</b>), adjacent to the BLM−Fe<sup>IV</sup>O complex, resembles the formation of the active species of cytochrome <i>c</i> peroxidase in the Poulos−Kraut proton-shuffle mechanism in heme peroxidases (Poulos, T. L.; Kraut, J. <i>J. Biol. Chem.</i> <b>1980</b>, <i>255</i>, 8199−8205). Experimental data are discussed and shown to be in accord with this proposal. It suggests that the high-valence Cpd I species of BLM participates in the DNA cleavage. This is an alternative mechanistic hypothesis to the exclusive reactivity scenario based on ABLM (Fe<sup>III</sup>−OOH).