Heme Peroxo Reaction Intermediates: Mimicking Biological Structure and Reactivity With Synthetic Model Systems

Heme enzymes are at the center of a mélange of salient transformations in biology, and for aerobic life, dioxygen binding and activation are by far the most critical. These enzymes typically channel through a panel of distinct heme-oxygen adducts, of which early- or mid-valent (i.e., Fe(III)-containing) intermediates have long remained underexplored. These ferric heme-oxygen intermediates demonstrate highly versatile chemistries, making the detailed comprehension of their chemistries a challenging endeavor. Intriguingly, however, systems with climacteric mid-valent intermediates have rapidly acquired interest in therapeutic applications, predominantly owing to their implications in human pathogenesis and prognosis. This review presents a comprehensive overview of mechanistic propositions for enzymes with heme-peroxo active species, and the key contributions from synthetic model compounds that have shed light on crucial subtleties geared toward a clear understanding of those biological pathways. Critical aspects of primary and secondary coordination spheres that directly assist in orchestrating the remarkably versatile reactivity attributes of heme peroxo complexes are also discussed. Most significantly, despite 40 years of interrogation, a crucial body of ambiguities pertaining to heme peroxo intermediates still remains to this day, warranting further in-depth biological surveying as well as meticulously designed bona fide functional model systems.