A novel, laser flash-quench approach to high oxidation state oxo-bridged diiron enzyme mimicry is described. Specifically, the enzymes soluble methane monooxygenase (MMO) and class I ribonucleotide reductase (RR) proceed through diiron(IV) (MMO) or iron(III)-iron(IV) (RR) intermediates at critical junctures in their catalytic cycles. Such unstably high oxidation states are difficult to achieve in model complexes. A brief review of relevant biochemistry is followed by the synthetic and photochemical strategy devised to generate diiron centers in the requisite oxidation states. An extensively characterized oxo-bondged diferric complex is tethered covalently, at both ends, to well-studied ruthenium or rhenium lumophores possessing long-lived triplet excited states. Electronic excitation of the photoactive part begins a redox cascade, where an external acceptor is reduced and one or both ferric sites are oxidized to iron(IV). The fate of the iron(IV) species can be monitored with time-resolved laser absorption spectroscopy. In additional experiments, enzyme substrates or substrate analogues are added, and their oxidation products, if any, are characterized. Control experiments are described to confirm that substrate reactivity indeed originates from iron(IV). The strategy, if successful, would allow a quantitative description of FeIV FeIV or FeIII FeIV -complex reactivity towards oxidizable substrates, with direct applicability to biochemical research in oxodiiron enzymes. The ideas within are applied to iron enzymes, but extend to other metals as well.
