Flash photolysis and pulse radiolysis were used to generate redtictants in situ to study the electron transfer (ET) reactivity of the FeIV=O heine centers in mt/oglobin and cijtochrome c peroxidase. Reduction of a5RuIII groups covalently bound to surface histidines allowed intramolecular RuII → FeIV=O ET rates to be measured. Protonation of the oxene ligand was found to be largely rate determining in myoglobin, consistent with the lack of proton donors in its heme pocket. The large distance (21-23 A) between surface histidines and the heme in wild-type cytochrome c peroxidase prevented the determination of the rate-limiting step(s) involved in FeIV=O reduction in this peroxidase, and strategies for attach men t of an a rtificial redox center closer to its heme a re ou tlin ed. From the work performed to date,: pulse radiolysis appears to be a more versatile technique than flash photolysis for the study ofFen'=O heme reactivity in protins.
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