Observation of the Fe–O2 and FeIV=O stretching Raman bands for dioxygen reduction intermediates of cytochrome bo isolated from Escherichia coli

Abstract
Reaction intermediates in dioxygen reduction by the E. coli cytochrome bo-type ubiquinol oxidase were studied by time-resolved resonance Raman spectroscopy using the artificial cardiovascular system. At 0-20 microseconds following photolysis of the enzyme-CO adduct in the presence of O2, we observed the Fe-O2 stretching Raman band at 568 cm-1 which shifted to 535 cm-1 with the 18O2 derivative. These frequencies are remarkably close to those of other oxyhemoproteins including dioxygen-bound hemoglobin and aa3-type cytochrome c oxidase. In the later time range (20-40 microseconds), other oxygen-isotope-sensitive Raman bands were observed at 788 and 361 cm-1. Since the 781 cm-1 band exhibited a downshift by 37 cm-1 upon 18O2 substitution, we assigned it to the FeIV=O stretching mode. This band is considered to arise from the ferryl intermediate, but its appearance was much earlier than the corresponding intermediate of bovine cytochrome c oxidase (> 100 microseconds). The 361 cm-1 band showed the 16O/18O isotopic frequency shift of 14 cm-1 similar to the case of bovine cytochrome c oxidase reaction.

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