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C(sp3)–H Bond Hydroxylation Catalyzed by Myoglobin Reconstituted with Manganese Porphycene

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journal contribution
posted on 20.11.2013, 00:00 by Koji Oohora, Yushi Kihira, Eiichi Mizohata, Tsuyoshi Inoue, Takashi Hayashi
Myoglobin reconstituted with manganese porphycene was prepared in an effort to generate a new biocatalyst and was characterized by spectroscopic techniques. The X-ray crystal structure of the reconstituted protein reveals that the artificial cofactor is located in the intrinsic heme-binding site with weak ligation by His93. Interestingly, the reconstituted protein catalyzes the H2O2-dependent hydroxylation of ethylbenzene to yield 1-phenylethanol as a single product with a turnover number of 13 at 25 °C and pH 8.5. Native myoglobin and other modified myoglobins do not catalyze C–H hydroxylation of alkanes. Isotope effect experiments yield KIE values of 2.4 and 6.1 for ethylbenzene and toluene, respectively. Kinetic data, log kobs versus BDE­(C­(sp3)–H) for ethylbenzene, toluene, and cyclohexane, indicate a linear relationship with a negative slope. These findings clearly indicate that the reaction occurs via a rate-determining step that involves hydrogen-atom abstraction by a Mn­(O) species and a subsequent rebound hydroxylation process which is similar to the reaction mechanism of cytochrome P450.