Redox Engineering of Myoglobin by Cofactor Substitution to Enhance Cyclopropanation Reactivity.

Design of metal cofactor ligands is essential for controlling the reactivity of metalloenzymes. We investigated a carbene transfer reaction catalyzed by myoglobins containing iron porphyrin cofactors with one and two trifluoromethyl groups at peripheral sites (FePorCF3 and FePor(CF3)2, respectively), native heme and iron porphycene (FePc). These four myoglobins show a wide range of Fe(II)/Fe(III) redox potentials in the protein of +147 mV, +87 mV, +42 mV and -198 mV vs. NHE, respectively. Myoglobin reconstituted with FePor(CF3)2 has a more positive potential, which enhances the reactivity of a carbene intermediate with alkenes, and demonstrates superior cyclopropanation of inert alkenes, such as aliphatic and internal alkenes. In contrast, engineered myoglobin reconstituted with FePc has a more negative redox potential, which accelerates the formation of the intermediate, but has low reactivity for inert alkenes. Mechanistic studies indicate that myoglobin with FePor(CF3)2 generates an undetectable active intermediate with a radical character. In contrast, this reaction catalyzed by myoglobin with FePc includes a detectable iron-carbene species with electrophilic character. This finding highlights the importance of redox-focused design of the iron porphyrinoid cofactor in hemoproteins to tune the reactivity of the carbene transfer reaction.

Intramolecular C-H bond amination catalyzed by myoglobin reconstituted with iron porphycene.

C-H bond amination is an effective way to obtain nitrogen-containing products. In this work, we demonstrate that myoglobin reconstituted with iron porphycene (rMb(FePc)) catalyzes intramolecular C(sp3)-H bond amination of arylsulfonyl azides to yield corresponding sultam analogs. The total turnover number of rMb(FePc) is up to 5.7 × 104 for the C-H bond amination of 2,4,6-triisopropylbenzenesulfonyl azide. Moreover, rMb(FePc) exhibits higher selectivity for the desired C-H bond amination than the competing azide reduction compared to native myoglobin. Kinetic studies reveal that the kcat value of rMb(FePc) is 4-fold higher than that of native myoglobin. Furthermore, H64A, H64V and H64I mutants of rMb(FePc) enhance the turnover number (TON) and enantioselectivity for the C-H bond amination of 2,4,6-triethylbenzenesulfonyl azide. The present findings indicate that iron porphycene is an attractive artificial cofactor for myoglobin toward the C-H bond amination reaction.

Photo-induced imine reduction by a photoredox biocatalyst consisting of a pentapeptide and a Ru bipyridine terpyridine complex.

Imine reduction is a useful reaction in the preparation of amine derivatives. Various catalysts have been reported to promote this reaction and photoredox catalysts are promising candidates for sustainable amine synthesis. Improvement of this reaction using biomolecule-based reaction scaffolds is expected to increase the utility of the reaction. In this context, we have recently investigated photoredox Ru complexes with pentapeptide scaffolds via coordination bonds as catalysts for photoreduction of dihydroisoquinoline derivatives. First, Ru bipyridine terpyridine complexes coordinated with five different pentapeptides (XVHVV: X = V, F, W, Y, C) were prepared and characterized by mass spectrometry. Catalytic activities of the Ru complexes with XVHVV were evaluated for photoreduction of dihydroisoquinoline derivatives in the presence of ascorbate and thiol compounds as sacrificial reagents and hydrogen sources. Interestingly, the turnover number of the Ru complex with VVHVV is 531, which is two-fold higher than that of a simple Ru complex with an imidazole ligand. The detailed emission lifetime measurements indicate that the enhanced catalytic activity provided by the peptide scaffold is caused by an efficient reaction with the thiol derivative to accelerate reductive quenching of Ru complex. The quenching behavior suggests formation of an active species such as a Ru(I) complex. These findings reveal that the simple pentapeptide serves as an effective scaffold to enhance the photocatalytic activity of a photoactive Ru complex.