Oxidative Cleavage of Cellobiose by Lytic Polysaccharide Monooxygenase (LPMO)-Inspired Copper Complexes.

The potentially tridentate ligand bis[(1-methyl-2-benzimidazolyl)ethyl]amine () was employed to prepare copper complexes [()Cu]OTf and ()Cu(HO) as bioinspired models of lytic polysaccharide copper-dependent monooxygenase (LPMO) enzymes. Solid-state characterization of [()Cu]OTf revealed a Cu(I) center with a T-shaped coordination environment and metric parameters in the range of those observed in reduced LPMOs. Solution characterization of ()Cu(HO) indicates that [()Cu(HO)] is the main species from pH 4 to 7.5; above pH 7.5, the hydroxo-bridged species [{()Cu(HO) }(μ-OH)] is also present, on the basis of cyclic voltammetry and mass spectrometry. These observations imply that deprotonation of the central amine of Cu(II)-coordinated is precluded, and by extension, amine deprotonation in the histidine brace of LPMOs appears unlikely at neutral pH. The complexes [()Cu]OTf and ()Cu(HO) act as precursors for the oxidative degradation of cellobiose as a cellulose model substrate. Spectroscopic and reactivity studies indicate that a dicopper(II) side-on peroxide complex generated from [()Cu]OTf/O or ()Cu(HO)/HO/NEt oxidizes cellobiose both in acetonitrile and aqueous phosphate buffer solutions, as evidenced from product analysis by high-performance liquid chromatography-mass spectrometry. The mixture of ()Cu(HO)/HO/NEt results in more extensive cellobiose degradation. Likewise, the use of both [()Cu]OTf and ()Cu(HO) with KO afforded cellobiose oxidation products. In all cases, a common Cu(II) complex formulated as [()Cu(OH)(HO)] was detected by mass spectrometry as the final form of the complex.