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

The potentially tridentate ligand bis[(1-methyl-2-benzimidazolyl)ethyl] amine (2BB) was employed to prepare copper complexes [(2BB)Cu-I]OTf and [(2BB)-Cu-II(H2O)(2)](OTf)(2) as bioinspired models of lytic polysaccharide copper-dependent monooxygenase (LPMO) enzymes. Solid-state characterization of [(2BB)Cu-I]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 [(2BB)-Cu-II(H2O)(2)](OTf)(2) indicates that [(2BB)Cu-II(H2O)(2)](OTf)(2), j is the main species from pH 4 to 7.5; above pH 7.5, the hydroxo-bridged species [{(2BB)Cu-II(H2O)(x)}(2)](mu-OH)(2)](2+) 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 2BB is precluded, and by extension, amine deprotonation in the histidine brace of LPMOs appears unlikely at neutral pH. The complexes [(2BB)Cu-I]OTf and [(2BB)Cu-II(H2O)(2)](OTf)(2) 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 [(2BB)Cu-I]OTf/O-2 or [(2BB)Cu-II(H2O)(2)](OTf)(2)/H2O2/NEt3 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 [(2BB)Cu-II (H2O)(2)](OTf)(2)/H2O2/NEt3 results in more extensive cellobiose degradation. Likewise, the use of both [(2BB)Cu-I]OTf and [(2BB)Cu-II (H2O)(2)](OTf)(2) with KO2 afforded cellobiose oxidation products. In all cases, a common Cu(II) complex formulated as [(2BB)Cu-II(OH)(H2O)](+) was detected by mass spectrometry as the final form of the complex.