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

Abstract

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 (H 2 O) 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 (H 2 O) 2 ](OTf) 2 indicates that [( 2BB )Cu II (H 2 O) 2 ] 2+ is the main species from pH 4 to 7.5; above pH 7.5, the hydroxo-bridged species [{( 2BB )Cu II (H 2 O) x } 2 (μ-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 (H 2 O) 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 (H 2 O) 2 ](OTf) 2 /H 2 O 2 /NEt 3 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 (H 2 O) 2 ](OTf) 2 /H 2 O 2 /NEt 3 results in more extensive cellobiose degradation. Likewise, the use of both [( 2BB )Cu I ]OTf and [( 2BB )Cu II (H 2 O) 2 ...

Cellulose Depolymerization with LPMO‐inspired Cu Complexes

“…Whether hydrogen peroxide acts as a shunt or as the true oxidant in biological systems, its use in conjunction with model copper complexes results in higher degradation activity in all cases. Two model systems containing a mixed imidazole/pyridylamine‐based donor set, [8] and the bis(benzimidazole)amine‐based 2BB [9] (Scheme 1), provide examples of confirmed oxidative degradation of model substrates. The more recently reported systems based on 2‐pyridyl/diazepane, [10] bis(imidazole)amine, [11] and bis(2‐picolyl)amine [12] ligands degrade p ‐nitrophenolate‐derivatized glucose and/or disaccharides, with the p ‐nitrophenolate leaving group quantified as the end product by UV‐vis spectroscopy.…”

Mimicking the Cu Active Site of Lytic Polysaccharide Monooxygenase Using Monoanionic Tridentate N-Donor Ligands

“… 10 − 12 Even though the specific Cu species responsible for the proposed rate-determining step of hydrogen atom transfer (HAT) from saccharide substrates is unknown, several Cu–oxygen cores have been implicated, such as [Cu–OO] + , [Cu–OH] 2+ , and [Cu–O] + . 8 , 10 , 13 − 20 These cores have been studied in model complexes, in the gas phase, and/or computationally and have been shown to be competent at HAT from various O–H and C–H bonds.…”

Generation and Aerobic Oxidative Catalysis of a Cu(II) Superoxo Complex Supported by a Redox-Active Ligand

“…Copperbased systems, particularly those coupled with redox-active additives such as aminoxyl radicals, have emerged as effective catalysts for aerobic oxidations (Figure 1A). [12][13][14][15][16][17][18][19][20][21][22][23][24][25][26] Enabled by Cu's accessible redox chemistry, facile ligand exchange, and broad functional group compatibility, synthetic C-H functionalization methods have been developed to access two-electron processes, especially when combined with co-catalysts. [27][28][29][30][31] The efficacy of Cu in aerobic oxidation chemistry is in large part motivated by its prevalence in biological oxidations.…”