Regioselective oxidative carbon-oxygen bond cleavage catalysed by copper(II) complexes: A relevant model study for lytic polysaccharides monooxygenases activity

Muthuramalingam, Sethuraman; Maheshwaran, Duraiyarasu; Velusamy, Marappan; Mayilmurugan, Ramasamy

doi:10.1016/j.jcat.2019.03.019

“…Although cupric–superoxo and oxyl complexes have been suggested as the reactive intermediates, a mechanistic insight may be obtained from synthetic models. In this regard, few biomimetic LPMO-inspired copper complexes have been reported to date, 7 despite the numerous reports on biologically inspired copper–oxygen systems. 8…”

Section: Introductionmentioning

confidence: 99%

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

Neira

¹

,

Martínez‐Alanis

²

,

Aullón

³

et al. 2019

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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 ...

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“…The absorption peak at 620 nm is reported to be specific to the 3N Cu coordination. 33 The position of these bands indicates a square-pyramidal geometry around the Cu(II) coordination sphere with the d x 2 – y 2 ground state 4 , 34 , 35 and is close to the d–d transition seen in LPMO (655 nm). 35 , 36 The absence of any absorption band at 520 nm further confirms the 3N geometry.…”

Section: Resultsmentioning

confidence: 59%

“… 35 , 37 The complex exhibits EPR parameters very close to those of the LPMOs ( g ∥ = 2.23–2.28; g ⊥ = 2.06–2.09). 35 A simulation of the EPR spectrum was obtained to validate the spectral features of the complex formed at pH 7 ( Figure S6 , Supporting Information). Comparison of the EPR spectra of the complex and copper salt at pH 7 ( g ∥ , 2.50 > g 2.11, Figure S8 , Supporting Information) suggests that there is no leaching of Cu(II) from the complex, which is in line with our observation in UV–vis analysis.…”

Section: Resultsmentioning

confidence: 85%

See 1 more Smart Citation

Minimalist De Novo Design of an Artificial Enzyme

Saikia

¹

,

Bhat

²

,

Potnuru

³

et al. 2022

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We employed a reductionist approach in designing the first heterochiral tripeptide that forms a robust heterogeneous short peptide catalyst similar to the “histidine brace” active site of lytic polysaccharide monooxygenases. The histidine brace is a conserved divalent copper ion-binding motif that comprises two histidine side chains and an amino group to create the T-shaped 3N geometry at the reaction center. The geometry parameters, including a large twist angle (73°) between the two imidazole rings of the model complex, are identical to those of native lytic polysaccharide monooxygenases (72.61°). The complex was synthesized and characterized as a structural and functional mimic of the histidine brace. UV–vis, vis-circular dichroism, Raman, and electron paramagnetic resonance spectroscopic analyses suggest a distorted square-pyramidal geometry with a 3N coordination at pH 7. Solution- and solid-state NMR results further confirm the 3N coordination in the copper center of the complex. The complex is pH-dependent and could catalyze the oxidation of benzyl alcohol in water to benzaldehyde with yields up to 82% in 3 h at pH 7 and above at 40 °C. The catalyst achieved 100% selectivity for benzaldehyde compared to conventional copper catalysis. The design of such a minimalist building block for functional soft materials with a pH switch can be a stepping stone in addressing needs for a cleaner and sustainable future catalyst.

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“…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. In the first two cases, D‐allose was also identified as product, instead of the expected gluconic acid upon hydrolysis of the initially formed gluconolactone; in the latter, the saccharide byproducts were not identified.…”

Section: Methodsmentioning

confidence: 99%

Cellulose Depolymerization with LPMO‐inspired Cu Complexes

Castillo

¹

,

Torres-Flores

²

,

Abad-Aguilar

³

et al. 2021

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Bis(benzimidazole)amine‐based copper complexes, with structural similarities to the active sites of Lytic Polysaccharide Monooxygenase enzymes (LPMOs), were tested for the oxidative degradation of cellulose. Spectroscopic characterization of the complexes, as well as structural authentication of one of them, confirm a tetragonal coordination environment with 3 nitrogen donors, as well as a thioether in axial positions. Aqueous oxidative degradation of cellulose was achieved with the CuII complexes and H2O2 as oxidant through putative cupric‐hydroperoxo intermediates. Conversion of cellulose was achieved in up to 67 % yield of soluble oligosaccharide derivatives at ambient temperature and pressure. The products were analyzed in aqueous solution by HPLC‐MS, confirming oxidative depolymerization of cellulose under ambient conditions, in an analogous fashion to LPMOs.

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Regioselective oxidative carbon-oxygen bond cleavage catalysed by copper(II) complexes: A relevant model study for lytic polysaccharides monooxygenases activity

Cited by 22 publications

References 61 publications

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

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

Minimalist De Novo Design of an Artificial Enzyme

Cellulose Depolymerization with LPMO‐inspired Cu Complexes

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