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

Lytic polysaccharide monooxygenases (LPMOs) are copper-containing monooxygenase and catalyzing the oxidative cleavage of recalcitrant polysaccharides using dioxygen. The copper(II) complexes [Cu(L1)(H₂O)ClO₄]ClO₄ 1, [L1 = 4-methyl-1-[(pyridin-2-yl-methyl)]-1,4-diazepane]; [Cu(L2)(H₂O)ClO₄]ClO₄ 2, [L2 = 4-methyl-1-[(2-(pyridine-2-yl)ethyl]-1,4-diazepane] and [Cu(L3)(H₂O)ClO₄]ClO₄ 3, [L3 = 1-(4-methoxy-3,5-dimethyl-pyridin-2-yl)methyl)-4-methyl-1,4-diazepane] have been synthesized and characterized as the novel models for LPMOs. The molecular structures exhibit distorted square pyramidal geometry (τ 0.183–0.388) as similar to LPMOs. The Cu–N (1.99–2.02 Å) bond distances of the model complexes are almost identical to those of native LPMOs enzyme (1.9–2.2 Å). The 1, 4-diazepane backbone and pyridine unit of complexes provide reasonable structural resemblances to ‘histidine brace’ and histidine residues of LPMOs respectively. The spectral, redox and kinetic studies were performed in water to mimic accurate enzymatic reaction conditions. The well-defined Cu(II)/Cu(I) reduction couples were observed around 8–112 mV versus NHE, which is lower than that of LPMOs. The electronic spectra of the complexes showed the d-d transitions around 600–635 nm and axial EPR parameter (g_||, 2.28–2.29; A_|| 160–168 × 10⁻⁴ cm⁻¹), which are almost identical to that of LPMOs. The model complexes were catalyzed oxidative cleavage of model substrate p-nitrophenyl-β-D-glucopyranoside into p-nitrophenol and D-allose with a maximum yield up to 78.4% and TON, 300. The kinetics of reaction monitored by following the formation of an absorption band around at 400 nm corresponds to p-nitrophenol, showed the rate of 3.19–5.26 × 10⁻³ s⁻¹. The oxidative cleavage reaction may occur via Cu^II-OOH intermediate, whose formation accompanied by an electronic spectral signature around 375 nm with the rate of 1.61–9.06 × 10⁻³ s⁻¹. The Cu^II-OOH intermediate was characterized by spectral methods and its geometry was optimized by DFT calculations.