Mimicking LPMO Active Sites with Synthetic copper(II) Complexes: Exploring the Minimal Requirements

Copper(II) complexes of the bidentate histamine (Hm) and its tridentate Schiff-base derivative (L) were studied as functional models of lytic polysaccharide monooxygenases (LPMOs) by X-ray crystallography, pH-potentiometry, UV-vis, and EPR methods. Although Schiff bases are rather sensitive to hydrolysis, in this case a strong intramolecular H-bond and the template effect of the metal ion allowed complete equilibrium and solution structural characterization of the Cu(II)-L species present between pH 2-10. The relatively stable mixed peroxo complexes of Hm and L undergo partial reduction to copper(I) at a high excess of HO, whereupon significant amounts of hydroxyl radicals were observed in these systems. In the presence of HO and the activated substrate p-nitrophenyl-β-d-glucopyranoside (PNPG), both systems acted as efficient functional models of LPMOs at pH 7.4 and 10.5, and the hydroxyl radical is proposed to be the main actor in the catalytic mechanism. Interestingly, the Cu(II)-Hm complexes are more active at both pH values than those present in the Cu(II)-L system, suggesting that the use of ligands suitable for tridentate T-shaped coordination is not a prerequisite for efficient LPMO-like molecular catalysts. This observation may open a new direction to develop a simple but robust LPMO-like molecular catalyst for potential practical applications.