Mimicking of the histidine brace structural motif in molecular copper(I) compounds

Abstract

Dedicated to Professor Hansgeorg Schnöckel on occasion of his 80 th birthday L-Nτ-methylhistidine methyl ester, MeHisOMe, has been employed as a potential ligand to mimic the histidine brace-type coordination of copper ions in enzymes such as the particulate methane monoxygenase or lytic polysaccharide monooxygenases. MeHisOMe was prepared by double-methylation of histidine methyl ester. Subsequently, its complexation by diphosphine copper(I) precursors [Cu(P^P)(MeCN) 2 ]BF 4 was tested, which led to the complexes [Cu(P^P)(MeHisOMe)]BF 4 (P^P = dpePhos: 1, P^P = XantPhos: 2, P^P = dppf: 3). 1-3 were fully characterized, also by single crystal X-ray analysis, thus providing first structural data for copper complexes with a synthetic, authentic histidine brace. The complexes proved inert in contact with dioxygen. To improve the biomimetic character attempts were made to formally replace the diphosphine ligands by bis(pyrazolyl)methanes, Bpm. Correspondingly, [BpmCu(NCMe) x ]BF 4 precursors were synthesized, with different substituents at the 3-positions of the pyrazolyl (i. e. Bpm = di(3-(phenyl)-1H-pyrazol-1-yl)diphenylmethane, di(3-(mesityl)-1Hpyrazol-1-yl)methane and di(3-(tert-butyl)-1H-pyrazol-1-yl) diphenylmethane). Addition of MeHisOMe to these complexes led to products that were so sensitive towards oxidation by the environment that they eluded isolation. One experiment provided blue crystals as a product of such a reaction. They belonged to a salt with a complex cation consisting of a Cu(μ-OH) 2 Cu core ligated by two MeHisOMe ligands, which dimerises in the solid state to give [Cu 4 (OH) 4 (MeHisOMe) 4 ] 4 + .

LPMO‐like Activity of Bioinspired Copper Complexes: From Model Substrate to Extended Polysaccharides

“…Since these first reports, several complexes modelling the coordination sphere of LPMO were prepared using conventional N ‐based ligands (pyridines, (benz)imidazoles, amines) [31–37] . or amino acid/peptide‐based ligands to reproduce the primary amine ligation found in the enzymatic system [38–40] . A variety of substrates were used to evaluate their activity ranging from simple benzyl alcohol [38] to soluble substrates ( p ‐NPG, [31,32,37,39] cellobiose, [35] p ‐NP‐glycosidic substrates) [33] and one report on the use of insoluble cellulosic substrate [34] …”

“…[31][32][33][34][35][36][37] or amino acid/peptide-based ligands to reproduce the primary amine ligation found in the enzymatic system. [38][39][40] A variety of substrates were used to evaluate their activity ranging from simple benzyl alcohol [38] to soluble substrates (p-NPG, [31,32,37,39] cellobiose, [35] p-NP-glycosidic substrates) [33] and one report on the use of insoluble cellulosic substrate. [34] The variety of substrates and reaction conditions used to evaluate LPMO-like activity of the mimics renders comparison and structure-activity relationships complicated.…”