No abstract

Cytochrome c oxidase (CcO) catalyzes the reduction of dioxygen to water utilizing a heterobinuclear active site composed of a heme moiety and a mononuclear copper center coordinated to three histidine residues, one of which is covalently cross-linked to a tyrosine residue via a post-translational modification (PTM). Although this tyrosine-histidine moiety has functional and structural importance, the pathway behind this net oxidative C-N bond coupling is still unknown. A novel route employing an iron(III) meso-substituted isoporphyrin derivative, isoelectronic with Cmpd-I ((Por •+ )Fe IV =O), is for the first time proposed to be a key intermediate in the Tyr-His cofactor biogenesis. Newly synthesized iron(III) meso-substituted isoporphyrins were prepared with azide, cyanide, and substituted imidazole functionalities, by adding nucleophiles to an iron(III) π-dication species formed via addition of trifluoroacetic acid to F 8 Cmpd-I (F 8 = (tetrakis(2,6-difluorophenyl)porphyrinate)). Isoporphyrin derivatives were characterized at cryogenic temperatures via ESI-MS and UV-vis, 2 H NMR, and EPR spectroscopies. Addition of 1,3,5-trimethoxybenzene or 4-methoxyphenol to the imidazolesubstituted isoporphyrin led to formation of the organic product containing the imidazole coupled to aromatic substrate via a new C-N bond, as detected via cryo-ESI-MS. Experimental evidence for the formation of an imidazole-substituted isoporphyrin and its promising reactivity to form the imidazole-phenol coupled product yields viability to the herein proposed pathway behind the PTM (i.e., biogenesis) leading to the key covalent Tyr-His cross-link in CcO.

Manganese complexes with polydentate quinol-containing ligands are found to catalyze the degradation of superoxide through inner-sphere mechanisms. The redox activity of the ligand stabilizes higher-valent manganese species.

The enzyme tyrosinase contains a reactive side‐on peroxo dicopper(II) center as catalytically active species in C−H oxygenation reactions. The tyrosinase activity of the isomeric bis(μ‐oxo) dicopper(III) form has been discussed controversially. The synthesis of bis(μ‐oxo) dicopper(III) species [Cu2(μ‐O)2(L1)2](X)2 ([O1](X)2, X=PF6−, BF4−, OTf−, ClO4−), stabilized by the new hybrid guanidine ligand 2‐{2‐((dimethylamino)methyl)phenyl}‐1,1,3,3‐tetramethylguanidine (L1), and its characterization by UV/Vis, Raman, and XAS spectroscopy, as well as cryo‐UHR‐ESI mass spectrometry, is described. We highlight selective oxygenation of a plethora of phenolic substrates mediated by [O1](PF6)2, which results in mono‐ and bicyclic quinones and provides an attractive strategy for designing new phenazines. The selectivity is predicted by using the Fukui function, which is hereby introduced into tyrosinase model chemistry. Our bioinspired catalysis harnesses molecular dioxygen for organic transformations and achieves a substrate diversity reaching far beyond the scope of the enzyme.