Bioinspired copper(I) complexes that exhibit monooxygenase and catechol dioxygenase activity

Acid-catalyzed Transformation of Nitrite to Nitric Oxide on Copper(II)-Cobalt(II) Centers in a Bimetallic Complex

Nitrite (NO2 -) serves as a pool of nitric oxide (NO) in biological systems under hypoxic conditions, and it is transformed to NO by nitrite reductase (NiR) enzyme in the presence of acid (H+ ions). However, NO synthases (NOSs) generate NO via L-arginine oxidation in normoxic conditions. Previously, acid-induced NO2 - reduction chemistry was modeled on mono-metallic 3d-metals, generating metal-nitrosyls or NO(g) with H2O or H2O2 products. Herein, to understand the relative potency of a bimetallic system, we report the acid-induced reductive conversion of η2-bound NO2 - to NO on CuII-CoII centers of a hetero-bimetallic CuII-nitrito-CoII complex, [(LN8H)CuII-NO2 --CoII]3+ (CuII-NO2 --CoII, 2) bearing an octadentate N8-cryptand ligand (LN8H). The CuII-NO2 --CoII generates [CuII(LN8H)CoII]4+ (1) upon reaction with one equiv. acid (HClO4, H+ ions source) with NO(g) via a presumed transient nitrousacid (ONOH) intermediate species. Likewise, this NO2 - reduction was found to form H2O, which is believed to be from the decomposition of H2O2, an intermediate species. In addition, complex 2, in the presence of more than one equiv. H+ ions also showed the formation of NO(g) with H2O. Mechanistic investigations, using 15N-labeled-15NO2 -, 18O-labeled-18O14N16O- and 2H-labeled-DClO4 (D+ source), revealed that the N-atom and O-atom in the 14/15NO and 14N18O gases are derived from NO2 - ligand and H-atom in H2O derived from H+-source, respectively.

Acid-induced nitrite reduction of nonheme iron(ii)-nitrite: mimicking biological Fe-NiR reactions

Nitrite reductase (NiR) catalyzes nitrite (NO₂ ^-) to nitric oxide (NO) transformation in the presence of an acid (H⁺ ions/pH) and serves as a critical step in NO biosynthesis. In addition to the NiR enzyme, NO synthases (NOSs) participate in NO production. The chemistry involved in the catalytic reduction of NO₂ ^-, in the presence of H⁺, generates NO with a H₂O molecule utilizing two H⁺ + one electron from cytochromes and is believed to be affected by the pH. Here, to understand the effect of H⁺ ions on NO₂ ^- reduction, we report the acid-induced NO₂ ^- reduction chemistry of a nonheme Fe^II-nitrito complex, [(12TMC)Fe^II(NO₂ ^-)]⁺ (Fe^II-NO₂ ^-, 2), with variable amounts of H⁺. Fe^II-NO₂ ^- upon reaction with one-equiv. of acid (H⁺) generates [(12TMC)Fe(NO)]²⁺, {FeNO}⁷ (3) with H₂O₂ rather than H₂O. However, the amount of H₂O₂ decreases with increasing equivalents of H⁺ and entirely disappears when H⁺ reaches ≅ two-equiv. and shows H₂O formation. Furthermore, we have spectroscopically characterized and followed the formation of H₂O₂ (H⁺ = one-equiv.) and H₂O (H⁺ ≅ two-equiv.) and explained why bio-driven NiR reactions end with NO and H₂O. Mechanistic investigations, using ¹⁵N-labeled-¹⁵NO₂ ^- and ²H-labeled-CF₃SO₃D (D⁺ source), revealed that the N atom in the {Fe^14/15NO}⁷ is derived from the NO₂ ^- ligand and the H atom in H₂O or H₂O₂ is derived from the H⁺ source, respectively.

A bio-inspired imidazole-functionalised copper cage complex

An endo-functionalized cage is presented that upon copper(i) complexation assembles to a well-defined structural and catalytically active biomimetic model compound.

Mechanistic insights into tyrosinase-mediated crosslinking of soy glycinin derived peptides

Tyrosinase from Bacillus megaterium (TyrBm) was previously used to modulate soy glycinin-based emulsions and gels. To study the crosslinking mechanism, TyrBm oxidation of three tyrosine-containing octapeptides derived from glycinin was analyzed by oxygen consumption measurements, absorbance and mass spectrometry. A significant lag period and lower activity were measured when tyrosine was located in the middle of the peptide chain. Mass spectrometry analysis showed that these peptides are crosslinked via the oxidative quinone ring of the tyrosine residue by aryl-alkylamine addition or aryloxy radical coupling to form di-DOPA (3,4-dihydroxyphenylalanine). In contrast, peptides containing tyrosine in the N- or C-terminus, were rapidly oxidized forming multimer units within thirty minutes. When small amino acids were adjacent to the terminus tyrosine, formation of di-tyrosine was observed. This work confirms that protein crosslinking by TyrBm occurs by several chemical mechanisms and may assist in designing peptide-based inhibitors for the food and cosmetic applications.

Rearrangements of organic peroxides and related processes

This review is the first to collate and summarize main data on named and unnamed rearrangement reactions of peroxides. It should be noted, that in the chemistry of peroxides two types of processes are considered under the term rearrangements. These are conventional rearrangements occurring with the retention of the molecular weight and transformations of one of the peroxide moieties after O-O-bond cleavage. Detailed information about the Baeyer-Villiger, Criegee, Hock, Kornblum-DeLaMare, Dakin, Elbs, Schenck, Smith, Wieland, and Story reactions is given. Unnamed rearrangements of organic peroxides and related processes are also analyzed. The rearrangements and related processes of important natural and synthetic peroxides are discussed separately.

Copper-catalyzed divergent oxidative pathways of 2-naphthol derivatives: ortho-naphthoquinones versus 2-BINOLs

Divergent reaction pathways of 2-naphthol derivatives to either ortho-naphthoquinones or BINOLs have been achieved using two different Cu(ii) catalyst systems.

Efficient Biomimetic Hydroxylation Catalysis with a Bis(pyrazolyl)imidazolylmethane Copper Peroxide Complex

Bis(pyrazolyl)methane ligands are excellent components of model complexes used to investigate the activity of the enzyme tyrosinase. Combining the N donors 3-tert-butylpyrazole and 1-methylimidazole results in a ligand that is capable of stabilising a (μ-η(2) :η(2) )-dicopper(II) core that resembles the active centre of tyrosinase. UV/Vis spectroscopy shows blueshifted UV bands in comparison to other known peroxo complexes, due to donor competition from different ligand substituents. This effect was investigated with the help of theoretical calculations, including DFT and natural transition orbital analysis. The peroxo complex acts as a catalyst capable of hydroxylating a variety of phenols by using oxygen. Catalytic conversion with the non-biological phenolic substrate 8-hydroxyquinoline resulted in remarkable turnover numbers. In stoichiometric reactions, substrate-binding kinetics was observed and the intrinsic hydroxylation constant, kox , was determined for five phenolates. It was found to be the fastest hydroxylation model system determined so far, reaching almost biological activity. Furthermore, Hammett analysis proved the electrophilic character of the reaction. This sheds light on the subtle role of donor strength and its influence on hydroxylation activity.

A Biomimetic Mechanism for the Copper-Catalyzed Aerobic Oxygenation of 4-tert-Butylphenol

Controlling product selectivity during the catalytic aerobic oxidation of phenols remains a significant challenge that hinders reaction development. This work provides a mechanistic picture of a Cu-catalyzed, aerobic functionalization of phenols that is selective for phenoxy-coupled ortho-quinones. We show that the immediate product of the reaction is a Cu(II)-semiquinone radical complex and reveal that ortho-oxygenation precedes oxidative coupling. This complex is the resting state of the Cu catalyst during turnover at room temperature. A mechanistic study of the formation of this complex at low temperatures demonstrates that the oxygenation pathway mimics the dinuclear Cu enzyme tyrosinase by involving a dinuclear side-on peroxodicopper(II) oxidant. Unlike the enzyme, however, the rate-limiting step of the ortho-oxygenation reaction is the self-assembly of the oxidant from Cu(I) and O2. We provide details for all steps in the cycle and demonstrate that turnover is contingent upon proton-transfer events that are mediated by a slight excess of ligand. Finally, our knowledge of the reaction mechanism can be leveraged to diversify the reaction outcome. Thus, uncoupled ortho-quinones are favored in polar, coordinating media, highlighting unusually high levels of chemoselectivity for a catalytic aerobic oxidation of a phenol.