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In various non‐heme iron oxygenases the Fe(II) center is coordinated by 2 N and 1 O atoms of the 2‐His‐2‐carboxylate facial triad; however, most artificial model complexes bear only N‐based ligands. In an effort to closely mimic the coordination environment in α‐ketoglutarate dependent oxygenases, we have now employed the Me2tacnO ligand (4,7‐dimethyl‐1‐oxa‐4,7‐diazacyclononane) in the synthesis of the complexes [(Me2tacnO)FeCl2]2 (1‐NNO), [(Me2tacnO)FeCl3] (1 b‐NNO) and [(Me2tacnO)Fe(BF)Cl] (2‐NNO; BF=benzoylformate). The weaker donation of the O atom in the ligand was found to result in stronger binding of the ligand in trans‐position to the O‐atom of the ancillary ligand as compared to the corresponding complexes involving the Me3tacn (1,4,7‐trimethyl‐1,4,7‐triazacyclononane) ligand. Furthermore, by stopped‐flow techniques we could detect an intermediate (3‐NNO) in the reaction of 2‐NNO with O2. The spectroscopic features of 3‐NNO agree with the involvement of an Fe(IV)‐oxo intermediate and hence this study represents the first detection of such an intermediate in the O2 activation of artificial α‐ketoglutarate Fe(II) complexes. [ABSTRACT FROM AUTHOR]

A conceptual approach for the synthesis of quasi‐heteroleptic complexes with properly endo‐functionalized cages as ligands is presented. The cage ligand reported here is of a covalent organic nature, it has been synthesized via a dynamic combinatorial chemistry approach, making use of a masked amine. Inspired by enzymatic active sites, the described system bears one carboxylate and two imidazole moieties as independent ligating units through which it is able to coordinate to transition metals. Analysis of the iron(II) complex in solution and the solid state validates the structure and shows that no undesired but commonly observed dimerization process takes place. The solid‐state structure shows a five‐coordinate metal center with the carboxylate bidentately bound to iron, which makes Fe@2 an unprecedentedly detailed structural model complex for this kind of non‐heme iron oxygenases. As, as confirmed by the crystal structure, sufficient space for other organic ligands is available, the biologically relevant ligand α‐ketoglutarate is implemented. We observe biomimetic reaction behavior towards dioxygen that opens studies investigating Fe@2 as a functional model complex. [ABSTRACT FROM AUTHOR]

Living organisms have adapted to atmospheric dioxygen by exploiting its oxidizing power while protecting themselves against toxic side effects. Reactive oxygen and nitrogen species formed during oxidative stress, as well as high-potential reactive intermediates formed during enzymatic catalysis, could rapidly and irreversibly damage polypeptides were protective mechanisms not available. Chains of redox-active tyrosine and tryptophan residues can transport potentially damaging oxidizing equivalents (holes) away from fragile active sites and toward protein surfaces where they can be scavenged by cellular reductants. Precise positioning of these chains is required to provide effective protection without inhibiting normal function. A search of the structural database reveals that about one third of all proteins contain Tyr/Trp chains composed of three or more residues. Although these chains are distributed among all enzyme classes, they appear with greatest frequency in the oxidoreductases and hydrolases. Consistent with a redox-protective role, approximately half of the dioxygen-using oxidoreductases have Tyr/Trp chain lengths ≥3 residues. Among the hydrolases, long Tyr/Trp chains appear almost exclusively in the glycoside hydrolases. These chains likely are important for substrate binding and positioning, but a secondary redox role also is a possibility. [ABSTRACT FROM AUTHOR]