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1. Cu( i )–O ₂ oxidation reactions in a fluorinated all-O-donor ligand environment

   https://doi.org/10.1039/C8DT05028G  

   Brazeau, Sarah E. ; Doerrer, Linda H. ( April 2019 , Dalton Transactions)

   Investigation of Cu–O 2 oxidation reactivity is important in biological and anthropogenic chemistry. Zeolites are one of the most promising Cu/O based oxidation catalysts for development of industrial-scale CH 4 to CH 3 OH conversion. Their oxidation mechanisms are not well understood, however, highlighting the importance of the investigation of molecular Cu( i )–O 2 reactivity with O-donor complexes. Herein, we give an overview of the synthesis, structural properties, and O 2 reactivity of three different series of O-donor fluorinated Cu( i ) alkoxides: K[Cu(OR) 2 ], [(Ph 3 P)Cu(μ-OR) 2 Cu(PPh 3 )], and K[(R 3 P)Cu(pin F )], in which OR = fluorinated monodentate alkoxide ligands and pin F = perfluoropinacolate. This breadth allowed for the exploration of the influence of the denticity of the ligand, coordination number, the presence of phosphine, and K⋯F/O interactions on their O 2 reactivity. K⋯F/O interactions were required to activate O 2 in the monodentate-ligand-only family, whereas these connections did not affect O 2 activation in the bidentate complexes, potentially due to the presence of phosphine. Both families formed trisanionic, trinuclear cores of the form {Cu 3 (μ 3 -O) 2 } 3− . Intramolecular and intermolecular substrate oxidation were also exploredmore »and found to be influenced by the fluorinated ligand. Namely, {Cu 3 (μ 3 -O) 2 } 3− from K[Cu(OR) 2 ] could perform both monooxygenase reactivity and oxidase catalysis, whereas those from K[(R 3 P)Cu(pin F )] could only perform oxidase catalysis.« less
2. Electronic structures and spectroscopic signatures of diiron intermediates generated by O ₂ activation of nonheme iron( ii )–thiolate complexes

   https://doi.org/10.1039/d1dt02286e  

   Ekanayake, Danushka M. ; Pham, Dao ; Probst, Andrew L. ; Miller, Joshua R. ; Popescu, Codrina V. ; Fiedler, Adam T. ( October 2021 , Dalton Transactions)

   The activation of O 2 at thiolate–ligated iron( ii ) sites is essential to the function of numerous metalloenzymes and synthetic catalysts. Iron–thiolate bonds in the active sites of nonheme iron enzymes arise from either coordination of an endogenous cysteinate residue or binding of a deprotonated thiol-containing substrate. Examples of the latter include sulfoxide synthases, such as EgtB and OvoA, that utilize O 2 to catalyze tandem S–C bond formation and S -oxygenation steps in thiohistidine biosyntheses. We recently reported the preparation of two mononuclear nonheme iron–thiolate complexes (1 and 2) that serve as structural active-site models of substrate-bound EgtB and OvoA ( Dalton Trans. 2020, 49 , 17745–17757). These models feature monodentate thiolate ligands and tripodal N 4 ligands with mixed pyridyl/imidazolyl donors. Here, we describe the reactivity of 1 and 2 with O 2 at low temperatures to give metastable intermediates (3 and 4, respectively). Characterization with multiple spectroscopic techniques (UV-vis absorption, NMR, variable-field and -temperature Mössbauer, and resonance Raman) revealed that these intermediates are thiolate-ligated iron( iii ) dimers with a bridging oxo ligand derived from the four-electron reduction of O 2 . Structural models of 3 and 4 consistent with the experimental data were generated viamore »density functional theory (DFT) calculations. The combined experimental and computational results illuminate the geometric and electronic origins of the unique spectral features of diiron( iii )-μ-oxo complexes with thiolate ligands, and the spectroscopic signatures of 3 and 4 are compared to those of closely-related diiron( iii )-μ-peroxo species. Collectively, these results will assist in the identification of intermediates that appear on the O 2 reaction landscapes of iron–thiolate species in both biological and synthetic environments.« less
3. Genomic analysis of siderophore β-hydroxylases reveals divergent stereocontrol and expands the condensation domain family

   https://doi.org/10.1073/pnas.1903161116  

   Reitz, Zachary L. ; Hardy, Clifford D. ; Suk, Jaewon ; Bouvet, Jean ; Butler, Alison ( October 2019 , Proceedings of the National Academy of Sciences)

   Genome mining of biosynthetic pathways streamlines discovery of secondary metabolites but can leave ambiguities in the predicted structures, which must be rectified experimentally. Through coupling the reactivity predicted by biosynthetic gene clusters with verified structures, the origin of the β-hydroxyaspartic acid diastereomers in siderophores is reported herein. Two functional subtypes of nonheme Fe(II)/α-ketoglutarate–dependent aspartyl β-hydroxylases are identified in siderophore biosynthetic gene clusters, which differ in genomic organization—existing either as fused domains (IβH Asp ) at the carboxyl terminus of a nonribosomal peptide synthetase (NRPS) or as stand-alone enzymes (TβH Asp )—and each directs opposite stereoselectivity of Asp β-hydroxylation. The predictive power of this subtype delineation is confirmed by the stereochemical characterization of β-OHAsp residues in pyoverdine GB-1, delftibactin, histicorrugatin, and cupriachelin. The l - threo (2 S , 3 S ) β-OHAsp residues of alterobactin arise from hydroxylation by the β-hydroxylase domain integrated into NRPS AltH, while l - erythro (2 S , 3 R ) β-OHAsp in delftibactin arises from the stand-alone β-hydroxylase DelD. Cupriachelin contains both l - threo and l - erythro β-OHAsp, consistent with the presence of both types of β-hydroxylases in the biosynthetic gene cluster. A third subtype of nonheme Fe(II)/α-ketoglutarate–dependent enzymes (IβH His )more »hydroxylates histidyl residues with l - threo stereospecificity. A previously undescribed, noncanonical member of the NRPS condensation domain superfamily is identified, named the interface domain, which is proposed to position the β-hydroxylase and the NRPS-bound amino acid prior to hydroxylation. Through mapping characterized β-OHAsp diastereomers to the phylogenetic tree of siderophore β-hydroxylases, methods to predict β-OHAsp stereochemistry in silico are realized.« less
4. Mechanochemical synthesis and structural analysis of trivalent lanthanide and uranium diphenylphosphinodiboranates

   https://doi.org/10.1039/d1dt01932e  

   Fetrow, Taylor V. ; Daly, Scott R. ( August 2021 , Dalton Transactions)

   Phosphinodiboranates (H 3 BPR 2 BH 3 − ) are a class of borohydrides that have merited a reputation as weakly coordinating anions, which is attributed in part to the dearth of coordination complexes known with transition metals, lanthanides, and actinides. We recently reported how K(H 3 BP t Bu 2 BH 3 ) exhibits sluggish salt elimination reactivity with f-metal halides in organic solvents such as Et 2 O and THF. Here we report how this reactivity appears to be further attenuated in solution when the t Bu groups attached to phosphorus are exchanged for R = Ph or H, and we describe how mechanochemistry was used to overcome limited solution reactivity with K(H 3 BPPh 2 BH 3 ). Grinding three equivalents of K(H 3 BPPh 2 BH 3 ) with UI 3 (THF) 4 or LnI 3 (Ln = Ce, Pr, Nd) allowed homoleptic complexes with the empirical formulas U(H 3 BPPh 2 BH 3 ) 3 (1), Ce(H 3 BPPh 2 BH 3 ) 3 (2), Pr(H 3 BPPh 2 BH 3 ) 3 (3), and Nd(H 3 BPPh 2 BH 3 ) 3 (4) to be prepared and subsequently crystallized in good yields (50–80%). Single-crystalmore »XRD studies revealed that all four complexes exist as dimers or coordination polymers in the solid-state, whereas 1 H and 11 B NMR spectra showed that they exist as a mixture of monomers and dimers in solution. Treating 4 with THF breaks up the dimer to yield the monomeric complex Nd(H 3 BPPh 2 BH 3 ) 3 (THF) 3 (4-THF). XRD studies revealed that 4-THF has one chelating and two dangling H 3 BPPh 2 BH 3 − ligands bound to the metal to accommodate binding of THF. In contrast to the results with K(H 3 BPPh 2 BH 3 ), attempting the same mechanochemical reactions with Na(H 3 BPH 2 BH 3 ) containing the simplest phosphinodiboranate were unsuccessful; only the partial metathesis product U(H 3 BPH 2 BH 3 )I 2 (THF) 3 (5) was isolated in poor yields. Despite these limitations, our results offer new examples showing how mechanochemistry can be used to rapidly synthesize molecular coordination complexes that are otherwise difficult to prepare using more traditional solution methods.« less
5. Plant Protein O-Arabinosylation

   https://doi.org/10.3389/fpls.2021.645219  

   Petersen, Bent Larsen ; MacAlister, Cora A. ; Ulvskov, Peter ( March 2021 , Frontiers in Plant Science)

   A wide range of proteins with diverse functions in development, defense, and stress responses are O -arabinosylated at hydroxyprolines (Hyps) within distinct amino acid motifs of continuous stretches of Hyps, as found in the structural cell wall extensins, or at non-continuous Hyps as, for example, found in small peptide hormones and a variety of plasma membrane proteins involved in signaling. Plant O -glycosylation relies on hydroxylation of Prolines to Hyps in the protein backbone, mediated by prolyl-4-hydroxylase (P4H) which is followed by O -glycosylation of the Hyp C 4 -OH group by either galactosyltransferases (GalTs) or arabinofuranosyltranferases (Ara f Ts) yielding either Hyp-galactosylation or Hyp-arabinosylation. A subset of the P4H enzymes with putative preference to hydroxylation of continuous prolines and presumably all Ara f T enzymes needed for synthesis of the substituted arabinose chains of one to four arabinose units, have been identified and functionally characterized. Truncated root-hair phenotype is one common denominator of mutants of Hyp formation and Hyp-arabinosylation glycogenes, which act on diverse groups of O -glycosylated proteins, e.g., the small peptide hormones and cell wall extensins. Dissection of different substrate derived effects may not be regularly feasible and thus complicate translation from genotype to phenotype. Recently, lackmore »of proper arabinosylation on arabinosylated proteins has been shown to influence their transport/fate in the secretory pathway, hinting to an additional layer of functionality of O -arabinosylation. Here, we provide an update on the prevalence and types of O -arabinosylated proteins and the enzymatic machinery responsible for their modifications.« less