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Translation initiation defines the identity and quantity of a synthesized protein. The process is dysregulated in many human diseases1,2. A key commitment step is when the ribosomal subunits join at a translation start site on a messenger RNA to form a functional ribosome. Here, we combined single-molecule spectroscopy and structural methods using an in vitro reconstituted system to examine how the human ribosomal subunits join. Single-molecule fluorescence revealed when the universally conserved eukaryotic initiation factors eIF1A and eIF5B associate with and depart from initiation complexes. Guided by single-molecule dynamics, we visualized initiation complexes that contained both eIF1A and eIF5B using single-particle cryo-electron microscopy. The resulting structure revealed how eukaryote-specific contacts between the two proteins remodel the initiation complex to orient the initiator aminoacyl-tRNA in a conformation compatible with ribosomal subunit joining. Collectively, our findings provide a quantitative and architectural framework for the molecular choreography orchestrated by eIF1A and eIF5B during translation initiation in humans.

The synthesis of housane derivatives from cyclopropenes is described. Under rhodium(II) catalysis, cyclopropenylvinyl carbinols can regioselectively generate a carbene intermediate which undergoes an intramolecular cyclopropanation to form a housane, a skeleton with similar ring strain as the cyclopropene precursor. The procedure shows a remarkable broad scope and efficiency. Moreover, the method served to prepare man‐made housane‐containing terpene derivatives, which are not accessible by Nature. [ABSTRACT FROM AUTHOR]

A computational approach to directly estimate the relative acidity of a given Lewis acid is presented. This approach is based on the strength of the π‐conjugation in trans‐crotonaldehyde‐Lewis acid complexes, the species used in the well‐known Childs’ Lewis acidity scale. It is found that the π‐conjugative strength values given by the Energy Decomposition Analysis – Natural Orbital for Chemical Valence method strongly correlate not only with the variation of the bond lengths in the system, which are greatly affected by the nature of the Lewis acid, but also with the downfield shifts experienced by the different nuclei of the conjugated system upon binding to the Lewis acid. These strong correlations indicate that the (easy‐to‐compute) π‐conjugation energies can be used as an alternative Lewis acidity scale. [ABSTRACT FROM AUTHOR]

The catalytic performance of a series of mono‐ and bimetallic Ir(I) and Rh(I) complexes bearing the (COC) bis‐triazolylidene ligand, coordinated in a bridging or chelating fashion, was evaluated in the cyclization of 4‐pentynoic acid. The chelated mononuclear cationic complex [Ir(cod)(COC)]PF6 (cod=1,5‐cyclooctadiene; COC=biscarbene ether) displayed the highest reaction rate for 4‐pentynoic acid to form γ‐methyl‐ene‐γ‐butyrolactone (5‐methylenedihydrofuran‐2(3H)‐one). Density Functional Theory calculations were carried out to rationalize the reaction mechanism which proceeds in three steps, namely coordination of the alkyne to the active catalyst followed by intramolecular cyclization and final reductive elimination. Its catalytic performance was further assessed against different substituted 4‐pentynoic acid derivatives. [ABSTRACT FROM AUTHOR]

New and simple ligand design strategies for the preparation of versatile metal-based catalysts capable of operating under greener and eco-friendly conditions in several industrially attractive processes are in high demand for society development. We present the first nucleophilic addition of an organolithium to ketenimines which incorporates a stereogenic centre in an N-donor atom to prepare new enantiopure NNN-donor scorpionates. We have also verified its potential utility as a valuable scaffold for chirality induction through the preparation of inexpensive, non-toxic and asymmetric zinc complexes. The pro-ligands and the corresponding zinc-based complexes have been characterized by X-ray diffraction studies. DFT studies were carried out to rationalize the different complexation abilities of these pro-ligands. These complexes have proved to act as highly efficient catalysts for a variety of sustainable bioresourced processes that are industrially attractive, with a wide substrate scope. Thus, complex 7 behaves as a highly efficient initiator for the well-behaved living ring-opening polymerization (ROP) of rac-lactide under very mild conditions. The PLA materials produced exhibited enhanced levels of isoselectivity, comparable to the highest value reported so far for zinc-based catalysts (Pi = 0.88). In addition, the combination of 7 with onium halide salts functioned as a very active and selective catalyst for CO2 fixation into five-membered cyclic carbonates through the cycloaddition of CO2 into epoxides under very mild and solvent-free conditions, reaching very good to excellent conversions (TOF = 227 h−1). Furthermore, this bicomponent system exhibits a broad substrate scope and functional group tolerance, including mono- and di-substituted epoxides, as well as the very challenging bio-renewable tri-substituted terpene-derived cis/trans-limonene oxide, whose reaction proceeds with high stereoselectivity. Finally, complex 7 also achieved high activity and selectivity as a one-component initiator for the synthesis of poly(cyclohexene carbonate)s via ring-opening copolymerization (ROCOP) of cyclohexene oxide and CO2 under very soft conditions, affording materials with narrow dispersity values. [ABSTRACT FROM AUTHOR]

Detailed quantum‐chemical calculations have been carried out to understand both the origin of the catalysis in the urea‐catalyzed nucleophilic ring‐opening of nitrocyclopropanes and the acceleration induced by the presence of an internal Lewis acid in the catalyst. To this end, the mode of activation of these ureas is quantitatively analyzed in detail by means of the Activation Strain Model of reactivity in combination with the Energy Decomposition Analysis method. It is found that the presence of the Lewis acid in the urea further enhances the interaction between the reactants, by increasing both the electrostatic and orbital attractions, which translates into a reduced activation barrier. This insight is also used to assess the impact of the nature of the Lewis acid on the transformation, facilitating the rational design of more active catalysts. [ABSTRACT FROM AUTHOR]

Bacteria use adaptive immune systems encoded by CRISPR and Cas genes to maintain genomic integrity when challenged by pathogens and mobile genetic elements.sup.1-3. Type I CRISPR-Cas systems typically target foreign DNA for degradation via joint action of the ribonucleoprotein complex Cascade and the helicase-nuclease Cas3.sup.4,5, but nuclease-deficient type I systems lacking Cas3 have been repurposed for RNA-guided transposition by bacterial Tn7-like transposons.sup.6,7. How CRISPR- and transposon-associated machineries collaborate during DNA targeting and insertion remains unknown. Here we describe structures of a TniQ-Cascade complex encoded by the Vibrio cholerae Tn6677 transposon using cryo-electron microscopy, revealing the mechanistic basis of this functional coupling. The cryo-electron microscopy maps enabled de novo modelling and refinement of the transposition protein TniQ, which binds to the Cascade complex as a dimer in a head-to-tail configuration, at the interface formed by Cas6 and Cas7 near the 3' end of the CRISPR RNA (crRNA). The natural Cas8-Cas5 fusion protein binds the 5' crRNA handle and contacts the TniQ dimer via a flexible insertion domain. A target DNA-bound structure reveals critical interactions necessary for protospacer-adjacent motif recognition and R-loop formation. This work lays the foundation for a structural understanding of how DNA targeting by TniQ-Cascade leads to downstream recruitment of additional transposase proteins, and will guide protein engineering efforts to leverage this system for programmable DNA insertions in genome-engineering applications. Cryo-electron microscopy structures of the TniQ-Cascade complex encoded by the Vibrio cholerae Tn6677 transposon reveal the mechanistic basis of the functional association of CRISPR- and transposon-associated machineries., Author(s): Tyler S. Halpin-Healy [sup.1] , Sanne E. Klompe [sup.1] , Samuel H. Sternberg [sup.1] , Israel S. Fernández [sup.1] Author Affiliations: (1) Department of Biochemistry and Molecular Biophysics, Columbia [...]

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We report on the synthesis and characterization of a series of (mostly) air‐stable diorganyl bis(pyridylimino) isoindolide (BPI) aluminum complexes and their chemistry upon visible‐light excitation. The redox non‐innocent BPI pincer ligand allows for efficient charge transfer homolytic processes of the title compounds. This makes them a universal platform for the generation of carbon‐centered radicals. The photo‐induced homolytic cleavage of the Al−C bonds was investigated by means of stationary and transient UV/Vis spectroscopy, spin trapping experiments, as well as EPR and NMR spectroscopy. The experimental findings were supported by quantum chemical calculations. Reactivity studies enabled the utilization of the aluminum complexes as reactants in tin‐free Giese‐type reactions and carbonyl alkylations under ambient conditions, which both indicated radical‐polar crossover behavior. A deeper understanding of the physical fundamentals and photochemical process was provided, furnishing in turn a new strategy to control the reactivity of bench‐stable aluminum organometallics. [ABSTRACT FROM AUTHOR]