Recoverable silica-gel supported binam-prolinamides as organocatalysts for the enantioselective solvent-free intra- and intermolecular aldol reaction

Silica-gel supported binam-derived prolinamides are efficient organocatalysts for the direct intramolecular and intermolecular aldol reaction under solvent-free conditions using conventional magnetic stirring. These organocatalysts in combination with benzoic acid showed similar results to those obtained under similar homogeneous reaction conditions using an organocatalyst of related structure. For the intermolecular process, the aldol products were obtained at room temperature and using only 2 equiv of the ketone with high yields, regio-, diastereo- and enantioselectivities. Under these reaction conditions, also the cross aldol reaction between aldehydes is possible. The recovered catalyst can be reused up to nine times providing similar results. More interestingly, these heterogeneous organocatalysts can be used in the intramolecular aldol reaction allowing the synthesis of the Wieland–Miescher and ketone analogues with up to 92% ee, with its reused being possible up to five times without detrimental on the obtained results.

A proposal of algorithm for solving definite descriptions through dialogue structure

En este trabajo proponemos un algoritmo para la resolución de las descripciones definidas en español a través de la estructura del diálogo, mediante la definición de un espacio de accesibilidad anafórico. Este algoritmo está basado en la hipótesis de que la resolución de la anáfora está relacionada con la estructura del diálogo. Así, la resolución de la anáfora mejora si se especifica un espacio de accesibilidad para cada tipo descripción definida según la estructura del diálogo. La utilización de este espacio de accesibilidad anafóico reduce tanto el tiempo de procesamiento como la posibilidad de obtener un antecedente erróneo. Además, la definición de este espacio de accesibilidad depende únicamente de la propia estructura textual del diálogo.

The structure of a PII signaling protein from a halophilic archaeon reveals novel traits and high-salt adaptations

To obtain insights into archaeal nitrogen signaling and haloadaptation of the nitrogen/carbon/energy-signaling protein PII, we determined crystal structures of recombinantly produced GlnK2 from the extreme halophilic archaeon Haloferax mediterranei, complexed with AMP or with the PII effectors ADP or ATP, at respective resolutions of 1.49 Å, 1.45 Å, and 2.60 Å. A unique trait of these structures was a three-tongued crown protruding from the trimer body convex side, formed by an 11-residue, N-terminal, highly acidic extension that is absent from structurally studied PII proteins. This extension substantially contributed to the very low pI value, which is a haloadaptive trait of H. mediterranei GlnK2, and participated in hexamer-forming contacts in one crystal. Similar acidic N-extensions are shown here to be common among PII proteins from halophilic organisms. Additional haloadaptive traits prominently represented in H. mediterranei GlnK2 are a very high ratio of small residues to large hydrophobic aliphatic residues, and the highest ratio of polar to nonpolar exposed surface for any structurally characterized PII protein. The presence of a dense hydration layer in the region between the three T-loops might also be a haloadaptation. Other unique findings revealed by the GlnK2 structure that might have functional relevance are: the adoption by its T-loop of a three-turn α-helical conformation, perhaps related to the ability of GlnK2 to directly interact with glutamine synthetase; and the firm binding of AMP, confirmed by biochemical binding studies with ATP, ADP, and AMP, raising the possibility that AMP could be an important PII effector, at least in archaea.

Elemental Anisotropic Growth and Atomic-Scale Structure of Shape-Controlled Octahedral Pt–Ni–Co Alloy Nanocatalysts

Multimetallic shape-controlled nanoparticles offer great opportunities to tune the activity, selectivity, and stability of electrocatalytic surface reactions. However, in many cases, our synthetic control over particle size, composition, and shape is limited requiring trial and error. Deeper atomic-scale insight in the particle formation process would enable more rational syntheses. Here we exemplify this using a family of trimetallic PtNiCo nanooctahedra obtained via a low-temperature, surfactant-free solvothermal synthesis. We analyze the competition between Ni and Co precursors under coreduction “one-step” conditions when the Ni reduction rates prevailed. To tune the Co reduction rate and final content, we develop a “two-step” route and track the evolution of the composition and morphology of the particles at the atomic scale. To achieve this, scanning transmission electron microscopy and energy dispersive X-ray elemental mapping techniques are used. We provide evidence of a heterogeneous element distribution caused by element-specific anisotropic growth and create octahedral nanoparticles with tailored atomic composition like Pt1.5M, PtM, and PtM1.5 (M = Ni + Co). These trimetallic electrocatalysts have been tested toward the oxygen reduction reaction (ORR), showing a greatly enhanced mass activity related to commercial Pt/C and less activity loss than binary PtNi and PtCo after 4000 potential cycles.