Computational studies of ethynyl- and diethynyl-expanded tetrahedranes, prismanes, cubanes, and adamantanes

B3LYP/6-31+G(d) and MP2/6-31+G(d) computations were performed on a series of ethynyl- and diethynyl-expanded tetrahedranes, prismanes, cubanes and adamantanes. Every ethynyl expansion reduces the ring strain energy of the cage. The deprotonation energies of the cage poly-ynes are exceptionally low; we estimate that the gas-phase deprotonation energy of the diethynyl-expanded cubane is about 309 kcal mol(-1). The ring and cage poly-ynes can serve as effective hosts of either lithium or sodium cation, where the best host maximizes the number of interactions of alkynyl groups with the cation at an ideal distance. Last, the vertical excitation energies of the poly-ynes and their conjugate bases suggest that the alkynyl groups are interacting through space. The poly-ynes express a broad range of absorption energies, indicating that these molecules are potential targets in expressly designed optical applications.     

Lithium trihydroxy/triisopropoxy-2-pyridylborate salts (LTBS): synthesis, isolation, and use in modified Suzuki-Miyaura cross-coupling reactions

We describe herein shelf-stable, isolable, and characterizable pyridyl lithium trihydroxy and triisopropoxy 2-borate salts (LTBS) for use in modified Suzuki-Miyaura cross-coupling reactions that can be produced in quantities greater than one hundred grams. Pyridyl LTBS provide a viable cross-coupling alternative to unstable 2-pyridylboronic acids, boronates, and trifluoroborate salt derivatives. We also demonstrate the synthesis and cross-coupling of shelf-stable LTBS reagents of other sp²-hybridized nitrogen-containing heterocycles including thiazole, pyrazine, quinoline, and isoquinoline heterocycles.     

Three notions of tropical rank for symmetric matrices

We introduce and study three different notions of tropical rank for symmetric and dissimilarity matrices in terms of minimal decompositions into rank 1 symmetric matrices, star tree matrices, and tree matrices. Our results provide a close study of the tropical secant sets of certain nice tropical varieties, including the tropical Grassmannian. In particular, we determine the dimension of each secant set, the convex hull of the variety, and in most cases, the smallest secant set which is equal to the convex hull.     

New Delhi metallo-β-lactamase: structural insights into β-lactam recognition and inhibition

The β-lactam antibiotics have long been a cornerstone for the treatment of bacterial disease. Recently, a readily transferable antibiotic resistance factor called the New Delhi metallo-β-lactamase-1 (NDM-1) has been found to confer enteric bacteria resistance to nearly all β-lactams, including the heralded carbapenems, posing a serious threat to human health. The crystal structure of NDM-1 bound to meropenem shows for the first time the molecular details of how carbapenem antibiotics are recognized by dizinc-containing metallo-β-lactamases. Additionally, product complex structures of hydrolyzed benzylpenicillin-, methicillin-, and oxacillin-bound NDM-1 have been solved to 1.8, 1.2, and 1.2 ?, respectively, and represent the highest-resolution structural data for any metallo-β-lactamase reported to date. Finally, we present the crystal structure of NDM-1 bound to the potent competitive inhibitor l-captopril, which reveals a unique binding mechanism. An analysis of the NDM-1 active site in these structures reveals key features important for the informed design of novel inhibitors of NDM-1 and other metallo-β-lactamases.     

Mathematical cognition: individual differences in resource allocation

Individuals scoring higher in tests of general cognitive abilities tend to perform better on novel and familiar mathematical tasks. It has been scarcely investigated how this superior mathematical performance relates to the amount of cognitive resources that is invested to solve a given task. In this study we propose that, on novel tasks, individuals with high cognitive abilities outperform less able individuals, because they allocate a higher amount of resources. On familiar tasks, however, individuals with higher abilities profit from more efficient processes compared to individuals of lower cognitive abilities. We tested this hypothesis by administering to 11th graders a geometric analogy task not practiced at school and an algebraic transformation task comprising operations that are routinely required during mathematical courses. General cognitive abilities were measured with Ravens Advanced Progressive matrices (fluid intelligence), the d2 (focused attention) and KAI-N (working memory capacity). Resource allocation was measured by assessing pupil diameter during the problem-solving process. Performance on both the analogy and the algebra task was correlated with general cognitive abilities, especially fluid intelligence. In line with our assumptions, a positive correlation between fluid intelligence and resource allocation was observed in the novel geometric analogy task, whereas the correlation was not significant in the more familiar algebra task.     

A LEED investigation of Xe adsorption on W(110)

Four ordered LEED patterns are observed for Xe adsorption on W(110) for temperatures between 77 and 90 K. A (2 × 2) structure with an area per Xe atom of 28.3 Ų is transformed into two coincidence structures which correspond to a disordered (100) Xe layer. The area per Xe atom in these structures is 17.6 and 20.2 Ų. Xe adsorption on oxygen covered W(110) leads to one-dimensional disorder in the structures observed on clean W(110) without the formation of new structures.     

DNA triangles and self-assembled hexagonal tilings

We have designed and constructed DNA complexes in the form of triangles. We have created hexagonal planar tilings from these triangles via self-assembly. Unlike previously reported structures self-assembled from DNA, our structures appear to involve bending of double helices. Bending helices may be a useful design option in the creation of self-assembled DNA structures. It has been suggested that DNA self-assembly may lead to novel materials and efficient computational devices.     

The vinylogous intramolecular Morita-Baylis-Hillman reaction: synthesis of functionalized cyclopentenes and cyclohexenes with trialkylphosphines as nucleophilic catalysts

The development of the vinylogous intramolecular Morita-Baylis-Hillman reaction for the synthesis of functionalized cyclopentenes and cyclohexenes is described. The reaction involves the trialkyphosphine-catalyzed cyclization of 1,6- or 1,7-diactivated 1,5-hexadienes or 1,6-heptadienes, containing carboxyaldehyde, methyl ketone, or methoxycarbonyl as the olefin activating groups. A representative example of this reaction is the Me(3)P-catalyzed cyclization of 1a in tert-amyl alcohol, which provides the substituted cyclopentene 2a in 95% yield and with 97:3 regioselectivity.     

A rapid solid-phase fluorescence-based protein assay for quantitation of protein electrophoresis samples containing detergents, chaotropes, dyes, and reducing agents

A new solid-phase, fluorescence-based protein assay was developed that quantifies proteins in the presence of detergents, urea and reducing agents (one-dimensional sodium dodecyl sulfate (1-D SDS) lysis buffers and urea isoelectric focusing (IEF) buffers). A specially designed 96-well microplate facilitates application of protein samples to the assay paper and allows easy quantitation of samples using fluorescence microplate readers (top or bottom reading format). Alternatively, stained membranes may be directly scanned using a variety of different laser or charge-coupled device (CCD)-based imaging devices with UV or visible imaging capabilities. Since protein is specifically bound to the membrane, contaminants are readily washed away, avoiding interference with the protein measurement. The protein assay has a dynamic range extending from 10 ng to 5 microg of protein per microliter and requires only 1 microL of sample, which is ideal for samples destined for electrophoresis. The protein-to-protein variability of staining of ten different proteins was determined to be comparable with that of the bicinchoninic acid (BCA) and Lowry assays (16%). Additionally, the quality of the assay according to Z-factor analyses is excellent.     

Thermal decomposition of 2‐ethylhexyl nitrate (2‐EHN)

The decomposition mechanism of 2‐ethyl‐hexylnitrate (2‐EHN), an important additive to diesel fuel to improve the cetane number, was investigated in solution and in the gas phase. In trans‐decalin as solvent, an activation barrier for the thermal decomposition of 39 kcal/mol was determined, and thus the decomposition is slow at temperatures below 100°C. Under high pressure conditions (2.4 kbar) the decomposition rates decrease, in accordance with a radical mechanism. Flash vacuum pyrolysis with subsequent detection of the products via mass spectroscopy or matrix IR spectroscopy allows to identify NO₂, formaldehyde, and several olefins as the major decomposition products. These data allow proposing a consistent mechanistic scheme for the 2‐EHN decay. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 34–38, 2002