177,363 research outputs found
Supramolecular structure of extrinsically chiral porphyrin hetero-assemblies and achiral analogues
No full textSupramolecular structures of porphyrin hetero-assemblies and the achiral analogous are determined through energy dispersive X-ray diffraction. In presence of phenylalanine, CuT4 cationic and H2TTPS anionic porphyrins yield a suprachiral alpha-helix persisting even after removing the chiral template. The analogous synthesis performed without the template-imprinting agent provides an amorphous arrangement, hence an achiral structure
Noncovalent interactions in ternary complexes of the spermine with copper(II) and adenosine-5'-triphosphate and tripolyphosphate
No full textAppropriate Similarity Measures for Author Cocitation Analysis
No full textWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Non-covalent interactions in thermodynamic stereoselectivity of mixed-ligand copper(II)-D- or L-histidine complexes with L-amino acids. A possible model of metal ion-assisted molecular recognition
No full textFormation constants of ternary complexes of copper(II), L/D-histidine and, in turn, glycine, L-alanine, L-valine, L-leucine, L-tryptophan, or L-phenylalanine have been determined potentio-metrically at 25 °C and I= 0.1 mol dm–3(KNO3). In the case of amino acids with aromatic side chains the ternary complexes containing ligands of opposite chirality are more stable than those having ligands of the same chirality; the opposite is true for amino acids with aliphatic residues. Calorimetric measurements have been carried out to obtain the enthalpy and entropy values associated with complex formation. Copper(II)–histamine ternary systems with L-alanine or L-phenylalanine have also been investigated. Comparison of the thermodynamic parameters pertinent to formation of the histamine complexes with those of the analogous histidine complexes allows one to ascertain the number of donor atoms involved in the co-ordination to copper(II), in the histidine systems. The determination of H and S values renders easier understanding of the factors determining Stereoselectivity in the above systems. The Stereoselectivity may be explained in terms of non-covalent interactions between side-chain residues. The role played by the histidine carboxylate in the molecular recognition of amino acids is also discussed
Effect of metformin on insulin binding to receptors in cultured human lymphocytes and cancer cells.
No full textPhenformin has opposite effects on insulin and growth hormone binding to IM-9 lymphocytes
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"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
Hierarchical self-assembly of water-soluble porphyrins
No full textHierarchy is a powerful and promising tool to rationally synthesize non-covalent complex species. In this paper we show two different examples of hierarchically driven aggregation processes allowing for the control of aggregation state or "shape" of the final species. In the first case we propose that water-soluble porphyrins presenting various protonation steps can follow kinetic routes alternative to the thermodynamic pathway; namely for each protonation step is allowed, in principle, an alternative kinetically driven homo-self-aggregation process. Our results indicate that the thermodynamic route leads to a monomeric, protonated final state, but the kinetic pathway ends with self-assemblies of the title porphyrin. The bias between the two final states can be easily pre-determined by going quickly or slowly throughout the thermodynamic-kinetic junction. The second case deals with the so-called "chiral memory" phenomenon. Also in this case the role of the kinetic control over the assembly process is remarkable. In particular, we have observed that self-aggregation of opposite charged, achiral porphyrins does not lead to induced chirality even if a chiral template is added after their aggregation. The final aggregates are kinetically inert, mostly because stabilized by a network of electrostatic interactions between net charges. However, if the two supramolecular components are mixed in the presence of a chiral template, then an induced circular dichroism signal (ICD) appears in the Soret region (the main absorption feature of porphyrins in the visible region whose maximum and intensity are strongly affected by the aggregation state). Removal of the template does not significantly affect the intensity of the ICD: the inert aggregates have memorized the chiral template shape. Remarkably, these species - now inherently chiral - act as very efficient templates for self-aggregation of additional non-chiral porphyrins. © 2004 Elsevier B.V. All rights reserved
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