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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
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe 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
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
Supramolecular structure of extrinsically chiral porphyrin hetero-assemblies and achiral analogues
Get PDFSupramolecular 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
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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