1,720,977 research outputs found
Noncovalent interactions in high-performance liquid chromatography enantioseparations on polysaccharide-based chiral selectors
No full textDesigned more than thirty years ago in order to improve and maximize the discrimination capability of native polysaccharides, cellulose- and amylose-based selectors have shown excellent and unequalled performances for the enantioseparation of chiral compounds. The successful story of these chiral selectors relies on a multi-site high-ordered chiral platform which is held up by intramolecular hydrogen bonds (HBs), and makes the polymer able to host and discriminate enantiomers. In this environment, both achiral and stereoselective intermolecular noncovalent interactions play a pivotal role, and HBs, halogen bonds (XBs), dipole-dipole, π-π stacking, steric repulsive, and van der Waals interactions underlie adsorption process and formation of transient diastereomeric assemblies between the polymer and the enantiomer pair. In the last decades, advances in computational chemistry and spectroscopic techniques have improved knowledge of noncovalent interactions, contributing to decode their functions in chemical systems. Significantly, over time the growing interplay between experimental and theoretical approaches has contributed to unravel intermolecular forces underlying selector-selectand association and to understand recognition patterns. On this basis, this review summarizes seminal and representative studies dealing with noncovalent interactions that function in HPLC enantioseparations promoted by cellulose benzoates and phenylcarbamates of amylose and cellulose. The importance of integrating theoretical and experimental approaches to profile mechanisms and interaction patterns is highlighted by discussing focused case studies. In particular, the advantageous utilization of electrostatic potential (V) analysis and molecular dynamics (MD) simulations in this field is evidenced. A systematic compilation of all published literature has not been attempted
Insights into the impact of shape and electronic properties on the enantioseparation of polyhalogenated 4,4'-bipyridines on polysaccharide-type selectors. Evidence of stereoselective halogen bonding interactions
No full textStarting from the high-performance liquid chromatography (HPLC) enantioseparation data collected by using twelve polyhalogenated 2,2-dichloro-3-substituted-5,5-dihalo-4,4-bipyridines as test probes on seven polysaccharide-based chiral stationary phases (CSPs) under multimodal elution, the impact of substitution pattern, shape and electronic properties of the molecules on the separation behaviour was investigated through the evaluation of the chromatographic parameters (k, alfa, Rs) and molecular properties determined by means of quantum chemistry calculations. The computational/chromatographic screening furnished relevant structure-chromatographic behaviour relationships and some molecular interactions involved in the chiral discrimination process could be identified. In particular, a halogen bonding interaction (I.O) could reasonably explain the high enantioseparation (alfa = 1.80, Rs = 8.2) observed for the 2,2-dichloro-3,5-diiodo-5-bromo-4,4-bipyridine on Lux Cellulose-1. To the best of our knowledge, this is the first report supporting the involvement of a stereoselective halogen bonding interaction in polysaccharide-based CSPs. Moreover, having at disposal a sufficient set of data, the unknown absolute configurations of the eluted enantiomers of 3-methyl-, 3-thiomethyl- and 3-diphenylphosphinoyl- 2,2-dichloro-5,5-dibromo-4,4-bipyridines could be deduced by chromatographic correlation with the enantiomer elution order (EEO) of the related compounds of known absolute configuration
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Comparative enantioseparation of planar chiral ferrocenes on polysaccharide-based chiral stationary phases
No full textPlanar chiral ferrocenes are well-known compounds that have attracted interest for application in synthesis, catalysis, material science, and medicinal chemistry for several decades. In spite of the fact that asymmetric synthesis procedures for obtaining enantiomerically enriched ferrocenes are available, sometimes, the accessible enantiomeric excess of the chiral products is unsatisfactory. In such cases and for resolution of racemic planar chiral ferrocenes, enantioselective high-performance liquid chromatography (HPLC) on polysaccharide-based chiral stationary phases (CSPs) has been used in quite a few literature articles. However, although moderate/high enantioselectivities have been obtained for planar chiral ferrocenes bearing polar substituents, the enantioseparation of derivatives containing halogens, or exclusively alkyl groups, remains rather challenging. In this study, the enantioseparation of ten planar chiral 1,2- and 1,3-disubstituted ferrocenes was explored by using five polysaccharide-based CSPs under multimodal elution conditions. Baseline enantioseparations were achieved for nine analytes with separation factors (α) ranging from 1.20 to 2.92. The presence of π-extended systems in the analyte structure was shown to impact affinity of the most retained enantiomer toward amylose-based selectors, observing retention times higher than 80 min with methanol-containing mobile phases (MPs). Electrostatic potential (V) analysis and molecular dynamics (MD) simulations were used in order to study interaction modes at the molecular level
- …
