1,721,108 research outputs found
Determination of enantiomerization barriers of hypericin and pseudohypericin by dynamic high-performance liquid chromatography on immobilized polysaccharide-type chiral stationary phases and off-column racemization experiments
No full textDirect enantiomer separation of hypericin, pseudohypericin, and protohypericin
was accomplished by high-performance liquid chromatography (HPLC) using
immobilized polysaccharide-type chiral stationary phases (CSPs). Enantioselectivities
up to 1.30 were obtained in the polar-organic elution mode whereby for hypericin and
pseudohypericin Chiralpak IC [chiral selector being cellulose tris(3,5-dichlorophenylcarbamate)]
and for protohypericin Chiralpak IA (chiral selector being the 3,5-dimethylphenylcarbamate
of amylose) gave favorable results. Enantiomers were distinguished by
on-line electronic circular dichroism detection. Optimized enantioselective chromatographic
conditions were the basis for determining stereodynamic parameters of the
enantiomer interconversion process of hypericin and pseudohypericin. Rate constants
delivered by computational simulation of dynamic HPLC elution profiles (stochastic
model, consideration of peak tailing) were used to calculate averaged enantiomerization
barriers (DG]
enant) of 97.6–99.6 kJ/mol for both compounds (investigated temperature
range 25–458C). Complementary variable temperature off-column (i.e., in solution) racemization
experiments delivered DG]
enant 5 97.1–98.0 kJ/mol (27–458C) for hypericin and
DG]
enant 5 98.9–101.4 kJ/mol (25–558C) for pseudohypericin. An activation enthalpy of
DH# 5 86.0 kJ/mol and an activation entropy of DS# 5 237.7 J/(K mol) were calculated
from hypericin racemization kinetics in solution, whereas for pseudohypericin these
figures amounted to 74.1 kJ/mol and 282.6 J/(K mol), respectively. Although the natural
phenanthroperylene quinone pigments hypericin and pseudohypericin as well as
their biological precursor protohypericin are chiral and can be separated by enantioselective
HPLC low enantiomerization barriers seem to prevent the occurrence of an
excess of one enantiomer under typical physiological conditions—at least as long as
stereoselective intermolecular interactions with other chiral entities are absen
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
Current Trends in Database Technology - EDBT 2004 Workshops, EDBT 2004 Workshops PhD, DataX, PIM, P2P&DB, and ClustWeb
No full textIn-line coupling of a reversed-phase column to cope with limited chemoselectivity of a quinine carbamate-based anion-exchange type chiral stationary phase
No full textA chiral stationary phase based on tert-butylcarbamoyl quinine has shown remarkable enantiomer separation capability for the thyroid hormone thyroxine (T(4)) and its structural analogue triiodothyronine (T(3)) employing hydroorganic buffered mobile phases (typical RP conditions). To overcome the problem of a somehow limited chemoselectivity for the critical peak pair between adjacent L-thyroxine (L-T(4)) and D-thyroxine (D-T(4)) peaks on the chiral anion-exchanger CSP when all four compounds need to be analysed simultaneously like in impurity profiling of L-T(4 )drug products, an RP column (Gemini C18) was serially coupled with the chiral anion-exchanger column to add a hydrophobic selectivity increment and to improve thereby the critical resolution between L-T(3) and D-T(4). Various parameters such as organic modifier content, pH, buffer concentration and type, type of achiral column as well as sequence of achiral and chiral column have been investigated with individual and tandem columns. With the optimized conditions and use of the tandem column a significantly improved separation, as compared to the chiral anion-exchanger column alone, with a critical resolution as high as 3.7 and an almost equal band spacing of the four components of the test mixture could be obtained. The sequence of the columns (achiral-chiral or chiral-achiral) had no significant effect on the separation performance
Enantioselective HPLC of potentially CNS active acidic amino acids with a cinchona carbamate based chiral stationary phase
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