1,720,961 research outputs found
Enzyme fingerprints of activity and stereo- and enantioselectivity from fluorogenic and chromogenic substrate arrays
No full textA series of stereochemically and structurally diverse fluorogenic and chromogenic substrates for hydrolytic enzymes has been synthesized and used to characterize enzyme activity profiles of esterases, lipases, proteases, pepti- dases, phosphatases, and epoxide hydro- lases. The substrates used are particu- larly resilient to nonspecific reactions due to their mechanism of activation.The activities recorded with the individ- ual substrates are therefore remarkably reproducible, and enable us to use the
overall pattern of activity as a specific fingerprint for the enzyme sample. Fin- gerprints of activity, and enantio- and stereoselectivity are displayed as arrays of color-scale squares that are easily analyzed visually. Such fingerprints might be useful for quality control, enzyme discovery, and possibly for addressing the issue of functional conver- gence in enzymes
Fluorescence Assay and Screening of Epoxide Opening Reactions by Nucleophiles
No full textTerminal epoxides such as 1 react with nucleophiles (H2O, Cl-, Br-, N3-, and CN-) at the primary oxirane carbon to give mostly anti-Markovnikov-type regioisomers 5a-d. The opening products of epoxide (R)-1 with chloride (5a), bromide (5b) and azide (5c) undergo enantio- and chemoselective oxidation by horse liver alcohol dehydrogenase and NAD+ to give the corresponding ketones 7a-c, and subsequently umbelliferone 4 by beta-elimination, leading to a fluorescence increase at landa em = 460 ± 20 nm (landa ex = 360 ± 20 nm). The epoxide hydrolysis products give no signal. This enantio- and chemo-selective fluorogenic assay for epoxide opening was used to search for catalytic antibodies for nucleophilic epoxide opening raised against 1,2-azidoammonium hapten 8 as a mimic for epoxide opening by azide and against chloromethyl phosphonate hapten 9 as a mimic for the transition state of chlorohydrin formation
Improved stereoselective synthesis of both methyl alpha- and beta-glycosides corresponding to the amino sugar component of the E ring of calicheamicin (I)(gamma 1) and esperamicin A(1)
No full textThe monosaccharide component (alpha and beta-anomer) of the E Ring of calicheamicin (I)(gamma 1) and esperamicin A(1) has been synthetized by an efficient and improved stereoselective procedure starting from methyl 2-deoxy-alpha- and beta-D-ribopyranoside. The synthetic procedure makes use, in each case, of a cyclic sulphate and of the regioselective ring opening of an intermediate activated aziridine. (C) 1997 Elsevier Science Ltd
Efficient enantioselective synthesis of (R)-2-acetyl-2-hydroxy-5,8-dimethoxy-1,2,3,4-tetrahydronaphthalene, the key intermediate in the synthesis of anthracycline antibiotics
No full textA simple, efficient, enantioselective synthesis of (R)-2-acetyl-2-hydroxy-5,8-dimethoxy-1,2,3,4-tetrahydronaphthalene, the key intermediate in the synthesis of anthracycline antibiotics, is described. The synthetic procedure starts with the Sharpless asymmetric dihydroxylation of 2-acetyl-5,8-dimethoxy-3,4-dihydronaphthalene: the diol obtained is regioselectively transformed into the corresponding chloroacetate which is dehalogenated and saponified to give the desired title compound in four steps with satisfactory yield (52%). No separation step is necessary at any point of the synthetic process. An efficient procedure for the synthesis of the starting enone and the stereoselectivity of the methanolysis of the intermediate chloroacetate are also reported. © 2002 Elsevier Science Ltd. All rights reserved
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
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