1,721,083 research outputs found
Synthesis of sialic acid derivatives and development of analytical method for their determination by GC-CI/MS
No full textThe First Synthesis of the N-acetylneuraminic Acid 1,7-Lactone a Possible Artefact in Determination of Sialic Acids from Glycoconjugate
No full textFirst synthesis of authentic 1,7-lactones of sialic acids and new reliable protocol for the analysis of these free and functionalized acids, allow to reject the reported identification of 1,7-lactones in the membrane of policythemia vera red blood cells since obtained by an inaccurate procedure
No full textThe work herein reported was inspired by the simple consideration that, in spite of the biological relevance of sialic acid (Sia), no analytical method has been reported to permit a suitable qualitative and quantitative mapping of the components of this acidic carbohydrate family. Indeed, the inspection of the literature reports, showed that the existing methods suffer of different drawback deriving, in some cases, from a methodological fault; the authors did not dispose of authentic reference standards and of suitable inner standards for the quantitative detection of sialic acids. Thus, the many lists of sialic acids derivatives with different substituens at the alcoholic groups (that can be differently O-acylated, O-alkilated, O-sulfate, etc), which have been formed free or linked to carbohydrates chains of glycolipids and glycoproteins, have not the validity of an absolute demonstration of their presence in various biological natural districts, from bacteria to humans. Moreover, in the best cases, the quantitative detection of Sias is not absolute but refers to a reciprocal relative ratio.
Our attention was directed to differently functionalized Sias derivatives from N-acetyl-neuraminic acid (Neu5Ac) (1) and N-glycolil-neuraminic acid (Neu5Gc) (2) and, in particular, to the 1,7 lactone of Neu5Ac (4). This lactone have never been isolated in pure form and in sufficient amounts to be identified beyond any reasonable doubt . The supposed presence of Neu5Ac1,7L (4) and of Neu5Gc1,7L (5) in biological media has still to be demonstrated too. It is, moreover reported that the Neu5Ac1,7L (4), present in the Bufo bufo mucins, is a ligand of human interleukin 4 and is well described as a component of sialoglycoproteins present in the membrane of red blood cells (RBCs) in Polycythemia Vera (PV), a malignant disorder of haematopoietic stem cells
In this case the authors, who did not isolate the lactone, identified it, as the 4,8,9-tri-heptafluorobutyrril derivative, applying a method associating gas chromatography and electron mass spectrometry to the samples obtained by acidic hydrolysis of the RBCs.
Successful results of the work, here described are:
• the first synthesis of the 1,7 lactones of Neu5Ac (4) and of Neu5Gc (5), and their isotopologues (8) and (9);
• the set-up of suitable analytical method for the analyses of these lactones by HPLC-MS methodology, which does not require any functionalization;
• a collection of evidences that impose to reject as demonstrated the presence of the lactone (4) in PV red blood cells.
Finally we could explain the mistake that deceived the authors who did not sufficiently demonstrated the presence of 1,7 lactone. In fact, we can suggest a possible artefactual formation of 1,7-lactones during derivatization of Sias.
Moreover, some accessory results obtained in the last part of our work impose to reconsider the analytical work reported by the same authors, and others, who used their methodology in the analysis of acylated amido sugars or acylated aminoacids
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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