1,721,096 research outputs found
Nitrite catalyzes ferriheme protein reductive nitrosylation
No full textNitrite ion is found to catalyze the NO reduction of met-hemoglobin and met-myoglobin in pH 7.0 buffered aqueous solution. The catalysis rate constants for these ferriheme proteins and for two water-soluble ferriheme model systems follow the same order as do the FeIII/II reduction potentials of the ferric nitrosyl complexes. This is consistent with a proposed mechanism occurring via outer sphere reduction of the FeIII(NO) center by NO2- to give the FeII(NO) product plus NO2. Although the first step is thermodynamically uphill, the NO2 generated would be rapidly trapped by excess NO to form N2O3, which would hydrolyze. We speculate that, if formed in the proximity of the protein, the strong nitrosating agent N2O3 could also result in protein modifications.</p
Nitrite catalyzes reductive nitrosylation of the water-soluble ferri-heme model Fe<sup>III</sup>(TPPS) to Fe<sup>II</sup>(TPPS)(NO)
No full textQuantitative investigation of the reaction of the ferri-heme model compound FeIII(TPPS)(H2O)2 (1) to give FeII(TPPS)(NO)(2) (TPPS = tetra(4-sulfonato-phenyl)porphinato) in buffered aqueous solution demonstrates a slow pH-independent reductive nitrosylation pathway in the pH range 4-6. The rate of this reaction is subject to modest general base catalysis. In the course of this study, a surprising catalytic pathway whereby nitrite ion (NO2-) strongly catalyzes the reduction of 1 to 2 under reductive nitrosylation conditions was demonstrated.</p
Mechanisms of ferriheme reduction by nitric oxide: nitrite and general base catalysis
No full textThe reductive nitrosylation (FeIII(P) + 2NO + H2O = FeII(P)(NO) + NO2- + 2H+) of the ferriheme model FeIII(TPPS) (TPPS = tetra(4-sulfonatophenyl) porphyrinato) has been investigated in moderately acidic solution. In the absence of added or adventitious nitrite, this reaction displays general base catalysis with several buffers in aqueous solutions. It was also found that the nitrite ion, NO2-, is a catalyst for this reaction. Similar nitrite catalysis was demonstrated for another ferriheme model system FeIII(TMPy) (TMPy = meso-tetrakis(N-methyl-4-pyridyl)porphyrinato), and for ferriheme proteins met-hemoglobin (metHb) and met-myoglobin (metMb) in aqueous buffer solutions. Thus, it appears that such catalysis is a general mechanistic route to the reductive nitrosylation products. Two nitrite catalysis mechanisms are proposed. In the first, NO2- is visualized as operating via nucleophilic addition to the FeIII-coordinated NO in a manner similar to the reactions proposed for FeIII reduction promoted by other nucleophiles. This would give a labile N2O 3 ligand that hydrolyzes to nitrous acid, regenerating the original nitrite. The other proposal is that FeIII reduction is effected by direct outer-sphere electron transfer from NO2- to Fe III(P)(NO) to give nitrogen dioxide plus the ferrous nitrosyl complex FeII(P)(NO). The NO2 thus generated would be trapped by excess NO to give N2O3 and, subsequently, nitrite. It is found that the nitrite catalysis rates are markedly sensitive to the respective FeIII(P)(NO) reduction potentials, which is consistent with the behavior expected for an outer-sphere electron-transfer mechanism. Nitrite is the product of NO autoxidation in aqueous solution and is a ubiquitous impurity in experiments where aqueous NO is added to an aerobic system to study biological effects. The present results demonstrate that such an impurity should not be assumed to be innocuous, especially in the context of recent reports that endogenous nitrite may play physiological roles relevant to the interactions of NO and ferriheme proteins.</p
Mechanisms of reductive nitrosylation in iron and copper models relevant to biological systems
No full textAn overview of reductive nitrosylation of iron and copper models relevant to biological systems is given. Studies on ferri-heme models and proteins, iron(III) complexes, and copper(II) systems are discussed. Finally, possible effects as a result of reductive nitrosylation reactions are outlined.</p
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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