1,720,985 research outputs found
Radical or electron transfer mechanism of oxidation with some laccase/mediator systems
No full textThe use of appropriate low molecular-weight compounds
(viz. mediators) in combination with fungal
laccase enables this enzyme to catalyse the oxidation
of ‘non-natural’ non-phenolic substrates, such as
benzyl alcohols, according to the reaction scheme outlined
below (Scheme 1).
The mediator needs to be easily oxidised by laccase
to the Medox status: the structure of the latter
is then crucial for the mechanism of the ensuing
non-enzymatic oxidation of the substrate. 1-Hydroxybenzotriazole
(HBT) and N-hydroxyphthalimide
(HPI) are two N–OH compounds that are known
to mediate the activity of laccase, driving it towards
non-phenolic substrates, and the relative efficiency
of these mediators has been recently evaluated.
Consensus has been reached upon the preliminary monoelectronic
oxidation of these N–OH mediators by
laccase to the corresponding N-oxyl radical (N–O•)
forms, through the fleeting intervention of
their radical cations, HPT•+ or HPI•+, respectively.
Two different hypotheses have been proposed to explain
the subsequent oxidation of the substrate. Either
these N–O• radicals perform a one-electron oxidation
(ET) of the substrate to a radical cation, or
they abstract a H-atom from the substrate (HAT), converting
it into a radical. The end-products of
oxidation would be formed from either one of these
two short-lived intermediates of the substrate. We
favour the HAT mechanism on the basis of evidence
previously described, but wanted to address this
key issue once more
Comparing the efficiency of some mediators of laccase
No full textThe mechanism of oxidation of non-phenolic substrates by laccase/mediators systems has been investigated. Oxidation of
4-methoxybenzyl alcohol (1), taken as a benchmark reaction, enabled us to compare and to rank the relative ability of twelve
mediators: TEMPO provedmost effective, and a ionic mechanism is suggested for its action. Data on intermolecular selectivity
of substrate oxidation are in favour of an electron transfer (ET) mechanism in the case of ABTS-mediated oxidations, and of a
radical mechanism in HBT- and HPI-mediated reactions. Investigation by cyclic voltammetry (CV) of some of the mediators
revealed that an important role in determining the mechanism of substrate oxidation may be played by the stability of the
oxidised form of the mediator, as well as by its redox potential
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
Aromatic iodination: a new investigation on the nature of the mechanism
No full textFollowing a suggestion by the late Lennart Eberson, we have employed the ICl-HFP (HFP being hexafluoropropan-2-ol) system in iodination reactions, and found unambiguous evidence for the occurrence of an ET-mechanism of halogenation. The evidence is based on the use of 'intelligent' substrates, which make it possible to fix the boundaries between the occurrence of an ET-mechanism and of a conventional polar mechanism. In an 'intelligent' substrate, in fact, the nature of the product(s) changes significantly depending on the operating mechanism. The ICl-HFP combination is instrumental to the onset of a one-electron oxidation with electron-rich substrates, followed by halogenation. The most prominent example is that of the electron-rich substrate durene (1,2,4,5-tetramethylbenzene, DUR), when compared to mesitylene (1,3,5-trimethylbenzene, MES): with a 'conventional' iodination system (i.e., I-2/ Ag+) and in common solvents, where the polar mechanism holds, durene is less reactive (k(MES)/k(DUR) = 46 +/- 3), but becomes more reactive (k(MES)/k(DUR) = 0.23) in HFP with ICI, where the ET-mechanism takes over. Other substrates also support the onset of ET-pathways in HFP. Finally, a preliminary survey of a biohalogenation reaction induced by laccase indicates the modest occurrence of a polar process of iodination with a few substrates
The radical rate-determining step in the oxidation of benzyl alcohols by two N-OH-type mediators of laccase: the polar N-oxyl radical intermediate
No full textDetermination of the effect of substituents in the aerobic oxidation of X-substituted benzyl alcohols by laccase,
with mediation by HPI or HBT, confirms the H-atom abstraction from the benzylic C–H bond as the ratedetermining
step (HAT route), and supports a polar nature for the N-oxyl radical intermediate originating from
the two N–OH mediators
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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