1,720,981 research outputs found
Chemically induced dynamic electron polarization of C-60 nitroxide bisadducts and spin multiplicity of their excited states
No full textA series of biradicals consisting of a C-60 linked to two nitroxide addends have been studied by time-resolved electron paramagnetic resonance (TR-EPR) spectroscopy. The series includes all four trans bisadducts and the equatorial isomer. After a visible laser pulse, the bisadducts in toluene liquid solution show chemically induced dynamic electron polarization (CIDEP) effects with a complex pattern of narrow Lines in emission and enhanced absorption. From the knowledge of the sign of exchange interaction between the two nitroxide moieties, it was possible to account for the CIDEP effect in terms of an intramolecular triplet-triplet annihilation. A broad signal attributed to the quintet excited state was observed at short time delay after the laser pulse. The TR-EPR spectra recorded at low temperature in glassy matrix are assigned to an excited triplet state localized on the C-60, with a small interaction with the spins of the two nitroxide fragments. The assignment is based on spectral simulation and analysis of the transient nutation frequency. The small exchange interaction between excited triplet C-60 and nitroxide is shown to depend on the nature of the nitroxide addend
The sign of the exchange interaction between triplet excited fullerene and nitroxide free radicals
No full textThe sign of the exchange interaction J in a series of radical triplet pairs (RTPs), formed
by a nitroxide free radical and a triplet excited fullerene, has been determined from the spin polarization
of time-resolved electron paramagnetic resonance spectra. Radical and fullerene are linked together
by covalent bonds in different geometries. It is shown that the sign of J depends on the overlap
between the orbital of nitroxide unpaired electron and the LUMO of fullerene, which is singly
occupied in the excited triplet state. When the overlap does not vanish, a negative contribution to J
arises from the admixing of a charge transfer structure in the wave function of the excited doublet
state D of the RTP, which does not take place in the excited quartet state Q. The mixing of D*
and Q' states lowers the energy of the former spin state and gives antiferromagnetic coupling
Intramolecular Exchange Interaction in many-spin systems. Determination of magnitude and sign by EPR, ENDOR and CIDEP effects due to the interaction with the excited triplet states
No full textSummary : Two series of derivatives containing C60 covalently linked to one and to two nitroxide groups have been investigated by EPR, ENDOR and time resolved EPR with pulsed laser excitation. In the first series, the sign of the electron exchange interaction between an unpaired electron spin and the spins of the triplet excited fullerene moiety has been determined. It is found that it depends on the overlap between the nitroxide unpaired electron wave function and the LUMO orbital of the C60 moiety. In the second series, magnitude and sign of the electron exchange interaction between the two nitroxide electron spins has been obtained from the EPR spectrum shape and the characteristics of the ENDOR spectra. The polarisation of the EPR lines observed when these derivatives are photoexcited by a laser light pulse is discussed in terms of C60 triplet annihilation by the nitroxide biradical triplet.
Abstract : Nous avons étudié le spectres RPE, ENDOR et RPE resolu dan le temps de deux series de molecules ou C60 est lié avec un ou deux radicaux libres nitroxides. Pour la première serie le signe de l’interaction d’èchange electronique radical-état de triplet a eté determiné. Elle dépand de la superposition entre la fonction d’onde de l’electron désaccouplé du nitroxide et l’orbital LUMO du C60.
Dans la deuxiemme serie, la valeur et le signe de l’interaction d’échange ont été obtenue a partir de la forme des spectres RPE et des caracteristiques du spectre ENDOR. La polarisation de RPE raies observè avec excitation laser est interprété suivant un model d’interaction triplet-triplet entre le triplet excité sur le C60 et le triplet formé par le deux nitroxides
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
Angiotensin II-induced over-activation of p47phox in fibroblasts friorn hypertensives: which role in the enhanced ERK1/2 responsiveness to angiotensin II?
No full textBackground Fibroblasts Eire involved in the remodeling of the heart and of the vasculature associated to arterial hypertension, and an abnormal extracellular signal-regulated kinase 1/2 (ERK1/2) activation by angiotensin II (Ang II) plays a pivotal role in this process. However, the intracellular pathways leading to cell hypertrophy and hyperplasia, as well as to collagen production, are still incompletely known.
Objective To investigate the role of superoxide anion (O-2(-)) and of nicotinamide adenine dinucleotide phosphate (NAD(P)H) oxidase in Ang II-stimulated ERK1/2 over-activation in fibroblasts from hypertensive patients.
Methods O-2(-) production was measured in skin fibroblasts from hypertensives (HT, n = 11) and from normotensive controls (NT, n = 10) by electron spin resonance technique. ERK1/2 phosphorylation and p47phox NAD(P)H oxidase subunit translocation were measured by western blot.
Results Ang II (1 mu mol/l) induced a larger p47phox subunit translocation and increased intracellular O-2(-) production to a larger extent in HT in comparison to NT and this effect was blocked by apocynin, an inhibitor of the NAD(P)H oxidase. Ang II increased ERK1/2 phosphorylation more in HT than in NT. The Ang III-induced ERK1/2 phosphorylation was inhibited by apocynin in a dose-dependent manner in NT, but not in HT.
Conclusions The chain of cellular events leading to increased ERK1/2 responsiveness to Ang II in hypertension include an exaggerated response of p47phox, NAD(P)H oxidase and O-2(-), but it is partially resistant to apocynin. Therefore, NAD(P)H-dependent reactive oxygen species (ROS) production is not the only determinant of the exaggerated ERK1/2 responsiveness in fibroblasts of hypertensives (HT)
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