1,721,002 research outputs found
Shadow bands, gap and pseudogap in high-Tc superconductors
No full textWithin the framework of the Charge Density Wave Quantum Critical Point (CDW-QCP] scenario for high-Tc superconductors (HTCS) we introduce a model for tight-binding electrons coupled to quasi-critical fluctuations. In the normal state our model reproduces features the Fermi Surface (FS) observed in ARPES measurements on optimality doped Bi2212, such as the anisotropic suppression of spectral weight around the M points of the Brillouin zone. The spectral density is characterized by a transfer of spectral weight from the main quasiparticle peak to dispersing shadow peaks which originate branches of a shadow FS. In the superconducting state our model reproduces the d-wave symmetry of the gap parameter, which results from a balance between small-q attraction and large-q repulsion. The gap parameter is enhanced due to cooperative effects of charge and spin fluctuations
Electron transfer induced dissociation of chloro-cyano-benzene radical anion: Driving chemical reactions via charge restraints
No full textWe introduce a new quantum mechanics/molecular mechanics based method to drive electron transfer reactions. Our approach uses the dynamically restrained electrostatic potential derived charges of the quantum atoms(1) as a reaction coordinate, and allows an estimation of the free energy barrier of the electron transfer process. Moreover, it provides an accurate description of the electronic structure changes and of the nuclear reorganization associated with the reaction. We use the method to describe the electron-transfer induced dissociation of the m-chloro-cyano-benzene radical anion in aqueous solution. The reaction is triggered by solvent reorganization by a change in the coordination water shell around the cyano nitrogen atom. At the onset of the reaction, charge-spin segregation is observed. The negative charge is transferred to the leaving Cl, while the spin density localizes on the non-saturated carbon atom of the benzene ring. The calculated free energy barrier of dissociation is in good quantitative agreement with the experimental data
Reaction mechanism of caspases: Insights from QM/MM Car-Parrinello simulations
No full textCaspases are fundamental targets for pharmaceutical interventions in a variety of diseases involving disregulated apoptosis. Here, we present a quantum mechanics/mol. mechanics Car-Parrinello study of key steps of the enzymic reaction for a representative member of this family, caspase-3. The hydrolysis of the acyl-enzyme complex is described at the d. functional (BLYP) level of theory while the protein frame and solvent are treated using the GROMOS96 force field. These calcns. show that the attack of the hydrolytic water mol. implies an activation free energy of .apprx.DFA ~ 19+-4 kcal/mol in good agreement with exptl. data and leads to a previously unrecognized gem-diol intermediate that can readily (DFA ~ 5+-3 kcal/mol) evolve to the enzyme products. Our findings assist in elucidating the striking difference in catalytic activity between caspases and other structurally well-characterized cysteine proteases (papains and cathepsins) and may help design novel transition-state analog inhibitors. [on SciFinder (R)]LCB
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
Reaction mechanism of caspases: insights from QM/MM Car-Parrinello simulations
No full textCaspases are fundamental targets for pharmaceutical interventions in a variety of diseases involving disregulated apoptosis. Here, we present a quantum mechanics/molecular mechanics Car-Parrinello study of key steps of the enzymatic reaction for a representative member of this family, caspase-3. The hydrolysis of the acyl-enzyme complex is described at the density functional (BLYP) level of theory while the protein frame and solvent are treated using the GROMOS96 force field. These calculations show that the attack of the hydrolytic water molecule implies an activation free energy of ca. DeltaF(A) approximately equal 19 +/- 4 kcal/mol in good agreement with experimental data and leads to a previously unrecognized gem-diol intermediate that can readily (DeltaF(A) approximately equal 5 +/- 3 kcal/mol) evolve to the enzyme products. Our findings assist in elucidating the striking difference in catalytic activity between caspases and other structurally well-characterized cysteine proteases (papains and cathepsins) and may help design novel transition-state analog inhibitors
Quantum Mechanical/Molecular Mechanical (QM/MM) Car-Parrinello Simulations in Excited States
No full textThe combination of time-dependent density functional theory (TDDFT) for the description of excited
states with a hybrid quantum mechanics/molecular mechanics (QM/MM) approach enables the study of photochemical
processes in complex environments. Here, we present a short overview of recent applications of TDDFT/
MM approaches to a variety of systems including studies of the optical properties of prototypical organic and
inorganic molecules in gas phase and solution, photoinduced electron transfer reactions in donor-bridge-acceptor
complexes, and in situ investigations of the molecular mechanisms of photoactive proteins. The application of
TDDFT/MM techniques to a wide range of systems enables an assessment of the current performance and limitations
of these methods for the characterization of photochemical processes in complex systems
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