1,720,975 research outputs found
The Color and Stability of Maya Blue: TDDFT Calculations
No full textCar-Parrinello structural optimizations of realistic models of the Maya Blue (MB) hybrid material are combined with TDDFT calculations of the electronic excitation spectra to identify the nature of the fundamental guest-host interactions leading to the unusual stability of this pigment. The comparison with the features of the experimental visible spectrum reveals that the main mode of interaction between the host solid (the palygorskite clay) and the guest molecule (the organic indigo dye) involves the coordination of the carbonyl group of the dye by Al3+ ions exposed at the edge of the palygorskite tunnels. Analogous Mg2+-dye interactions which do not strongly affect the MB visible spectrum can also be present. Thermal treatment used in the preparation of the pigment appears therefore essential to release some of the structural water molecules tightly bound to the Al3+ ions in the internal clay surface, thus leaving them available to coordinate the organic molecule. Moderate heating also favors the oxidation of indigo to dehydroindigo (DHI): the spectral features of the latter complex with Al3+ are in remarkable agreement with the experimental spectrum, thus confirming the substantial role of DHI in the properties of Maya Blue
Application of correlated and uncorrelated models to the diffusion of linear molecules in silicalite
No full textTwo- and N-step correlated models for the analysis of MD trajectories of linear molecules in silicalite
No full textStatics and dynamics of Ethane molecules in AlPO4-5: a molecular dynamics simulation study
No full text
First-principles molecular dynamics investigation of the D-amino acid oxidative half-reaction catalyzed by the flavoenzyme D-amino acid oxidase
No full textLarge-scale Car-Parrinello molecular dynamics simulations Of D-alanine oxidation catalyzed by the flavoenzyme D-ammo acid oxidase have been carried out. A model of the enzyme active site was built by starting from the enzyme X-ray structure, and by testing different subsystems comprising different sets of aminoacyl residues. In this process, the stability of the enzyme-substrate complex was taken as a measure of the accuracy of the model. The activated transfer of the amino acid a-hydrogen from the substrate to the flavin N5 position was then induced by constraining a suitable transfer reaction coordinate, and the free energy profile of the reaction was calculated. The evolution of electronic and structural properties of both enzyme-bound substrate and flavin cofactor along the reaction path is consistent with a hydride-transfer mechanism. The calculated free energy barrier for this process (13 kcal/mol) is in excellent agreement with the activation energy value derived from the experimentally determined rate constant for the corresponding enzyme-catalyzed reaction. The electronic distribution of the reduced flavin shows that the transferred electrons tend to be centered near the C4a position rather than delocalized over the flavin pyrimidine ring. This feature is mechanistically relevant in that such an electronic distribution may promote the subsequent enzyme-catalyzed reduction of molecular oxygen to yield hydrogen peroxide via a postulated flavin 4a-peroxide intermediate. These results also show that a first-principles molecular dynamics approach is suitable to study the mechanism of complex enzymatic processes, provided that a smaller, yet reliable, subsystem of the enzyme can be identified, and special computational techniques are employed to enhance the sampling of the reactive event
Statics and dynamics of ethane molecules in AlPO4-5 studied by molecular dynamics simulation
No full text
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
- …
