1,724,254 research outputs found
On understanding proton transfer to the biocatalytic [Fe―Fe]H sub-cluster in [Fe―Fe]H2ases: QM/MM MD simulations
No full textAbstractProton transfer to the [Fe―Fe]H sub-cluster in the Desulfovibrio desulfuricans (DdH) and Clostridium pasteurianum (CpI) [Fe―Fe] hydrogenases was investigated by a combination of first principles and empirical molecular dynamics simulations. Pathways that can be inferred from the X-ray crystal structures of DdH and CpI, i.e., (Glu159→Ser198→Glu156→water460→Cys178→DTMA([Fe―Fe]H) and (Glu282→Ser319→Glu279→water612→Cys299), respectively, were considered. Proton transfer from Cys178 to DTMA in the [Fe―Fe]H sub-cluster in DdH was readily observed in our results, specifically when [Fe―Fe]H was in the reduced state ([FeI―FeI]) or in the mixed valence state for the protonated distal iron Fed ([FeI―FeII―H−]H). A concerted mechanism is proposed, where proton transfer in DdH from Glu159 to Glu156 via Ser198 and Glu156 to Cys178 via water460 readily occurred, as well as from Glu282 to Glu279 via Ser319 and Glu279 to Cys299 via water612 in CpI. The theoretical prediction of the proton transfer characteristics is consistent with the assumed biocatalytic mechanism of the [Fe―Fe] hydrogenases in which the proton binds at Fed, providing confirmation that has not been explored so far. The computational results were qualitatively validated by the agreement with experimental hydrogen production activity data for mutated CpI enzymes, relative to the wild-type protein. Finally, the insight provided by the simulations, combined, in part, with experimental validation, are important for establishing an approach in future exploration of proton transfer to the active site in this class of enzymes, and possibly also for biomimetic analogs
La natrodufrénite (Na, ⃞) (Fe+ + +, Fe+ + ) (Fe+ + +, Al)5 (PO4)4 (OH)6 • 2H2O, une nouvelle espèce minérale du groupe de la dufrénite
No full textThe Rochefort-en-Terre dufrénite, described by Lacroix (1910), is remarkable for its high Na content which justifies the naming of a new mineral species : natrodufrénite (approved by the IMA Commission). This mineral has been found in a limonitic ore layer, associated with cyrilovite and goethite. The X-ray powder pattern is identical with that of the dufrénite crystals from Wheal Phoenix. Pleochroïsm distinct with X = pale yellow and Z = dark green ; parallel or sub-parallel extinction, positive elongation ; α' = 1.756 ± 0.005 γ' = 1.775 ± 0.005 ; Gmeas = 3.20, Gcalc = 3.23. Wet chemical analysis gave Na₂O 2.36, CaO 0.14 ; FeO 0.42 ; Al₂O₃ 5.51 ; Fe₂O₃ 46.21 ; P₂O₅ 32.67 ; H₂O 13.04 ; total : 100.35.
The simplified structural formula, based on Moore's model (1970) :
(Na,⃞) (Fe⁺⁺⁺, Fe⁺⁺) (Fe⁺⁺⁺, Al)₅ (PO₄)₄ (OH)₆ • 2 H₂O
TGA and DTA curves are given and discussed.La dufrénite de Rochefort-en-Terre, décrite par Lacroix (1910), est remarquable par sa teneur en Na qui justifie l'appellation : natrodufrénite marquant une nouvelle espèce. Ce minéral provient d'une couche de limonite où elle est associée à de la cyrilovite et de la goethite. Le diagramme de poudre est pratiquement identique à celui de la dufrénite de Wheal Phoenix. Pléochroïsme net : jaune (X), vert foncé (Z). Extinction droite ou presque, allongement positif, n'g = 1,775 ± 0,005, n'p = 1,765 ± 0,005. dmes = 3,20, dx = 3,23. Analyses chimiques par voie humide : Na₂O 2,36 ; CaO 0,14 ; FeO 0,42 ; Al₂O₃ 5,51 ; Fe₂O₃ 46,21 ; P₂O₅ 32,67 ; H₂O 13,04 ; total : 100,35. La formule structurale basée sur le type proposé par Moore (1970) est essentiellement :
(Na,⃞) (Fe⁺⁺⁺, Fe⁺⁺) (Fe⁺⁺⁺, Al)₅ (PO₄)₄ (OH)₆ • 2 H₂O.
Les courbes d'analyses thermiques, thermopondérales et différentielles sont données et discutées.Fontan François, Pillard François, Permingeat François. La natrodufrénite (Na, ⃞) (Fe+ + +, Fe+ + ) (Fe+ + +, Al)5 (PO4)4 (OH)6 • 2H2O, une nouvelle espèce minérale du groupe de la dufrénite. In: Bulletin de Minéralogie, volume 105, 4, 1982. pp. 321-326
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
Thermal behaviour of polymetallic metal carbonyls with Fe-Fe, Fe-Hg and Hg-Hg bonds containing Lewis bases.
No full textThe thermal behaviour of polymetallic metal carbonyls containing Fe-Fe, Fe-Hg and Hg-Hg bonds and Lewis bases, such as [Fe-3(CO)(8)(L)(2)] (L = 1,10-phenantroline,2,2'-bipyridine), [Fe(CO)(4)(HgCl)(2)] and [Fe(CO)(4)(HgCl)(2)(L)(2)] (L = 1,10-phenantroline,2-quinolinethiol), have been investigated by thermal analysis (TG), Thermal studies give evidence that the thermal decomposition mechanisms and starting temperatures are strongly influenced by the Lewis bases. The thermal decompositions under synthetic air yielded, in all cases, the final solid product Fe2O3 which presence was confirmed by X-ray powder diffraction technique.UNIV ESTADUAL PAULISTA,INST QUIM ARARAQUARA,BR-14800900 ARARAQUARA,SP,BRAZILUNIV ESTADUAL PAULISTA,INST QUIM ARARAQUARA,BR-14800900 ARARAQUARA,SP,BRAZI
Sorpcja jonow Fe²⁺, Fe³⁺ i Mn²⁺ w niedotlenionym materiale gleby lessowej
No full textThis study examined courses of sorption and of leaching with distilled water of ions Fe²⁺, Fe³⁺, and Mn²⁺ in non-deoxidized and unoxygenated loessial soil material. The sorption was measured using a dynamic method. ODR and Eh were measured as indices of soil oxygenation. It was found that unoxygenation of loessial soil material increases the sorption of ions Fe²⁺, Fe³⁺, and Mn²⁺ in samples of pH 4.8, while in samples having pH 6.5 an increased sorption was only found for Fe³⁺ ions.Zbadano przebieg sorpcji jonów Fe²⁺, Fe³⁺ i Mn²⁺ i wymywania ich wodą destylowaną, w materiale gleby lessowej - nieodtlenionym i niedotlenionym. Sorpcję przeprowadzono metodą dynamiczną. Jako wskaźniki natlenienia mierzono ODR i Eh. Stwierdzonie materiału gleby lessowej zwiększa sorpcję jonów Fe²⁺, Fe³⁺ i Mn²⁺ w próbkach o pH 4,8 a w próbkach o pH 6,5 jedynie jonów Fe³⁺
Mechanism of Hydrogen Production in [Fe−Fe]-Hydrogenase: A Density Functional Theory Study
No full text[Fe−Fe]-hydrogenases are a class of metalloenzymes that catalyze the production of H2 from two protons
and two electrons. In this work, we used density functional theory (DFT) calculations to analyze the mechanism
of hydrogen production, providing insight into the role of the intermediates in the catalysis. We also validated
the exchange-correlation functional applied within DFT for model compounds of the active site in [Fe−Fe]-hydrogenase, enabling us a reliable application for understanding previously established hydrogen production
hypotheses, as well as providing a starting point for a future investigation of the effects of the protein environment
on the catalytic mechanism of [Fe−Fe]-hydrogenases
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
Hitherto Unknown [Fe-Fe]-Hydrogenase Gene Diversity in Anaerobes and Anoxic Enrichments from a Moderately Acidic Fen
Full text linkNewly designed primers for [Fe-Fe]-hydrogenases indicated that (i) fermenters, acetogens, and undefined species in a fen harbor hitherto unknown hydrogenases and (ii) Clostridium- and Thermosinus-related primary fermenters, as well as secondary fermenters related to sulfate or iron reducers might be responsible for hydrogen production in the fen. Comparative analysis of [Fe-Fe]-hydrogenase and 16S rRNA gene-based phylogenies indicated the presence of homologous multiple hydrogenases per organism and inconsistencies between 16S rRNA gene- and [Fe-Fe]-hydrogenase-based phylogenies, necessitating appropriate qualification of [Fe-Fe]-hydrogenase gene data for diversity analyses. Copyright © 2010, American Society for Microbiology. All Rights Reserved
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
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