1,720,967 research outputs found
Crystal structures and superoxide dismutase mimetic activity of [CuL2(Him)2].MeOH and [CuL2(mim)2].H2O [HL = 4-amino-N-(thiazol-2-yl)benzenesulfonamide, Him = imidazole, mim = M-methylimidazole].
No full textThe enzymatic properties of octopus vulgaris hemocyanin: o-Diphenol oxidase activity
No full textHemocyanin and tyrosinase are dinuclear copper proteins capable of reversibly binding dioxygen.
Despite the great similarity of structure and properties of their active site, the two proteins perform different biological functions (oxygen transport/storage versus monooxygenase and oxidase activity). In this paper, we show that Octopus Vulgaris hemocyanin exhibits a tyrosinase-like activity; namely, it is capable of utilizing dioxygen for the oxidation of o-diphenol to quinone. The reaction is specific for this isomer of diphenol, the meta and para isomers being unreactive, and is strongly controlled by steric factors. Dioxygen represents a cosubstrate of the reaction, and it is involved in the catalytic turnover by binding to the dinuclear copper site of the protein to form, under steady-state conditions, oxy-Hc, which is the active species. The generation of semiquinone radicals, detected by EPR and by their reaction with N,N,N¢,N¢-tetramethyl-1,4-phenylenediamine, strongly supports a reaction mechanism in which such radicals represent the reaction products of one-electron oxidation of the substrate, quinone being generated by dismutation of semiquinones. Met-Hc is regenerated by the substrate to the deoxy form. To close the catalytic cycle, the proposed reaction mechanism also involves the participation of two transient protein forms with the total oxidation state of the active site (V and IV) intermediate between that of oxy-Hcy, [CuIIO22-CuII]VI, and deoxy-Hc, [CuICuI]II. A mathematical model has been elaborated to describe the reaction kinetics. The differences in reaction mechanisms between hemocyanin and tyrosinase are discussed in terms of accessibility to exogenous molecules of their active sites
The oxidation of hemocyanin: kinetics, reaction mechanism and characterization of the met-hemocyanin product
No full textThe reaction that gives met-hemocyanin from Octopus vulgaris oxy-hemocyanin has been reinvestigated under several experimental conditions. Various anions including azide, fluoride and acetate have been found to promote this reaction. Kinetic data indicate that the reaction mechanism is different from that currently accepted involving a peroxide displacement of hound dioxygen through an associative chemistry on an open axial position of the copper ions [Hepp, A. F., Himmelwright, R. S., Eickman, N. C. and Solomon, E. I. (1979) Biochem. Biophys. Res. Commun. 89, 1050-1057; Solomon, E. I, in Copper proteins (Spiro, T. G., ed.) pp. 43-108, J. Wiley, New York]. Our study suggests that the protonated form of the anion is likely to be the species reacting with the oxygenated form of the protein. Furthermore, it is also proposed that protonation of bound dioxygen generates an intermediate hydroperoxo-dicopper(II) complex to which the exogenous anion is also bound. This intermediate in not accumulated and precedes the release of hydrogen peroxide by reaction with water. Upon dialysis it leads to the met-hemocyanin form, The structure of this dinuclear copper(II) derivative contains a di-mu-hydroxo bridge but there is evidence from optical and circular dichroism spectra for partial protonation of these bridges at low pH. As a consequence, while one azide molecule binds in the bridging mode to met-hemocyanin with low affinity (K = 30 M(-1)) at pH 7.0, it binds with much higher affinity at pH 5.5 (K = 1500 M(-1)), where a second azide ligand also binds in the terminal mode (K = 20 M(-1)). The coordination mode of the azide ligands is deduced from the optical and circular dichroism spectra of the protein complexes
Metal complexes of the carbonic anhydrase inhibitor methazolamide (Hmacm). Crystal structure of the Zn(macm)2(NH3)2. Anticonvulsant properties of the Cu(macm)2(NH3)2(H2O)
No full textCopper monooxygenase models. Aromatic hydroxylation by a dinuclear copper(I) complex containing methionine sulfur ligands
No full textFunctional Superoxide Dismutase Mimics. Structural Characterization and Magnetic Exchange Interactions of Copper(II)-N-substituted Sulfonamide Dimer Complexes
No full textDinuclear copper(II) complexes with N-substituted sulfonamide ligands as superoxide dismutase (SOD) mimics
have been investigated. The new N-(thiazol-2-yl)toluenesulfonamide (Htz-tol) and N-(thiazol-2-yl)naphthalenesulfonamide
(Htz-naf) ligands have been prepared and structurally characterized. The complexes derived from
these ligands, [Cu2(tz-tol)4] (1) and [Cu2(tz-naf)4] (2), have been synthesized, and their crystal structure, magnetic
properties, and EPR spectra were studied in detail. In both compounds the metal centers are bridged by four
nonlinear triatomic NCN groups. The coordination geometry of the coppers in the dinuclear entity of 1 and 2 is
distorted square planar with two N-thiazole and two N-sulfonamido atoms. Magnetic susceptibility data show a
strong antiferromagnetic coupling, with -2J ) 121.3 cm-1 for compound 1 and -2J ) 104.3 cm-1 for compound
2. The EPR spectra of the polycrystalline samples of compounds 1 and 2 have been measured at the X- and
Q-band frequencies at different temperatures. Above 20 K the spectra are characteristic of S ) 1 species with
zero-field splitting parameter D ) 0.230 cm-1 for compound 1 and 0.229 cm-1 for compound 2. The EPR parameters
are discussed in terms of the known binuclear structures. The complexes exhibit high SOD activity, as shown by
the low IC50 values obtained with the xanthine/xanthine oxidase/NBT assay: 0.13 íM for compound 1; 0.17 íM
for compound
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
- …
