1,720,989 research outputs found
DETERMINATION OF ABSOLUTE CONFIGURATION OF HELICENES AND RELATED BIARYLS FROM CALCULATION OF HELICAL TWISTING POWERS BY THE SURFACE CHIRALITY MODEL
No full textThe sign and the magnitude of the helical twisting power β for a series of helicenes have been calculated by the Surface Chirality model. The principal contribution to β derives from the helicity in the direction perpendicular to what can be defined as the main molecular plane; the cholesteric axis is also predicted to be along this direction, in agreement with a view of the cholesteric induction by helical molecules based on empirical observations. In the case of non-rigid biphenanthryl derivatives, the value of β is predicted to vary with the dihedral angle between the phenanthryl moieties, changing sign at about 90° where the conformation passes from s-cis to s-trans, i.e. when the stereochemical descriptor of the biaryl moiety exchanges the M and P indices. Comparison between experimental and calculated values indicates an s-trans conformation for the flexible dopants, in agreement with a previous conclusion drawn from empirical correlations. The Surface Chirality model appears to be a promising technique to assess the absolute configuration of rigid molecules by the comparison of experimental and calculated β values. For flexible molecules, the quality of the information depends critically on the degree of knowledge of their conformational freedom
The chirality of the cholesteric phase of DNA and G-wires: its connection to their molecular structures
No full textWhile the handedness of the cholesteric phases formed by assembled guanosine derivatives (G-wires) follow the correlation right-handed helices --> right-handed cholesterics (left-handed --> left-handed), the cholesteric phase formed by B-DNA (right-handed helix) is left-handed. This apparent discrepancy is overcome by considering pitch (p) variations with temperature. Plots of p(-1) versus T-1 have, in fact, the same trend (positive intercept and negative slope) in the case of right-handed G-wires and B-DNA, while for the left-handed G-wire of deoxyguanosine monophosphate (dGMP), the opposite behavior is observed (negative intercept and positive slope). Therefore, the relation between molecular handedness and cholesteric helicity cannot, in general, be assessed by using measurements based on a single temperature; hence the temperature variation of the cholesteric parameters should be investigated. In all cases there is no remarkable salt effect on the cholesteric parameters
Oligodeoxyguanylates: A case of self–assembly leading to lyotropic liquid crystals
No full text. The guanosine derivatives d(Gp)1-5G, dissolved in water, give rise to cholesteric and hexagonal mesophases. The results of X-ray Diffraction, Optical Microscopy, Circular Dichroism and Small Angle Neutron Scattering measurements indicate that the building block of the liquid-crystalline phases is a chiral rod, composed of a stacked array of Hoogsteen-bonded guanosine tetramers. The concentration at which the cholesteric phase appears increases with the oligomerisation degree, a pattern that seems to be related to the change of the ratio of negative charge/guanine units along the series. © 1993 IUPA
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
Effect of Δ9-tetrahydrocannabinol on phosphorylated CREB in rat cerebellum: An immunohistochemical study
No full textSeveral converging lines of evidence indicate that drugs of abuse may exert their long-term effects on the central nervous system by modulating signaling pathways controlling gene expression. Cannabinoids produce, beside locomotor effects, cognitive impairment through central CB1 cannabinoid receptors. Data clearly indicate that the cerebellum, an area enriched with CB1 receptors, has a role not only in motor function but also in cognition. This immunohistochemical study examines the effect of Δ9-tetrahydrocannabinol (Δ9-THC), the principal psychoactive component of marijuana, on the levels of phosphorylated CREB (p-CREB) in the rat cerebellum. Acute treatments with Δ9-THC at doses of 5 or 10 mg/kg induced a significant increase of p-CREB in the granule cell layer of the cerebellum, an effect blocked by the CB1 receptor antagonist SR 141716A. Following chronic Δ9-THC administration (10 mg/kg/day for 4 weeks), the density of p-CREB was markedly attenuated compared to controls, and this attenuation persisted 3 weeks after withdrawal from Δ9-THC. These data provide evidence for the involvement of cerebellar granule cells in the adaptive changes occurring during acute and chronic Δ9-THC exposure. This might be a mechanism by which Δ9-THC interferes with motor and cognitive functions. © 2005 Elsevier B.V. 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
Delta-9-tetrahydrocannabinol differently affects striatal c-Fos expression following haloperidol or clozapine administration
No full textIt was previously shown that haloperidol, but not clozapine, induced intense rat catalepsy when co-administered with delta-9-tetrahydrocannabinol. The present study investigated whether similar alterations could be observed on striatal c-Fos immunoreactivity after administration of the same drug combinations. Western Blot and immunocytochemistry stereological analyses indicated that delta-9-tetrahydrocannabinol (0.5 mg/kg) increased striatal c-Fos immunoreactivity induced by haloperidol (0.1 mg/kg). Conversely, no significant alterations of striatal c-Fos immunoreactivity were observed after injections of clozapine (10 mg/kg) + vehicle, clozapine + delta-9-tetrahydrocannabinol or vehicle + delta-9-tetrahydrocannabinol. The present results indicate that the behavioral effects induced by delta-9-tetrahydrocannabinol in haloperidol- and clozapine-treated rats are associated with different striatal c-Fos expressions. © 2008 Elsevier B.V. All rights reserved
A Study of the Enolization of Some Alkyl Ketones in Lyomesophases formed by Alkylammonium Surfactants
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