1,720,988 research outputs found
Self-assembly in organic solvents of a deoxyguanosine derivative induced by alkali metal picrates
No full text3′,5′-Didecanoyl-2′-deoxyguanosine in chlorinated organic solvents undergoes self-assembly mediated by alkali metal picrates which leads to octameric or polymeric species according to the amount of ions present
The use of CD spectroscopy for the study of the self-assembly of guanine derivatives
No full textCircular dichroism (CD) spectroscopy can be successfully used to study the assembly of homoguanylic and guanosine-rich oligonucleotides. The CD corresponding to the low-energy bands of the guanine spectrum in the 250-290 nm region is mainly determined by exciton interaction between the guanine chromophores; it gives information on the relative skew of G-quartets and on their distortion from the classical structure found in the four-stranded helix of poly(G). The handedness of the sugar-phosphate backbone can be deduced indirectly only if the conformation of the bases around the glycosyl bond is known
A new axially-chiral photochemical switch
No full textAxially chiral bis(azo) derivative 1 undergoes photochemical isomerisation, which can be seen with circular dichroism and pitch measurements of the induced cholesteric phases. © 2003 The Royal Society of Chemistry
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
Columnar lyomesophases formed in hydrocarbon solvents by chiral lipophilic guanosine-alkali metal complexes
No full textThe lipophilic guanosine derivative 1 acts as a self-assembled ionophore and, in the presence of alkali metal ions, forms chiral polymeric structures in organic solvents, These polymeric columnar aggregates are comprised of G-quartets held together by alkali metal ions which are located inside the tubular structure; the quartets are surrounded by hydrocarbon chains. In hydrocarbon solvents, these columnar ag gregates form lyomesophases of the cholesteric and hexagonal type
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
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