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Cubic phases of lipid - containing systems: structure analysis and biological implications
No full textOf the six cubic phases identified so far in lipid-containing systems, the structures of only two have been determined unambiguously. We tackle the structure determination of the other four. We use for that purpose a novel pattern recognition approach, which consists of generating all the sets of phase angles (phi-sets) compatible with the observed reflections, and of screening them in a search for the "best" one. Two criteria are used for screening: both involve the parameter [(delta rho)4] (delta rho is a dimensionless function proportional to the Fourier transform of the set of observed structure factors). One is a test of smoothness, based upon the postulate that the "best" phi-set is that whose [(delta rho)4] is minimum; this criterion, equivalent to maximum entropy, is fulfilled when the system is devoid of heavy atoms, and when the polar and the hydrocarbon moieties occupy almost equal volumes. The other criterion is based upon the notion that [(delta rho)4] takes the same (or similar) values in thermodynamic phases with the same (or similar) chemical composition, whatever the structure of the phases. The validity of the two criteria is verified using numerous examples. The six cubic phases are analysed using this approach. The structure of three of them (Q230, Q224, Q229) can be described in terms of two three-dimensional networks of connected rods, mutually intertwined and unconnected: in Q230 the rods are coplanarly joined 3 by 3; in Q224 the rods are tetrahedrally joined 4 by 4; in Q229 the rods are cubically joined 6 by 6. The structures of Q212 and Q227 are related to those of Q230 and Q224, respectively; one of the two networks of rods is preserved, the other is replaced by a lattice of closed micelles. The structure of Q223 appears to consist of a cage-like continuous three-dimensional network of connected globules, coplanarly joined 3 by 3 at one end and 4 by 4 at the other, enclosing a three-dimensional lattice of closed micelles. The analogies of the structures of Q230, Q224, Q229 with the three fundamental cubic infinite periodic minimal surfaces are discussed. More interestingly, the structures of, on the one hand Q230, Q224, Q229 and of Q212, Q227, Q223 on the other, are shown to provide topological generalizations of the two paradigms of lipid organization; namely, the bilayer and the monolayer
X-ray crystallography at macromolecular resolution: A solution of the phase problem
No full textThe problem of crystal structure analysis at macromolecular resolution is tackled using notions borrowed from small‐angle solution‐scattering studies. The mathematical procedure ‐ essentially a pattern recognition approach ‐ consists of generating all the phase combinations compatible with the data and screening them: the critical steps are the choice of the “pattern” and of the criteria which preside the “recognition”. Two strategies are adopted. One is based upon use of the histogram of the electron density maps, a function more dependent on chemical composition than on physical structure: the histograms of samples of different structure and similar chemical composition are compared in search for similarity. The other strategy operates on the electron density maps and involves the principle of maximum smoothness. From the practical viewpoint, one single parameter, (Δp)4, is found to play an outstanding role in the two cases. Algorithms are worked out, implemented on the computer and tested using a variety of lipid‐water phases. The comparison of samples of different structure and similar chemical composition is found to provide a powerful screening criterion. The principle of maximum smoothness is also of great help, at least to the extent that the electron density distribution is a fairly smooth function, devoid of sharp peaks (associated with heavy atoms). The practical aspects of the algorithms are discussed, as well as their possible application to crystallographic problems of more general interes
Freeze fracture electron microscope study of lipid system. The cubic phases of space group Pm3n
No full textCubic phase of lipid-containing systems. Element of a theory and biological connotations
No full textPolymorphism of a lipid extract from Pseudomonas Fluorescens : structure analysis of a hexagonal phase and of a novel cubic phase of extinction symbol Fd--
No full textStructure and polymorphism of tetraether lipids from< i> Sulfolobus solfataricus</i>: I. Crystallographic analysis
No full textCubic phases of lipid containing systems. The structure of phase Q223 (space group Pm3n). An X-ray scattering study
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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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