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High Electron Density (high + electron_density)

Distribution by Scientific Domains

Chemistry	80%

Selected Abstracts

Ultrastructural and histochemical study of the salivary glands of Aplysia depilans (Mollusca, Opisthobranchia)

ACTA ZOOLOGICA, Issue 3 2001
Alexandre Lobo-da-Cunha
Abstract The digestive system of the sea hare, Aplysia depilans, includes a pair of ribbon-shaped salivary glands. A central duct and a large blood vessel run close to each other along the length of these glands and both are surrounded by a layer of muscle cells. Three cell types form the glandular epithelium: granular cells, vacuolated cells and mucocytes. The granular cells possess cilia and spherical secretion granules, located primarily in the apical region. The granules of immature cells have a low electron density and are mainly formed by neutral polysaccharides with small amounts of proteins. The granules of mature cells are larger, have a high electron density and are mainly formed by proteins with lower amounts of neutral polysaccharides. Transition stages between immature and mature granular cells are observed. The vacuolated cells are large and frequently pyramidal in shape, but after the application of histochemical techniques almost all vacuoles remain uncoloured. The numerous vacuoles contain flocculent material in a clear background and the mitochondria possess large crystalline structures in the matrix. A pyramidal shape is also typical of the mucocytes, which are filled with vesicles containing granular masses surrounded by a network of secretion material. These large cells are strongly stained by Alcian blue, revealing the presence of acidic mucopolysaccharides. This is the first ultrastructural study of the salivary glands in opisthobranch gastropods. [source]

Ultrastructure, development and histochemistry of the polysaccharide-containing subcuticular compartments in Origanum dictamnus L. peltate glandular hairs

FLAVOUR AND FRAGRANCE JOURNAL, Issue 4 2010
Artemios M. Bosabalidis
Abstract Peltate glandular hairs of Origanum dictamnus at the stage of secretion create two subcuticular chambers; one large and bladder-like, at the apex of the head (containing essential oil), and one small and ring-like, at the bottom of the head (containing polysaccharides). In the apical chamber, along with the essential oil, a small lateral compartment containing polysaccharides, also exists. This compartment surrounds peripherally the apical chamber creating a second ring-like structure. The apical plasmalemma of the head cells exhibits a high electron density and presumably has a specific substructure to facilitate passing to the subcuticular chamber of the secretory product. The latter probably exists in the form of glucosides, which, after passing across the plasmalemma and entering the apical chamber, become hydrolysed into the aglycone fraction (essential oil) and the sugar fraction (polysaccharides). Copyright © 2010 John Wiley & Sons, Ltd. [source]

Der Mechanismus der Alken-Ozonolyse , eine kritische Betrachtung

HELVETICA CHIMICA ACTA, Issue 8 2004
Kurt Schank
The Mechanism of Alkene Ozonolysis , a Critical Examination. Alkene ozonolyses have been carried out in experimental organic chemistry for more than a century, and a lucid description of the underlying reaction mechanism has been developped and integrated in most chemistry textbooks. Nevertheless, there are a great number of exceptions hard to rationalize by the accepted mechanism. Attempts in the past to give more-convincing answers to this problem failed. Therefore, the search for an alternative mechanistic model without contradictions is compelling. The present paper offers a redox model based upon the known properties of ozone both as a strong oxidant and an O-atom-transferring agent, especially when reacted with alkenes of sufficiently high electron density to act as reducing partners. [source]

The application of structure envelopes in structure determination from powder diffraction data

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2 2002
Simon Brenner
A structure envelope is a special type of periodic nodal surface that separates regions of high electron density from those of low electron density. Once such a surface has been generated, it can be used in combination with direct-space methods to facilitate structure solution from powder data. To generate an informative structure envelope, the phases of the structure factors of a few strong low-order reflections must be determined; an algorithm has been developed for this purpose. The program SayPerm combines (a) the use of error-correcting codes (e.c.c.'s) to sample phase space efficiently, (b) a pseudo-atom approximation of structure fragments to simulate atomic resolution at ca 2.5,Å, and (c) phase extension and phase set ranking using the Sayre equation. The effect of using a structure envelope in structure solution was first tested in combination with a subroutine for finding zeolite topologies in the program FOCUS. Then extension to molecular structures in combination with a simulated-annealing program was explored. This resulted in the development of the program Safe and the subsequent determination of the structure of a tri-,-peptide (C32N3O6H53) with 17 variable torsion angles. [source]

Rapid chain tracing of polypeptide backbones in electron-density maps

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 3 2010
Thomas C. Terwilliger
A method for the rapid tracing of polypeptide backbones has been developed. The method creates an approximate chain tracing that is useful for visual evaluation of whether a structure has been solved and for use in scoring the quality of electron-density maps. The essence of the method is to (i) sample candidate C, positions at spacings of approximately 0.6,Å along ridgelines of high electron density, (ii) list all possible nonapeptides that satisfy simple geometric and density criteria using these candidate C, positions, (iii) score the nonapeptides and choose the highest scoring ones, and (iv) find the longest chains that can be made by connecting nonamers. An indexing and storage scheme that allows a single calculation of most distances and density values is used to speed up the process. The method was applied to 42 density-modified electron-density maps at resolutions from 1.5 to 3.8,Å. A total of 21,428 residues in these maps were traced in 24 CPU min with an overall r.m.s.d. of 1.61,Å for C, atoms compared with the known refined structures. The method appears to be suitable for rapid evaluation of electron-density map quality. [source]