Intermediate Electron Density Research Articles

Electron-self-confinement in an electron gas with an intermediate electron density

The electronic and magnetic structures of thin films with intermediate electron densities, described by the planar uniform jellium model, are investigated using the local spin density approximation. A fully polarized ferromagnetic state, which has a periodic electron density along the perpendicular direction to the surfaces of a film, is the most stable among possible states. In such a state, electrons are self-confined to the valleys of a periodic potential induced by the electrons. This suggests that a 3D electron gas reveals phase transitions from an unpolarized or fully polarized fluid to a 1D electron-self-confined state, to a 2D one, and finally to a 3D Wigner crystal, as the electron density is lowered.

Phase diagram of the weak-coupling two-dimensionalt−t′Hubbard model at low and intermediate electron density

We study the stability of the ferromagnetic phase of the $t\ensuremath{-}{t}^{\ensuremath{'}}$ weak-coupling Hubbard model at low and intermediate electron density within the T-matrix approximation. The superconducting instability of the paramagnetic phase is discussed by a perturbative evaluation of the superconducting vertex.

The structural organization of the PsaC protein in Photosystem I from single crystal EPR and X-ray crystallographic studies

In Photosystem I (PS I) the terminal electron acceptors, F A and F B, are iron–sulfur (4Fe–4S) centers, which are bound to the stromal subunit PsaC. The orientation of PsaC is determined relative to the whole PS I complex (see Schubert, W.-D. et al. (1995) in From Light to Biosphere (Mathis, P. ed.), Vol. II, pp. 3–10, Kluwer) from which a molecular model for the structure of PsaC within PS I is derived. Two strategies are followed: (i) PS I single crystal EPR data on the orientation of the g tensors of both F − A and F − B relative to each other and relative to the crystal axes (see preceding paper) are used in conjuction with the central structural part of the bacterial 2 [Fe 4S 4] ferredoxins, the cysteine binding motifs of which are known to be homologous to those of PsaC; (ii) the same core structure is fitted into the intermediate resolution electron density map of PS I. The PsaC orientation obtained both ways agree well. The local twofold symmetry axis inherent to the ferredoxin model leaves a twofold ambiguity in the structural conclusion. Deviations from this C 2-symmetry in the amino acid sequence of PsaC are analyzed with respect to observable properties which would resolve the remaining structural ambiguity. Arguments both for and against F A being the distal iron–sulfur center (to F X) are discussed.

The organisation of the axonal reticulum at a ligation, in in vitro incubated bovine splenic nerves

From previous studies we concluded that in noradrenergic neurons the axonal reticulum can be considered to be an extension of the Golgi apparatus, directly involved in the condensation and packaging of neurosecretion. But the precise ultrastructure of the organisation of the axonal reticulum in relation to neurosecretory granule formation remained to be elucidaded. This conversion was studied in ligated bovine splenic nerve incubated in vitro for three hours. The ultrastructure of the material accumulating proximally and distally was examined and its nature was determined by phosphotungstic acid staining and immunocytochemistry on glycolmethacrylate sections. Proximal to the ligation predominantly electron-lucent vesicles and tubules were found. Tubules of intermediate electron density appeared in between. The latter, especially in thicker sections, were seen to form complexes with tubules and granules of high electron density. All those elements were shown to be positive for dopamine-β-hydroxylase and cytochrome b561. In the distal part multivesicular bodies accumulated and they were also positive for both enzymes. From these findings it is concluded that the different types of structures accumulating proximally belong to a neurosecretory axonal reticulum. At a block the axonal reticulum is triggered to generate a reticular differentiation, in which granular densities of different size are found. This configuration compares well with that in nerve terminals and strongly suggests that granule formation is basically a local process.

Realistic, detailed images of proteins and tertiary structure elements: ab initio quality electron density calculations for bovine insulin

Detailed, ab initio quality electron density calculations are reported for the elements of the tertiary structure of bovine insulin, a protein containing 773 atoms in 51 residues. The high resolution 3D images of the molecular body and tertiary structure elements of insulin are computed using the new Molecular Electron Density Lego Assembler (MEDLA) method. The high, intermediate, and low electron density images show detailed, intricate shape features not revealed before. The MEDLA technique serves as a computational microscope for chemical, biochemical, pharmaceutical, and drug design applications.

Magneto-luminescence study of carrier-induced excitonic to free-carrier transition of radiative recombination in a semiconductor quantum well

We have investigated the magnetic field dependence of the photoluminescence of a modulation-doped n-AlGaAs/GaAs single quantum well structure while changing the electron density in the well from zero to 7 × 10 11 cm −2 via gate electric fields. A clear transition from excitonic to free-carrier recombination was observed. At intermediate electron density, the existence of magneto-excitons is suggested.

Carrier-induced transition from excitonic to free-carrier-like radiative recombination in a semiconductor quantum well studied by magnetoluminescence.

We have investigated the magnetic-field dependence of the photoluminescence of an $n$-type modulation-doped ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}/\mathrm{GaAs}$ single-quantum-well structure while changing the electron density in the well from zero to 7 \ifmmode\times\else\texttimes\fi{} ${10}^{11}$ ${\mathrm{cm}}^{\ensuremath{-}2}$ via an applied gate voltage. A clear transition from excitonic to free-carrier recombination was observed. At intermediate electron density, the existence of magnetoexcitons is suggested.

Ultrastructure of the basal lamina and its relationship to extracellular matrix of embryos of the starfish Pisaster ochraceus as revealed by anionic dyes.

The ultrastructure of the 51/2-6-day-old embryonic asteroid basal lamina (BL) was studied by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) and after treatment with anionic dyes. Conventional fixation in glutaraldehyde and osmium reveals a BL consisting of a lamina densa separated from the basal cell surface by a lamina lucida. Little or no reticular lamina is present. Material similar in appearance to the basal lamina extends into the blastocoel, forming an extracellular matrix (ECM). Following fixation in the presence of the dye ruthenium red, proteoglycan (PG) granules are visible in the lamina lucida and immediately beneath the lamina densa. The ECM consists of granules of a similar appearance, which are associated with fibers of an intermediate electron density resembling invertebrate collagen. After fixation in the presence of alcian blue under polyanionic conditions, all aspects of the basal lamina and the ECM stain very densely. The use of alcian blue in 0.3 M MgCl2 (monoanionic condition) or in low concentrations reveals a lamina densa consisting of a fine feltwork and tubule-like structures. A meshwork composed of thick, densely stained and thinner, intermediately stained strands is embedded in the inner aspect (that adjacent to the blastocoel) of the ectodermal lamina densa. Similar elements are present in the endodermal BL, but the dense material is represented by short regions that do not form a meshwork. The dense and intermediate strands of both basal laminae also extend into the blastocoel as ECM. The tubule-like structures extend from the dense material of the inner meshwork into the lamina densa. They also cross both the lamina densa and lucida to associatee with the basal cell membranes. The fact that the basal cell surfaces are often puckered outward at the points of contact suggests that this configuration might be providing a means whereby forces can be transferred from the ECM through the basal lamina to the cells.

Eye color pigment granules in wild-type and mutant Drosophila melanogaster

Eye color pigment granules were studied in ultrathin sections of the wild-type fruit fly Drosophila melanogaster and the following eye-color mutants, cinnabar (cn), brown (bw), cinnabar brown (cn bw), and white (w). Ommatin-containing granules of the primary pigment cells are electron lucent in newly emerged flies but are dense in aged flies. The intraretinular granules are of intermediate or high electron density and also contain ommatins. The content of these granules was deduced from comparisons between wild type and cn, which blocks ommatin synthesis. The bw mutant was used to show that drosopterins reside throughout the secondary pigment cells while drosopterin granules monopolize the distal portion. The secondary pigment cell's granules, especially the most distal ones, are electron lucent in our work as well as in most earlier publications. Here we show that these granules are manifested as holes in the section. Both ommatins and drosopterins reside more proximally in the compound eye's pigment cells. We show that white-eyed flies have unusually large granules, and the possible function of these structures is discussed.

Unified theory of the power spectrum of intermediate wavelength ionospheric electron density fluctuations

An extension of the Kolmogorov theory of fluid turbulence is used to develop a unified treatment of the power spectrum of intermediate wavelength electron density fluctuations in the equatorial and high‐latitude ionosphere. Favorable comparison is made between theory and experimental observations.

Ultrastructural responses of rat exocrine pancreas to cholecystokinin octapeptide and secretin

The study examines the ultrastructural changes in the rat pancreas stimulated in vivo to secrete zymogen and fluid by the hormones cholecystokinin and secretin, administered either separately or in combination. The octapeptide of cholecystokinin (CCK-OP) (2.5 × 10 −7 g/kg) 5 min after injection produced discharge of electron-dense yymogen into the acinar lumen and intercellular canaliculi (ICC), leaving misshagen, collapsed zymogen granule profiles around the lumen. Five minutes after secretin (7.5 clinical units/kg), acinar cells were distended, rough endoplasmic reticulum was dilated, acinar lumina and ICC were expanded and filled by electron-lucent and flocculent contents, and there were “halo” zymogen granules and pale “vacuoles.” Electron-lucent zones surrounding acinar and duct cell microvilli indicated trasccellular fluid secretion. When secretin was administered with CCK-OP, the picture was a composite between zymogen and fluid secretory patterns. Zymogen granules took up fluid producing a halo appearance, pale vacuoles formed in acinar cells, and acinar lumina and discharging zymogen granules were of intermediate electron density. The results demonstrated that, although fluid is secreted by duct cells in response to secretin, a major site of secretin-stimulated fluid secretion is acinar cells. Fluid is transported across both cell types by transcellular routes, and the acinar cell fluid secretion is integrated with zymogen discharge. CCK-OP produces partial discharge of undiluted zymogen by exocytosis.

Phagocytosis of Bacteroides melaninogenicus and Bacteroides gingivalis in vitro by human neutrophils.

Phagocytosis of Bacteroides melaninogenicus (five strains) and Bacteroides gingivalis (two strains) was studied by electron microscopy. The chemiluminescent response of bacteria‐challenged polymorphonuclear leukocytes was also measured. There was a correlation between the ability of the bacteria to induce chemiluminescence in polymorphonuclear leukocytes and the cellular uptake of the bacteria. Four of the five B. melaninogenicus strains were phagocytized within 10 min, while for one strain the phagocytic response was delayed. Cells of this strain were surrounded by a loosely bound layer of intermediate electron density. One of the B. gingivalis strains was phagocytized to the same extent as the B. melaninogenicus strains. The other B. gingivalis strain resisted phagocytosis. Cells of this strain were surrounded by an electron dense layer. When comparing previous results on the pathogenicity of these strains in experimental polymicrobial infections in guinea pigs with the present results on phagocytosis, it is apparent that all the pathogenic B. metaninogenicus strains were phagocytized in vitro, while the non‐pathogenic strain of B. gingivalis was phagocytized and the pathogenic strain was not.

Spermatozoa and spermatogenesis in homalorhagha kinorhyncha

Mature spermatozoa and stages in the spermatogenesis of the homalorhagid kinorhynch Pycnophyes communis ( K. Zelinka, 1928 , Monographie der Echinodera, Englemann, Leipzig) were studied by light microscope and transmission electron microscope. The spermatozoon of P. communis is of a highly modified type. It consists of a long sausage-shaped head, with high motility and a short flagellum with very low power of locomotion. A midpiece is lacking. The head contains a thread-formed nucleus surrounded by oval corpuscles, here termed platelets. Three types of platelets, type 1, type 2, and type 3, with high, low, and intermediate electron density, respectively, have been distinguished. Type 1 and type 2 platelets are formed during the whole spermiogenesis, probably from the Golgi apparatus. Type 3 platelets arise late in spermiogenesis probably from mitochondria. A new organelle, a membrane-surrounded “filament”, is described. The filaments, which are arranged in a bundle along one side of the nucleus of the mature spermatozoon, may be responsible for the locomotion of the head. The early spermatid is spherical and has a motile flagellum.

Freeze-fracture and thin-section study of condensing vacuoles in rat pancreatic acinar cells.

Early and late developmental forms of condensing vacuoles are prominent in the relatively low rate secreting acinar cells of suckling rat pancreas. These vacuoles, when studied in freeze-fracture replicas and ultrathin sections under standard processing conditions, showed a biphasic evolution. During the first stage the condensing vacuoles (referred to as CV1) enlarge, accumulating contents of rather low electron density. Fracture faces with irregular patterns, possibly the result of fusion (pinching off) of microvesicles with (from) the condensing vacuoles, were occasionally encountered. The infrequency of such images indicates that fusion-fission during the growth stage must be a very rapid event. One common type of surface irregularity is gibbosities (or convexities) in the P fracture face with complementary images in the E fracture face. The significance of these irregularities, which are in apparent discordance with the theory of microvesicular transport, is unclear. By the end of the growing period the condensing vacuoles are large and smooth-surfaced (referred to as CV2) with contents of intermediate electron density (between that of the initial growing stage and that of the mature zymogen granule). The number of intercalated particles on both the large irregularly surfaced CV1) and large smooth-surfaced condensing vacuoles (CV2) membranes is high and comparable to that of the Golgi saccule and endoplasmic reticulum membranes. During the second stage, the smooth-surfaced condensing vacuoles undergo volume reduction associated with progressive increase in the electron density of their contents, thus becoming zymogen granules. Concomitant with size reduction, the number of intercalated particles in the membranes with CV2 diminishes markedly. The process of membrane retrieval appears to be accomplished selectively by pinching off coated microvesicles heavily studded with intercalated particles.

Changes in the ultrastructure of Chlamydia psittaci produced by treatment of the host cell with DEAE-Dextran and cycloheximide

The ultrastructure of two Chlamydia psittaci serotypes was investigated in infected L-cell monolayers fixed and embedded in situ . This preparation preserved the relationship of the parasite to the internal structure of the host cells. Chlamydial inclusions in cells treated with diethylaminoethyl-dextran (DEAE-D) and cycloheximide were compared to inclusions in untreated cells. C. psittaci serotype 1 had the same or larger yields of infectious progeny per infected cell in treated cells as in untreated cells. This serotype developed normal inclusions following both treatments. C. psittaci serotype 2 had decreased infectivity yields per infected cell and developed aberrant chlamydial forms in treated cell monolayers. Serotype 2 inclusions contained pycnotic chlamydial forms and numerous miniature reticulate bodies in cycloheximide-treated cells. In DEAE-Dtreated cells, some serotype 2 reticulate bodies were abnormally pleomorphic and contained amorphous material of intermediate electron density. Other reticulate bodies had lost electron density, degenerated, and contained only fragments of the reticulum. Maturation of reticulate bodies into infectious elementary bodies was greatly decreased in serotype 2 inclusions that contained aberrant forms.

Bark beetle – fungal symbiosis. II. Fine structure of a basidiomycetous ectosymbiont of the southern pine beetle

A basidiomycetous yeast is a dimorphic fungal ectosymbiont associated with the southern pine beetle, Dendroctonus frontalis. Within the mycangium (a glandular integumental invagination) of the southern pine beetle, the fungus grows in a yeast-like manner, while on some media or in a plant host, it forms mycelial masses. The yeast stage has a lamellate cell wall and forms new cross walls in a manner rather similar to the basidiomycetous yeasts. Like certain ambrosial fungi (Ascoidea), the yeast stage is multinucleate. The mycelial stage, initially produced in culture and within the plant host, shows clamp connections and dolipore septa, characteristic of many Basidiomycetes, but its cell wall is not lamellate.The dolipore septa have septal pore caps and also mirror images of these caps. The septal pore caps (about 400 Å in thickness) have five layers: the two outermost (40 Å) are probably continuous with the 'unit membranes' of the endoplasmic reticulum; the middle layer is similar to the outermost; and the intervening zones (150–170 Å) are homogeneous and of intermediate electron density. The endoplasmic reticulum is both tubular and cisternal, and some portions of the membranes are associated with ribosomes. The multinucleate condition is regarded as a special adaptation for effective colonization; otherwise the ectosymbiont is consistently similar to known basidiomycetous yeasts.

Corrections to the Dielectric Constant of a Degenerate Electron Gas

The Lindhard approximation to the frequency- and wavenumber dependent dielectric constant, epsilon ( kappa , omega ), provides a good description of many properties of the degenerate electron gas. However, it is known that the short-range behavior of the gas is not adequately represented by this function and it is necessary to include certain additional terms. DuBois incorporated some exchange terms into kappa , omega ) and was able to obtain the correction to the plasmon excitation frequency. Though his final results are reasonable and have been corroborated using alternative approaches the corrected'' dielectric constant is found to violate certain a priori restrictions. In this paper a more accurate dielectric constant is derived. In order to obtain an acceptable function which does not violate the sum rule and positive definiteness restrictions on the imaginary part it is necessary to account for three types of corrections. These corrections originate in (1) the effective screening of the long-range interaction between particles; (2) the shift in single-particle energies of electrons and holes; and (3) the tendency of particles and holes to form bound states when any repulsive interparticle interaction is present. With these corrections all spurious singularities in the dielectric constant disappear. Numericalmore » calculations of epsilon ( kappa , omega ) and of moments of the imaginary part of this function have been carried out for an intermediate electron density equal to the density of conduction electrons in aluminum. The resulting dielectric constant departs by as much as 50% from the Lindhard form for low frequencies, but has similar qualitative features. The moments can be used to determine the high-frequency behavior and other properties of the electron gas. (auth)« less