3,488 research outputs found
Ergosterol Effect on the Desaturation of 14C-Cis-Vaccenate in Tetrahymena
Supplement of ergosterol to the growth medium of the ciliated protozoan Tetrahymena pyriformis W leads to incorporation of the foreign sterol within cell membranes and suppression of synthesis of the native sterol-like compound tetrahymanol, as well as to changes in the fatty acid compositions of several major classes of membrane lipid. Alteration of fatty acid composition is thought to represent a regulatory mechanism whereby optimum membrane fluidity is maintained when the slightly dissimilar foreign sterol is added into the phospholipid bilayer of the membranes.
The present study, using several different conditions of growth temperature, substrate concentrations and incubation time, and ergosterol concentrations and exposure time, is an attempt to provide evidence supporting a hypothetical regulatory mechanism. This mechanism proposes that there is a feedback regulation by membrane-bound sterol on an enzyme or enzymes involved in synthesis of the long chain fatty acids contained in membrane phospholipid. Such a mechanism could account for the balance between sterol and fatty acid content of membrane. The data presented here show that a statistically significant increase in desaturation of 14C-cis-vaccenate can be demonstrated in Tetrahymena cell cultures whose membranes contain the foreign sterol, when growth temperature is maintained at 20° or 29.5°.
Tetrahymena desaturated 14C-cis-vaccenate substrate in both ergosterol supplemented and normal cultures. The 14C labeled product, 6,11-18:2 was recovered and separated by silver nitrate-Unisil column chromatography
Nanoscale grains, high irreversibility field, and large critical current density as a function of high energy ball milling time in C-doped magnesium diboride
Magnesium diboride (MgB2) powder was mechanically alloyed by high energy ball
milling with C to a composition of Mg(B0.95C0.05)2 and then sintered at 1000 C
in a hot isostatic press. Milling times varied from 1 minute to 3000 minutes.
Full C incorporation required only 30-60 min of milling. Grain size of sintered
samples decreased with increased milling time to less than 30 nm for 20-50 hrs
of milling. Milling had a weak detrimental effect on connectivity. Strong
irreversibility field (H*) increase (from 13.3 T to 17.2 T at 4.2 K) due to
increased milling time was observed and correlated linearly with inverse grain
size (1/d). As a result, high field Jc benefited greatly from lengthy powder
milling. Jc(8 T, 4.2 K) peaked at > 80,000 A/cm2 with 1200 min of milling
compared with only ~ 26,000 A/cm2 for 60 min of milling. This non-compositional
performance increase is attributed to grain refinement of the unsintered powder
by milling, and to the probable suppression of grain growth by milling-induced
MgO nano-dispersions.Comment: 12 pages, 11 figure
Potential, core-level and d band shifts at transition metal surfaces
We have extended the validity of the correlation between the surface
3d-core-level shift (SCLS) and the surface d band shift (SDBS) to the entire 4d
transition metal series and to the neighboring elements Sr and Ag via accurate
first-principles calculations. We find that the correlation is quasilinear and
robust with respect to the differencies both between initial and final-state
calculations of the SCLS's and two distinct measures of the SDBS's. We show
that despite the complex spatial dependence of the surface potential shift
(SPS) and the location of the 3d and 4d orbitals in different regions of space,
the correlation exists because the sampling of the SPS by the 3d and 4d
orbitals remains similar. We show further that the sign change of the SCLS's
across the transition series does indeed arise from the d band-narrowing
mechanism previously proposed. However, while in the heavier transition metals
the predicted increase of d electrons in the surface layer relative to the bulk
arises primarily from transfers from s and p states to d states within the
surface layer, in the lighter transition metals the predicted decrease of
surface d electrons arises primarily from flow out into the vacuum.Comment: RevTex, 22 pages, 5 figures in uufiles form, to appear in Phys.Rev.
Effects of Neutron Irradiation on Carbon Doped MgB2 Wire Segments
We have studied the evolution of superconducting and normal state properties
of neutron irradiated Mg(BC) wire segments as a function
of post exposure annealing time and temperature. The initial fluence fully
suppressed superconductivity and resulted in an anisotropic expansion of the
unit cell. Superconductivity was restored by post-exposure annealing. The upper
critical field, H(T=0), approximately scales with T starting with an
undamaged T near 37 K and H(T=0) near 32 T. Up to an annealing
temperature of 400 C the recovery of T tends to coincide with a
decrease in the normal state resistivity and a systematic recovery of the
lattice parameters. Above 400 C a decrease in order along the c- direction
coincides with an increase in resistivity, but no apparent change in the
evolution of T and H. To first order, it appears that carbon doping
and neutron damaging effect the superconducting properties of MgB
independently
Re-entrant ferroelectricity in liquid crystals
The ferroelectric (Sm C) -- antiferroelectric (Sm C) -- reentrant
ferroelectric (re Sm C) phase temperature sequence was observed for system
with competing synclinic - anticlinic interactions. The basic properties of
this system are as follows (1) the Sm C phase is metastable in temperature
range of the Sm C stability (2) the double inversions of the helix
handedness at Sm C -- Sm C and Sm C% -- re-Sm C phase
transitions were found (3) the threshold electric field that is necessary to
induce synclinic ordering in the Sm C phase decreases near both Sm
C -- Sm C and Sm C -- re-Sm C phase boundaries, and it has
maximum in the middle of the Sm C stability region. All these properties
are properly described by simple Landau model that accounts for nearest
neighboring layer steric interactions and quadrupolar ordering only.Comment: 10 pages, 5 figures, submitted to PR
Correlated defects, metal-insulator transition, and magnetic order in ferromagnetic semiconductors
The effect of disorder on transport and magnetization in ferromagnetic III-V
semiconductors, in particular (Ga,Mn)As, is studied theoretically. We show that
Coulomb-induced correlations of the defect positions are crucial for the
transport and magnetic properties of these highly compensated materials. We
employ Monte Carlo simulations to obtain the correlated defect distributions.
Exact diagonalization gives reasonable results for the spectrum of valence-band
holes and the metal-insulator transition only for correlated disorder. Finally,
we show that the mean-field magnetization also depends crucially on defect
correlations.Comment: 4 pages RevTeX4, 5 figures include
Local Isoelectronic Reactivity of Solid Surfaces
The quantity w^N(r) = ( 1/ k^2 T_el)[partial n(r, T_el) / partial
T_el]_(v(r),N) is introduced as a convenient measure of the local isoelectronic
reactivity of surfaces. It characterizes the local polarizability of the
surface and it can be calculated easily. The quantity w^N(r) supplements the
charge transfer reactivity measured e.g. by the local softness to which it is
closely related. We demonstrate the applicability and virtues of the function
w^N(r) for the example of hydrogen dissociation and adsorption on Pd(100).Comment: RevTeX, 13 pages, 3 figures, to appear in Phys. Rev. Let
Robustness and epistasis in mutation-selection models
We investigate the fitness advantage associated with the robustness of a
phenotype against deleterious mutations using deterministic mutation-selection
models of quasispecies type equipped with a mesa shaped fitness landscape. We
obtain analytic results for the robustness effect which become exact in the
limit of infinite sequence length. Thereby, we are able to clarify a seeming
contradiction between recent rigorous work and an earlier heuristic treatment
based on a mapping to a Schr\"odinger equation. We exploit the quantum
mechanical analogy to calculate a correction term for finite sequence lengths
and verify our analytic results by numerical studies. In addition, we
investigate the occurrence of an error threshold for a general class of
epistatic landscape and show that diminishing epistasis is a necessary but not
sufficient condition for error threshold behavior.Comment: 20 pages, 14 figure
Experimental study exploring the factors that promote rib fragility in the elderly
Rib fractures represent a common injury type due to blunt chest trauma, affecting hospital stay and mortality especially in elderly patients. Factors promoting rib fragility, however, are little investigated. The purpose of this in vitro study was to explore potential determinants of human rib fragility in the elderly. 89 ribs from 13 human donors (55\u201399 years) were loaded in antero-posterior compression until fracture using a material testing machine, while surface strains were captured using a digital image correlation system. The effects of age, sex, bone mineral density, rib level and side, four global morphological factors (e.g. rib length), and seven rib cross-sectional morphological factors (e.g. cortical thickness, determined by \u3bcCT), on fracture load were statistically examined using Pearson correlation coefficients, Mann\u2013Whitney U test as well as Kruskal\u2013Wallis test with Dunn-Bonferroni post hoc correction. Fracture load showed significant dependencies (p < 0.05) from bone mineral density, age, antero-posterior rib length, cortical thickness, bone volume/tissue volume ratio, trabecular number, trabecular separation, and both cross-sectional area moments of inertia and was significantly higher at rib levels 7 and 8 compared to level 4 (p = 0.001/0.013), whereas side had no significant effect (p = 0.989). Cortical thickness exhibited the highest correlation with fracture load (r = 0.722), followed by the high correlation of fracture load with the area moment of inertia around the longitudinal rib cross-sectional axis (r = 0.687). High correlations with maximum external rib surface strain were detected for bone volume/tissue volume ratio (r = 0.631) and trabecular number (r = 0.648), which both also showed high correlations with the minimum internal rib surface strain (r = 12 0.644/ 12 0.559). Together with rib level, the determinants cortical thickness, area moment of inertia around the longitudinal rib cross-sectional axis, as well as bone mineral density exhibited the largest effects on human rib fragility with regard to the fracture load. Sex, rib cage side, and global morphology, in contrast, did not affect rib fragility in this study. When checking elderly patients for rib fractures due to blunt chest trauma, patients with low bone mineral density and the mid-thoracic area should be carefully examined
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