Stable Beta Research Articles

Combined atomistic–crystal plasticity analysis of the effect of beta phase precipitates on deformation and fracture of lamellar γ + α2 titanium aluminide

Atomistic simulations based on the use of interatomic potentials and a finite element method based on the crystal plasticity theory are combined to investigate the deformation and fracture behaviour of polycrystalline lamellar γ-TiAl + α2-Ti3Al material containing 10 vol % of body centred cubic beta phase precipitates. The effects of both stable beta phase precipitates, which deform by slip, and metastable beta phase precipitates, which deform by a combination of stress-induced martensitic transformation and slip, are studied. To model the cracking along the grain boundaries and the matrix–precipitate interfaces, the grain boundaries and interfaces are modelled using a cohesive zone approach. The grain boundary–interface potentials are determined by carrying out atomistic simulations of the grain boundary–interface normal separation (decohesion) and sliding.

Plasma transport control and self-sustaining fusion reactor

The possibility of a high-performance/low-cost fusion reactor concept which can simultaneously satisfy (1) high beta, (2) high bootstrap fractio (self-sustaining) and (3) high confinement is discussed. In CDX-U, a tokamak configuration was created and sustained solely by internally generated bootstrap currents, in which a 'seed' current is created through a nonclassical current diffusion process. Recent theoretical studies of MHD stability limits in spherical tori [e.g. the National Spherical Torus Experiment (NSTX)] produced a promising regime with stable beta of 45% and bootstrap current fraction of . Since the bootstrap current is generated by the pressure gradient, to satisfy the needed current profile for MHD stable high beta regimes, it is essential to develop a means to control the pressure profile. It is suggested that the most efficient approach for pressure profile control is through the creation of transport barriers (localized regions of low plasma transport) in the plasma. As a tool for creating the core transport barrier, poloidal-sheared-flow generation by ion Bernstein waves (IBW) near the wave absorption region appears to be promising. In PBX-M, application of IBW power produced a high-quality internal transport barrier where the ion energy and particle transport became neoclassical in the barrier region. The observation is consistent with the IBW-induced-poloidal-sheared-flow model. An experiment is planned on TFTR to demonstrate this concept with D - T reactor-grade plasmas. For edge transport control, a method based on electron ripple injection (ERI), driven by electron cyclotron heating (ECH), is being developed on CDX-U. It is estimated that both the IBW and ERI methods can create a transport barrier in reactor-grade plasmas (e.g. ITER) with a relatively small amount of power .

A further examination of the effect of diversification on the stability of portfolio betas

Although many studies have found a non-trivial incidence of beta instability for individual common stocks, there exists controversy over the beta stability characteristics to expect in portfolios formed from these stocks. The extent is examined to which portfolio formation can diversify away beta instability. Specifically, although a constant beta for the market portfolio is acknowledged as the trivial limiting case, particular attention is devoted to shedding light on the speed with which diversification can deliver a reasonable expectation of stable portfolio betas. The issue is examined by forming portfolios which mix together constant beta stocks and varying beta stocks. The evidence is generally consistent with the presence of a diversification effect, but as the portfolio size increases, results do show a greater proportion of constant beta stocks are needed to maintain the relative stability of the portfolio.

Molecular anatomy of the beta' subunit of the E. coli RNA polymerase: identification of regions involved in polymerase assembly.

The E. coli RNA polymerase is a multisubunit enzyme, which is present in two different forms: the catalytic competent core enzyme (alpha2beta beta') and the promoter selective holoenzyme (alpha2beta beta' sigma). Correct assembly of individual subunits into core or holoenzyme is essential for the function of this enzyme. Mutant beta' proteins truncated near the centre or at the C-terminus were able to form stable core enzyme-like complexes under reconstitution conditions. Mutant beta' proteins lacking the region between amino acids 201-477 failed to form holoenzyme complexes while retaining the ability to form core enzyme complexes. Furthermore, free beta' subunit interacted with free sigma subunit to form a stable beta' sigma subassembly. Removal of amino acids 201-477 from the beta' subunit strongly interfered with this interaction. Our results suggest that the N-terminal region of the beta' subunit is involved in the assembly of core enzyme. The region between amino acids 201 and 477 on beta' may be directly or indirectly involved in the interaction between the beta' subunit and the sigma subunit.

Expression and Functional Role of Syntaxin 1/HPC-1 in Pancreatic β Cells: SYNTAXIN 1A, BUT NOT 1B, PLAYS A NEGATIVE ROLE IN REGULATORY INSULIN RELEASE PATHWAY

Syntaxin 1/HPC-1 is an integral membrane protein, which is thought to be implicated in the regulation of synaptic neurotransmitter release. We investigated syntaxin 1 expression in pancreatic beta cells and the functional role of syntaxin 1 in the insulin release mechanism. Expression of syntaxin 1A, but not 1B, was detected in mouse isolated islets by the reverse transcriptase-polymerase chain reaction procedure. An immunoprecipitation study of metabolically labeled islets with an anti-rat syntaxin 1/HPC-1 antibody demonstrated syntaxin 1A protein with an apparent molecular mass of approximately 35 kDa. Immunohistochemistry of the mouse pancreas demonstrated that syntaxin 1/HPC-1 was present in the plasma membranes of the islets of Langerhans. In order to determine the functional role of syntaxin 1 in pancreatic beta-cells, rat syntaxin 1A or 1B was overexpressed in mouse beta TC3 cells using the transient transfection procedure. Transfection of beta TC3 cells with either syntaxin 1 resulted in approximately 7-fold increases in their immunodetectable protein levels. Glucose-stimulated insulin release by syntaxin 1A-overexpressing cells was suppressed to about 50% of the level in control cells, whereas insulin release by syntaxin 1B-overexpressing and control cells did not differ. Next, we established stable beta TC3 cell lines that overexpressed syntaxin 1A and used them to evaluate the effect of syntaxin 1A on the regulatory insulin release pathway. Two insulin secretogogues, 4-beta-phorbol 12-myristate 13-acetate or forskolin, increased insulin release by untransfected beta TC3 cells markedly, but their effects were diminished in syntaxin 1A-overexpressing beta TC3 cells. Glucose-unstimulated insulin release and the proinsulin biosynthetic rate were not affected by syntaxin 1A overexpression, indicating a specific role of syntaxin 1A in the regulatory insulin release pathway. Finally, in vitro binding assays showed that syntaxin 1A binds to insulin secretory granules, indicating an inhibitory role of syntaxin 1A in insulin exocytosis via its interaction with vesicular proteins. These results demonstrate that syntaxin 1A is expressed in the islets of Langerhans and functions as a negative regulator in the regulatory insulin release pathway.

Identification of a three-amino-acid region in G protein gamma 1 as a determinant of selective beta gamma heterodimerization.

Guanine-nucleotide-binding protein beta and gamma subunits belong to large protein families encompassing at least five and ten members, respectively, from mammalian cells. The formation of stable beta gamma heterodimers is a selective process determined by the primary sequences of both the beta and gamma subunit. For example, gamma 2 dimerizes with both beta 1 and beta 2, gamma 1 with beta 1, but not with beta 2. To identify the structural elements of gamma subunits relevant to the selectivity of beta gamma dimerization, we have used the baculovirus-insect cell-expression system to produce chimeric beta and gamma subunits and have studied their dimerization using an assay based on the ability of isoprenylation-resistant gamma subunit mutants to draw beta subunits into the cytosol and including sucrose density gradient analysis of soluble recombinant beta gamma dimers. The results show that replacement of three consecutive residues of gamma 1, Cys36-Cys37-Glu38, by the corresponding residues of gamma 2, Ala33-Ala34-Ala35, suffices to render the mutant gamma 1 subunit capable of forming heterodimers with beta 2. The ability of mutant gamma 1 subunits to dimerize with beta 2 does not correlate with the probability of the mutated region to participate in coiled-coil structures. The tripeptide region identified here as a critical determinant of the selectivity of beta gamma dimer formation is distinct from, but partially overlaps with, the region reported by Lee et al. [Lee, C., Murakami, T. & Simonds, W. F. (1995) J. Biol. Chem. 270, 8779-8784]. The results of this study, therefore, not only extend the region of gamma 1 selecting between beta 1 and beta 2 to the five-residue sequence between Cys36 and Phe40, but also argue against the notion that the hydrophobic terminal residue of this motif represents the key determinant of selective beta gamma interaction.

Specificity of the PTB Domain of Shc for β Turn-forming Pentapeptide Motifs Amino-terminal to Phosphotyrosine

Shc phosphorylation in cells following growth factor, insulin, cytokine, and lymphocyte receptor activation leads to its association with Grb2 and activation of Ras. In addition to being a cytoplasmic substrate of tyrosine kinases, Shc contains an SH2 domain and a non-SH2 phosphotyrosine binding (PTB) domain. Here we show that the Shc PTB domain, but not the SH2 domain, binds with high affinity (ID50 approximately equal to 1 microM) to phosphopeptides corresponding to the sequence surrounding Tyr250 of the polyoma virus middle T (mT) antigen (LLSNPTpYSVMRSK). Truncation studies show that five residues amino-terminal to tyrosine are required for high affinity binding, whereas all residues carboxyl-terminal to tyrosine can be deleted without loss of affinity. Substitution studies show that tyrosine phosphorylation is required and residues at -5, -3, -2, and -1 positions relative to pTyr are important for this interaction. 1H NMR studies demonstrate that the phosphorylated mT antigen-derived sequence forms a stable beta turn in solution, and correlations between structure and function indicate that the beta turn is important for PTB domain recognition. These results show that PTB domains are functionally distinct from SH2 domains. Whereas SH2 domain binding specificity derives from peptide sequences carboxyl-terminal to phosphotyrosine, the Shc PTB domain gains specificity by interacting with beta turn-forming sequences amino-terminal to phosphotyrosine.

Wall stabilization of high beta plasmas in DIII-D

Detailed analysis of recent high beta discharges in the DIII-D [Plasma Physics Controlled Nuclear Fusion Research, 1986 (International Atomic Energy Agency, Vienna, 1987), Vol. I, p. 159] tokamak demonstrates that the resistive vacuum vessel can provide stabilization of low n magnetohydrodynamic (MHD) modes. The experimental beta values reaching up to βT=12.6% are more than 30% larger than the maximum stable beta calculated with no wall stabilization. Plasma rotation is essential for stabilization. When the plasma rotation slows sufficiently, unstable modes with the characteristics of the predicted ‘‘resistive wall’’ mode are observed. Through slowing of the plasma rotation between the q=2 and q=3 surfaces with the application of a nonaxisymmetric field, it has been determined that the rotation at the outer rational surfaces is most important, and that the critical rotation frequency is of the order of Ω/2π=1 kHz.

Present Status of Systems and their Prospects

Recent research has made significant progress for the plasma confinement in helical systems. Experiments have been carried out in W7AS, CHS, ATF, H-E and others. The plasmas, whose electron temperature is around 2-3 keV, ion temperature is 1 keV and density is 2-3x10{sup 20}m{sup -3}, are well confined with the energy confinement time {tau}{sub E}=10-40 ms and the stable beta value reaches at 2% ({beta}(0)=7%). The obtained scalings for the global energy confinement time such as LHD, GRB(Gyro-Reduced Bohm), L-G(Lackner-Gottardi) scalings are comparable with the tokamak L-mode scaling in the above parameters` range. The improved confinement modes are also explored in helical systems as H mode or the control of plasma profile. In this paper, firstly presented experimental results are reviewed on the confinement, MHD equilibrium/stability beta limit, and secondly the outline of LHD and W7X program is described. Finally, a discussion is given on the feasibility of helical systems as a fusion reactor, based on the present experimental results. 21 refs., 8 figs., 3 tabs.

Adoption of beta structure by the inactivating "ball" peptide of the Shaker B potassium channel

The conformation of the inactivating peptide of the Shaker B K+ channel (ShB peptide) and that of a noninactivating mutant (ShBL7E peptide) have been studied. Under all experimental conditions explored, the mutant peptide remains in a predominantly nonordered conformation. On the contrary, the inactivating ShB peptide has a great tendency to adopt a highly stable beta structure, particularly when challenged "in vitro" by anionic phospholipid vesicles. Because the putative peptide binding elements at the inner mouth of the channel comprise a ring of anionic residues and a hydrophobic pocket, we hypothesize that the conformational restrictions imposed on the ShB peptide by its interaction with the anionic lipid vesicles could partly imitate those imposed by the above ion channel elements. Thus, we propose that adoption of beta structure by the inactivating peptide may also occur during channel inactivation. Moreover, the difficulties encountered by the noninactivating ShBL7E peptide mutant to adopt beta structure and the observation that trypsin hydrolysis of the ShB peptide prevent both structure formation and channel inactivation lend further support to the hypothesis that adoption of beta structure by the inactivating peptide in a hydrophobic environment is important in determining channel blockade.

Stability of high beta tokamak plasmas*

Stability at high beta (the ratio of plasma pressure to magnetic field pressure) is an important requirement for a compact, economically attractive fusion reactor. It is also important in present large tokamak experiments, where the best performance is now often limited by instabilities rather than by energy transport. The past decade has seen major advances in our understanding of the stability of high beta tokamak plasmas, as well as in the achievement of high values of beta. Ideal magnetohydrodynamic (MHD) theory has been remarkably successful in predicting the stability limits, and the scaling of maximum stable beta with the normalized plasma current predicted by Troyon and others has been confirmed in many experiments, yielding a limit βmax≊3.5 (%-m-T/MA) I/aB (where I is the plasma current, a is the minor radius, and B is the toroidal field). The instabilities which are predicted to limit beta have been observed experimentally, in good agreement with theoretical predictions, including long-wavelength kink modes and short-wavelength ballooning instabilities. Advances in understanding of tokamak stability have opened several paths to higher values of beta. The use of strong discharge shaping, approaching the limits of axisymmetric stability, has allowed beta values as high as 12% to be reached in agreement with Troyon scaling. Recent experimental results and ideal MHD modeling have shown that the beta limit depends on the form of the pressure and current density profiles, and modification of the current density to create a centrally peaked profile has allowed beta values up to 6I/aB to be achieved experimentally. Recent experiments have also begun to explore both local and global access to the predicted second stable regime for ballooning modes, with the potential for very high values of β/(I/aB). Preliminary experimental investigations of wall stabilization and radio-frequency (RF) current profile control hold the promise of further improvements in beta through passive and active control of instabilities. The developing understanding of high beta stability and the application of this understanding to present experiments and future fusion devices hold the potential for production of stable, steady state plasmas at high beta with good confinement.

In vitro processing of recombinant G protein gamma subunits. Requirements for assembly of an active beta gamma complex.

The gamma subunits of the heterotrimeric G proteins (G gamma) are subject to carboxyl-terminal processing. This processing involves prenylation of a cysteine residue initially 4 amino acids from the carboxyl terminus, endoproteolytic truncation of the 3 terminal amino acids, and methylation of the now carboxyl-terminal prenylcysteine residue. The significance of each of these modifications in the ultimate properties of G proteins is not yet clear. We have developed in vitro systems for the production of the three processing intermediates (unmodified, prenylated, and truncated-prenylated) for two G gamma subunits, one which is subject to farnesylation (G gamma 1) and one which is geranylgeranylated (G gamma 2). Assessment of the functional consequences of the processing of G gamma was found to require reconstitution of the polypeptides with a G protein beta subunit (G beta). The ability of recombinant G beta, produced in Sf9 cells, to assemble into stable beta gamma complexes (G beta gamma) with each of the G gamma processing intermediates was assessed. Both prenylated and unprenylated G gamma subunits formed stable complexes with G beta, but surprisingly, neither of the truncated-prenylated G gamma subunits were competent for this assembly. The G beta gamma complexes which were formed were examined for their ability to interact with a G protein alpha subunit (G alpha). Only those G beta gamma complexes containing a prenylated G gamma subunit were functional in this assay. These data indicate that: 1) prenylation of G gamma is not required for G beta gamma assembly; 2) assembly of the G beta gamma complex occurs prior to the proteolytic processing of G gamma; and 3) G beta gamma complexes require prenylated G gamma for interaction with G alpha.

Molecular dissection of the beta subunit of F1-ATPase into peptide fragments.

Partial digestion of the native beta subunit of F1-ATPase from the thermophilic Bacillus strain PS3 by three different proteases produced a limited number of peptide fragments. In most cases, the peptides remained associated, and the gross structure of the beta subunit was not destroyed. Furthermore, most peptides were able to reassociate into the form of the beta subunit after denaturating urea treatment. Therefore, the cleaved sites are most likely located in water-exposed loop regions in the tertiary structure of the protein. Almost all peptides were analyzed, and 17 cleaved sites were determined. From the analysis of the distribution of cleaved sites and deletions or insertions in the multiple amino acid sequence alignment of proteins homologous to the beta subunit, locations of five loops and four candidate loops in the beta subunit are suggested. There are two large loops in the central region of the beta subunit sequence, and dicyclohexylcarbodiimide-reactive Glu190 is located in one of them. Tyr341, involved in putative catalytic ATP binding, is also found in one of the loops. Then, taking cleaved sites as a reference, two kinds of expression plasmids, each of which carried genes of two complementary peptide fragments, 1-193 and 198-473 or 1-284 and 285-473, were constructed and expressed in Escherichia coli. For each plasmid, two peptides were coexpressed, associated into a stable beta subunit form in E. coli cells, and purified without dissociation. When these beta subunits were denatured by urea and applied to polyacrylamide gel without denaturant, a protein band with the same mobility as that of the beta subunit appeared, indicating that reassociation of peptide fragments into the form of the beta subunit occurred upon removal of urea. These beta subunits retained the ability to reconstitute the alpha 3 beta 3 gamma complexes even though the efficiency of reconstitution and the recovered ATPase activities were decreased. These complexes were stable at high or low temperature, and ATPase activities were sensitive to inhibition by N3-.

Portfolio risk, adjustment costs, and security dealers' bid/ask spreads

This article notes that dealers' bid/ask spreads should vary directly with their costs of adjusting to inventory imbalances. Thus, well-diversified dealers are expected to quote lower bid/ask spreads on stocks with substantial total risk caused by undiversifiable risk. Furthermore, the effect of systematic risk on bid/ask spreads should be negligible if dealers are compensated for systematic risk by market returns. This article shows that, empirically, bid/ask spreads of OTC stocks are insensitive to the systematic risk of individual stocks—even when only stocks with stable betas are considered. Furthermore, bid/ask spreads are not sensitive to changes in market variance, as would be expected if systematic risk affected spreads. While unsystematic risk affects bid/ask spreads, its effect is pronounced for stocks traded by small, undiversified dealers. If stocks are only traded by large dealers with low diversification costs, unsystematic risk does not affect bid/ask spreads.

In vitro synthesis of G protein βγ dimers

Abstract The guanine nucleotide-binding proteins (G proteins), which play a central role in coupling membrane-bound receptors to intracellular effectors, are heterotrimers composed of alpha, beta, and gamma subunits. The beta and gamma subunits form a functional monomer that does not appear to separate under physiological conditions. This has made it difficult to differentiate the individual roles of beta and gamma subunits in signal transduction. To characterize the individual subunits, the 36-kDa beta subunit (beta 1), brain gamma (gamma 2), and transducin gamma (gamma t) were translated in vitro in a rabbit reticulocyte lysate system. Hydrodynamic studies and tryptic proteolysis were used to compare the physical properties of the in vitro translation products with those of beta gamma dimers purified from bovine brain. The hydrodynamic studies indicate that, without gamma subunits, the beta subunits are not stable but tend to aggregate into high molecular weight complexes. When beta and gamma subunits were co-translated, stable beta gamma dimers formed that bound alpha 0 in a guanine nucleotide-dependent manner. The beta gamma dimers were less hydrophobic than those purified from bovine brain. This may reflect a lack of post-translational modification in the reticulocyte lysate or other differences between the in vitro translation products and the purified beta gamma. When beta and gamma were translated separately and then mixed, beta gamma dimers also formed. Analysis of in vitro translated beta gamma subunits will provide ways to assess the function of these subunits and to determine the structural requirements for beta gamma formation.

Degradation of Alpha-, Beta-, and Gamma-Hexachlorocyclohexane by a Soil Bacterium under Aerobic Conditions

A Pseudomonas sp., isolated from sugarcane rhizosphere soil, readily metabolized not only alpha and gamma isomers of hexachlorocyclohexane, but also the thermodynamically more stable beta isomer, under aerobic conditions. Bacterial degradation of the three isomers led to the accumulation of a transitory metabolite and eventual release of covalently linked chlorine as chloride in stoichiometric amounts.

Detection of ampicillin resistant Haemophilus influenzae in United Kingdom laboratories.

Susceptibility of Haemophilus influenzae clinical isolates to ampicillin reported by 23 laboratories, using a variety of methods, was compared with results obtained following retesting at The London Hospital Medical College. Beta lactamase production was not detected on initial isolation in 25 of 157 isolates (16%) found to be positive on retest. One hundred beta lactamase negative isolates, which gave reduced zone diameters (less than 20 mm) around 2 micrograms discs and required 1-64 mg/l ampicillin for inhibition, were detected at The London Hospital. Eighty five of these had been reported as sensitive to ampicillin by the laboratories of origin. Many of these 100 isolates showed reduced susceptibility to other beta lactam antibiotics. Accurate detection of non-enzymic reduced susceptibility to ampicillin may emerge as an important guide to the likely sensitivity of H influenzae isolates to the enzyme stable beta lactams.

Amino acid sequence and comparative antigenicity of chicken metallothionein.

The complete amino acid sequence of metallothionein (MT) from chicken liver is reported. The primary structure was determined by automated sequence analysis of peptides produced by limited acid hydrolysis and by trypsin digestion. The comparative antigenicity of chicken MT was determined by radioimmunoassay using rabbit anti-rat MT polyclonal antibody. Chicken MT consists of 63 amino acids as compared to 61 found in MTs from mammals. One insertion (and two substitutions) occurs in the amino-terminal region, a region considered invariant among mammalian MTs. Eighteen of the 20 cysteines in chicken MT were aligned with cysteines from other mammalian sequences. Two cysteines near the carboxyl terminus are shifted by one residue due to the insertion of proline in that region. Overall, the chicken protein showed approximately equal to 68% sequence identity in a comparison with various mammalian MTs. The affinity of the polyclonal antibody for chicken MT was decreased by 2 orders of magnitude in comparison to that of a mammalian MT (rat MT isoforms). This reduced affinity is attributed to major substitutions in chicken MT in the regions of the principal determinants of mammalian MTs. Theoretical analysis of the primary structure predicted the secondary structure to consist of reverse turns and random coils with no stable beta or helix conformations. There is no evidence that chicken MT differs functionally from mammalian MTs.

Inductive inhibition of cold-plasma stabilization of curvature-driven modes in finite-length plasmas

A modified quasistatic theory that incorporates inductive effects in earlier electrostatic models connects the conventional electrostatic and magnetohydrodynamic (MHD) pictures of line tying by cold plasma. The modified theory predicts that curvature-driven flute modes in mirror-confined plasmas can be stabilized by moderate concentrations of cold plasma if the beta of the hot, mirror-confined plasma is less than a critical value. The maximum stable beta for an idealized stratified model of a hot-ion plasma, separated from conducting end walls by cold plasma, is given approximately by βcrit≂(4RpRc/LhLc) ×[ω2pe(cold)/ (k2⊥c2+ω2pe(cold))]. For ω2pe(cold) ≪k2⊥ c2, this result is identical with the predictions of electrostatic models. In the opposite limit, the result resembles the predictions of ideal MHD models, namely, that unstable curvature-driven modes can occur even in the presence of dense cold plasmas if the hot-plasma beta exceeds a limiting value estimated to be π2RpRc/L2.

Variable versus stationary beta in the market model: A comparative analysis

The assumption of stable beta coefficient within the context of the market model, and the consequent use of ordinary least squares. estimation, has been widely criticized. In this study an alternative market model is offered in which beta is allowed to vary over time. Varying parameters regression is used to estimate the variable beta and the estimates are compared to those produced by ordinary least squares in terms of accuracy of predictions. The results indicate that neither model shows a clear advantage over the other. Even when a large change in beta was simulated, the differences in prediction errors were rather small though slightly in favor of the varying parameters model.