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At least 379 records · Page 21Linked to original sources
PubMed2001

Convection patterns in end-heated inclined enclosures.

The natural convection in inclined side-heated rectangular boxes with adiabatic walls is theoretically and numerically investigated. The study is focused on the characterization of the convection patterns arising at the core of the basic steady unicellular flow and covers the whole range of Prandtl numbers (0< or = Pr < or = infinity) and inclinations (from alpha=0 degrees, heated-from-below vertical cavities, to alpha=180 degrees ). The onset of the flow instabilities depends on the core Rayleigh number R identical with K Ra, defined in terms of the local streamwise temperature gradient, K Delta T/L. The critical value of R for transversal and longitudinal modes is determined by the linear stability analysis of the basic plane-parallel flow, which also provides the stability diagram in the (Pr-alpha) chart. Anyhow, the effect of confinement can decisively change the stability properties of the core: if the steady unicell reaches the boundary layer regime (BLR) the local temperature gradient vanishes at the core leaving a completely stable core region. A theoretical determination of the frontier of the BLR in the space of parameters (alpha, R, and cavity size) yields an extra criterion of stability that has been displayed in the stability diagram. As confirmed by numerical calculations, the core-flow instabilities can only develop for Pr<O(1) whereas, at larger Pr the core region remains stable and the instabilities may only develop at the boundary layers. The analysis of the instability mechanisms reveals several couplings between the momentum and temperature fields that are not possible in the horizontal (alpha=90 degrees ) or vertical limits. For instance, by tilting the cavity with respect to alpha=0 degrees, the (Rayleigh-Bérnard) stationary thermal mode is suppressed in cavities whose depth is smaller than a theoretically predicted cutoff wavelength. The inclination also alters the properties of the oscillatory longitudinal instability, extensively investigated in the horizontal Hadley configuration at low Pr (liquid metals). An analytical relationship for its frequency in terms of alpha, Ra, and Pr is derived. Throughout the paper, numerical calculations in two- and three-dimensional enclosures illustrate each type of multicellular flow and examples of instability interactions near the codimension-2 lines predicted by the theory.

Journal Article↗
PubMedDate not supplied

High performance liquid chromatographic analysis of liposome stability.

Two techniques have been studied for their suitability for the analysis of the stability of liposomes: (1) High Performance Gel Permeation Liquid Chromatography (HPGPLC), a TSK G5000PW Ultrogel column; (2) Gel Permeation Chromatography (GPC), a Sepharose 4B column. The stability of dual radio-labelled, cholesterol-poor and cholesterol-rich, negatively charged liposomes in vitro (in saline and in serum), and in vivo, have been investigated using these two techniques. The HPGPLC TSK G5000PW column proved to be the superior technique for the analysis of liposome stability with the advantages of rapid run times, increased sample recovery, and smaller sample volumes were required. The results obtained confirm that inclusion of a high ratio of cholesterol into the liposome structure prevents phospholipid loss when exposed to serum, and that the cholesterol-poor liposome structure is dramatically altered under the same conditions. In conclusion, the TSK G5000PW column is ideal for monitoring movement of phospholipid between liposomes and serum proteins and for detecting changes in liposome size.

Animals↗
PubMed2004

Anomalous diffusion and collapse of self-gravitating Langevin particles in D dimensions.

We address the generalized thermodynamics and the collapse of a system of self-gravitating Langevin particles exhibiting anomalous diffusion in a space of dimension D. This is a basic model of stochastic particles in interaction. The equilibrium states correspond to polytropic configurations similar to stellar polytropes and polytropic stars. The index n of the polytrope is related to the exponent of anomalous diffusion. We consider a high-friction limit and reduce the problem to the study of the nonlinear Smoluchowacute;ski-Poisson system. We show that the associated Lyapunov functional is the Tsallis free energy. We discuss in detail the equilibrium phase diagram of self-gravitating polytropes as a function of D and n, and determine their stability by using turning point arguments and analytical methods. When no equilibrium state exists, we investigate self-similar solutions of the nonlinear Smoluchowski-Poisson system describing the collapse. Our stability analysis of polytropic spheres can be used to settle the generalized thermodynamical stability of self-gravitating Langevin particles as well as the nonlinear dynamical stability of stellar polytropes, polytropic stars and polytropic vortices. Our study also has applications concerning the chemotactic aggregation of bacterial populations.

Journal Article↗
PubMed2004

Stability of membrane bound reactions.

We present a novel approach to the dynamics of reactions of diffusing chemical species with species fixed in space, e.g., by binding to a membrane. The nondiffusing reaction partners are clustered in areas with a diameter smaller than the diffusion length of the diffusing partner. The activated fraction of the fixed species determines the size of an active subregion of the cluster. Linear stability analysis reveals that diffusion is one of the major determinants of the stability of the dynamics. We illustrate the model concept with Ca2+ dynamics in living cells, which has release channels as fixed reaction partners. Our results suggest that spatial and temporal structures in intracellular Ca2+ dynamics are caused by fluctuations due to the small number of channels per cluster.

Calcium↗
PubMed1997

Temporal stability of node-based internal bone adaptation simulations.

Jacobs et al. (1995, J. Biomechanics 28, 449-459) introduced a new implementation of the remodeling theory developed by Beaupre et al. (1990, J. Orthop. Res. 8, 651-661) that eliminates certain spurious spatial instabilities of previous implementations. Due to the highly nonlinear, coupled nature of multi-dimensional adaptation simulations, direct stability analyses of this method are not practical. In this manuscript, a linearized stability analysis was used to derive an expression for the critical time step for the stability of a simple model problem. In addition to accurately predicting the temporal response of the single degree-of-freedom problem, the analysis provided a conservative estimate of the critical time step for a more realistic, multiple degree-of-freedom adaptation simulation.

Adaptation, Physiological↗
PubMed2004

Feedback error learning and nonlinear adaptive control.

In this paper, we present our theoretical investigations of the technique of feedback error learning (FEL) from the viewpoint of adaptive control. We first discuss the relationship between FEL and nonlinear adaptive control with adaptive feedback linearization, and show that FEL can be interpreted as a form of nonlinear adaptive control. Second, we present a Lyapunov analysis suggesting that the condition of strictly positive realness (SPR) associated with the tracking error dynamics is a sufficient condition for asymptotic stability of the closed-loop dynamics. Specifically, for a class of second order SISO systems, we show that this condition reduces to K2D > K(P) where K(P) and K(D) are positive position and velocity feedback gains, respectively. Moreover, we provide a 'passivity'-based stability analysis which suggests that SPR of the tracking error dynamics is a necessary and sufficient condition for asymptotic hyperstability. Thus, the condition K2D > K(P) mentioned above is not only a sufficient but also necessary condition to guarantee asymptotic hyperstability of FEL, i.e. the tracking error is bounded and asymptotically converges to zero. As a further point, we explore the adaptive control and FEL framework for feedforward control formulations, and derive an additional sufficient condition for asymptotic stability in the sense of Lyapunov. Finally, we present numerical simulations to illustrate the stability properties of FEL obtained from our mathematical analysis.

Artificial Intelligence↗
PubMed2003

Analysis of Tikhonov regularization for function approximation by neural networks.

This paper is devoted to the convergence and stability analysis of Tikhonov regularization for function approximation by a class of feed-forward neural networks with one hidden layer and linear output layer. We investigate two frequently used approaches, namely regularization by output smoothing and regularization by weight decay, as well as a combination of both methods to combine their advantages. We show that in all cases stable approximations are obtained converging to the approximated function in a desired Sobolev space as the noise in the data tends to zero (in the weaker L(2)-norm) if the regularization parameter and the number of units in the network are chosen appropriately. Under additional smoothness assumptions we are able to show convergence rates results in terms of the noise level and the number of units in the network. In addition, we show how the theoretical results can be applied to the important classes of perceptrons with one hidden layer and to translation networks. Finally, the performance of the different approaches is compared in some numerical examples.

Algorithms↗
PubMed2003

[Study on stability of beta-cyclodextrin to chlorogenic acid].

To investigate the stabilizing effect of chlorogenic acid against beta-cyclodextrin, beta-cyclodextrin inclusion compound of chlorogenic acid was prepared by liquid phase method, and affirmed by UV and different thermal analysis. Stability in solution was contrasted between chlorogenic acid and its inclusion complex with beta-cyclodextrin, and the stability of the latter was increased.

Chlorogenic Acid↗
PubMed2006

Instability of creeping flow past a deformable wall: the role of depth-dependent modulus.

Linear stability analysis is carried out to examine the effect of a depth-dependent modulus on the stability of creeping flow of a Newtonian fluid past an incompressible and impermeable linear elastic solid. Two different systems are considered: (i) Couette flow past a solid with a continuously varying modulus, and (ii) Couette flow past two adjacent solids with different thicknesses and moduli. For the first system, we find that between two configurations having the same average modulus, the more stable configuration is the one that has the higher modulus at the interface. In the case of two different configurations having the same interfacial modulus and the same average modulus, the more stable configuration is the one that has the higher modulus right below the interface. For the second system, we find that stability depends in a non-monotonic way on the modulus ratio (top modulus to bottom modulus) of the two solids. If the thickness of the top solid is less than a critical value, then increasing the modulus ratio initially causes the system to be less stable. Since this critical thickness decreases as the modulus ratio increases, increasing the modulus ratio beyond a certain point causes the system to be more stable. An analysis of the solid-solid interfacial boundary conditions suggests that the relationship between the stiffness of the top solid and the stability of the system is due to a jump in the base-state displacement gradient at the interface which creates a net perturbation displacement.

Journal Article↗
PubMed2002

Impact of self-assembled surfactant structures on rheology of concentrated nanoparticle dispersions.

Rheological behavior of surfactant-stabilized colloidal dispersions of silica particles under extreme conditions (low pH, high ionic strength) has been investigated in relation to interparticle forces and stability of the dispersion. The surfactant used as the dispersing agent was C(12)TAB, a cationic surfactant. Stability analysis through turbidity measurements indicated that there is a sharp increase in the stability of the dispersion when the surfactant concentration is in the range of 8 to 10 mM in the system. The state of the dispersion changes from an unstable regime to a stable regime above a critical concentration of C(12)TAB in the system. In the case of interaction forces measured between the silica substrate and AFM tip, no repulsive force was observed up to a surfactant concentration of 8 mM and a transition from no repulsive forces to steric repulsive forces occurred between 8 and 10 mM. Rheological measurements as a function of C(12)TAB concentration indicated a significant decrease in the viscosity and linear viscoelastic functions of the dispersion over the same range of surfactant concentration (8 to 10 mM C(12)TAB), showing a strong correlation between the viscosity behavior, interparticle forces, and structure development in the dispersion.

Journal Article↗
PubMed2003

Interplay of Rayleigh and Peierls instabilities in metallic nanowires.

A quantum-mechanical stability analysis of metallic nanowires within the free-electron model is presented. The stability is determined by an interplay of electron-shell effects, the Rayleigh instability due to surface tension, and the Peierls instability. Although the latter effect limits the maximum length also for wires with "magic radii," it is found that nanowires in the micrometer range can be stable at room temperature.

Journal Article↗
PubMed1990

Effect of L-carnitine and acetyl-L-carnitine on the human erythrocyte membrane stability and deformability.

In this study we examined the effect of carnitine and acetylcarnitine on the human erythrocyte membrane stability and membrane deformability. Since erythrocyte membranes are impermeable to these compounds, we resealed erythrocyte ghosts in the presence of different concentrations of carnitine or acetylcarnitine. Resealed ghosts can be adequately studied in their cellular deformability and membrane stability properties by means of ektacytometry. Both carnitine and acetylcarnitine alter the membrane stability but not membrane deformability of the red cell membrane. Resealed ghosts containing 20, 50, 150, and 300 microM carnitine had 1.1, 1.6, 0.9, and 0.7 times the normal stability. While resealed ghosts containing 20, 50, 150, and 300 microM acetylcarnitine had 1.1, 1.5, 1.3, and 1.2 times the normal stability. Such changes were found to be reversible. We also conducted SDS PAGE of cytoskeletal membrane proteins from membrane fragments and residual membranes produced during membrane stability analysis, and unsheared resealed membranes in those samples where we observed an increase or a decrease of membrane stability. No changes in the cytoskeletal membrane proteins were noticed, even when the samples, prior SDS PAGE analysis, were treated with or without dithiothreitol. In addition, fluorescence steady state anisotropy of DPH in the erythrocyte membrane treated with carnitine or acetylcarnitine shows no modification of the lipid order parameter. Our results would suggest that both carnitine and its acetyl-ester, at physiological concentrations, may increase membrane stability in mature erythrocytes, most likely via a specific interaction with one or more cytoskeletal proteins, and that this effect would manifest when the erythrocytes are subjected to high shear stress.

Acetylcarnitine↗
PubMed1981

Derivatization of estrogen conjugates for analysis by capillary gas chromatography.

By using 20 meter wall-coated open tubular glass capillary columns of high stability, analysis of methyl ester, methyloxime trimethylsilyl ether derivatives of estrogen glucuronides had been achieved. Relative retention times of five glucuronide conjugates on OV-1 stationary phase are reported. Estrogen sulfates conjugated at the 3-position were shown to be quantitatively hydrolyzed and derivatized in a single trimethylsilylation step, and this method of direct derivatization was compared to two solvolysis methods. These analytical methods could be further developed to allow rapid and quantitative analysis of estrogens in biological fluids, and may prove particularly useful for analysis of labile compounds.

Chromatography, Gas↗
PubMed2026

A fitness advantage from the pLVPK plasmid fuels the global spread of a carbapenem-resistant hypervirulent Klebsiella pneumoniae high-risk clone: ST11-KL64.

BACKGROUND: The global emergence of carbapenem-resistant hypervirulent Klebsiella pneumoniae (CR-hvKP), particularly the ST11-KL64 subclone acquiring pLVPK-like virulence plasmids, represents a critical public health threat. This study investigates the epidemiological dominance and molecular mechanisms underlying ST11-KL64's fitness advantage over KL47 variants. METHODS: We performed comparative genomic analysis on 43,722 K. pneumoniae genomes (2011-2022) from 112 countries, focusing on ST11-CRKP strains. Capsular typing (KL64 vs. KL47), virulence gene profiling (aerobactin, RmpADC), and plasmid stability analysis were conducted using Kleborate, RAST, and PlasmidFinder. Plasmid-chromosome interactions were characterized through hybrid assembly approaches. RESULTS: ST11-KL64 demonstrated rapid expansion post-2016, surpassing KL47 as China's dominant CRKP subtype (40.5% vs. 28.9%), with regional predominance in Zhejiang (62.3%) and Sichuan (58.7%) provinces. Notably, 94.8% of KL64 strains maintained intact pLVPK plasmids with high aerobactin carriage (60.5%), while KL47 exhibited frequent plasmid fusion (58.8% with IncFIB[pNDM-Mar]) or chromosomal integration (41.4%), resulting in lower virulence potential (27.3% aerobactin+). Genomic analysis revealed KL64's superior plasmid stability (71.2% gene retention vs. KL47's 43.6%) and clinical correlation with severe outcomes (OR&#x2009;=&#x2009;2.34, 95%CI 1.67-3.28). CONCLUSION: The ST11-KL64 subclone's epidemiological success stems from stable pLVPK plasmid maintenance, enabling simultaneous carbapenem resistance and hypervirulence. These findings highlight the urgent need for genomic surveillance targeting plasmid-mediated virulence in CRKP outbreaks, particularly in critical care settings where horizontal gene transfer may accelerate strain evolution.

Klebsiella pneumonia↗
PubMed2003

Influence of a nonlinear reference temperature profile on oscillatory Bénard-Marangoni convection.

We analyze oscillatory instabilities in a fluid layer of infinite horizontal extent, heated from above or cooled from below, taking into account the nonlinearity of the reference temperature profile during the transient state of heat conduction. The linear stability analysis shows that a nonlinear reference temperature profile can have a strong effect on the system, either stabilizing or destabilizing, depending on the relative importance of buoyancy and surface tension forces. For the nonlinear analysis we use a Galerkin-Eckhaus method leading to a finite set of amplitude equations. In the two-dimensional (2D) case, we show the solution of these amplitude equations are standing waves.

Journal Article↗
PubMed2003

Surface glycoprotein PSA (GP46) expression during short- and long-term culture of Leishmania chagasi.

The mRNAs encoding promastigote surface antigen (PSA) of Leishmania chagasi have previously been shown to increase about 30-fold as in vitro cultured parasites progress from logarithmic to stationary phase, growth phases that are, respectively associated with parasites having low and high infectivity to mammals. Experiments reported here establish by western blot analysis that PSA proteins of 44 and 66 kDa also increase about 30-fold as parasite cultures reach stationary phase. Serial passage of parasite cultures resulted in a progressive reduction in PSA protein and RNA abundance to levels less than 3% that of cultures newly-initiated with parasites derived from a parasitized rodent. Loss of PSA mRNA abundance in serially passaged cells was not due to reduced PSA gene transcription rates, as determined by nuclear run-on assays. Neither was the loss associated with a marked decrease in PSA mRNA stability. Analysis of PSA RNA stability in the presence of actinomycin D, an inhibitor of transcription elongation, failed to detect a difference in fully processed cytosolic PSA mRNA stability regardless of the number of times a culture was passaged or the growth phase of the culture. Based on the lack of detectable difference in (cytosolic) mature PSA mRNA stability during promastigote development, the data indirectly suggest that the regulated expression of PSA in cells from low-passage cultures and the loss of PSA expression in high-passage cultures may be mediated by nuclear events that occur after transcription of the PSA genes and before arrival of the mature mRNAs in the cytoplasm.

Animals↗
PubMed2006

Reanalysis of the Grønnerød (2003) Rorschach temporal stability meta-analysis data set.

I reanalyzed the Grønnerød (2003) Rorschach temporal stability meta-analysis data with a theoretically driven hierarchical regression analysis. I also examined the intrarater reliability of initial coding decisions and verified their consistency. In the new regression analyses, retest period strengthened its influence on stability, whereas methodological factors showed a tendency toward influence. The new results are more in line with the results from Grønnerød's (2004) psychotherapy change meta-analysis results.

Humans↗
PubMed2006

Small-subunit cysteine-65 substitutions can suppress or induce alterations in the large-subunit catalytic efficiency and holoenzyme thermal stability of ribulose-1,5-bisphosphate carboxylase/oxygenase.

In the green alga Chlamydomonas reinhardtii, an L290F substitution in the chloroplast-encoded large-subunit of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) causes decreases in carboxylation Vmax, CO2/O2 specificity, and thermal stability. Analysis of photosynthesis-competent revertants selected at the 35 degrees C restrictive temperature identified a rare C65S suppressor substitution in the nuclear-encoded small subunit. C65S enhances catalysis and CO2/O2 specificity in the absence of other wild-type small subunits, and restores thermal stability in vivo. C65S, C65A, and C65P mutant strains were created. C65S and C65A enzymes have normal catalysis, but C65P Rubisco, which contains land-plant Pro, has decreases in carboxylation Vmax/Km and CO2/O2 specificity. In contrast to other small-subunit substitutions that affect specificity, Cys-65 contacts the large subunit, and the C65P substitution does not cause a decrease in holoenzyme thermal stability in vivo or in vitro. Further analysis of the C65P protein may identify structural alterations that influence catalysis separate from those that affect stability.

Amino Acid Sequence↗