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PubMed2006

Pickering emulsions stabilized solely by layered double hydroxides particles: the effect of salt on emulsion formation and stability.

The formation and stability of liquid paraffin-in-water emulsions stabilized solely by positively charged plate-like layered double hydroxides (LDHs) particles were described here. The effects of adding salt into LDHs dispersions on particle zeta potential, particle contact angle, particle adsorption at the oil-water interface and the structure strength of dispersions were studied. It was found that the zeta potential of particles gradually decreased with the increase of salt concentration, but the variation of contact angle with salt concentration was very small. The adsorption of particles at the oil-water interface occurred due to the reduction of particle zeta potential. The structural strength of LDHs dispersions was strengthened with the increase of salt and particle concentrations. The effects of particle concentration, salt concentration and oil phase volume fraction on the formation, stability and type of emulsions were investigated and discussed in relation to the adsorption of particles at the oil-water interface and the structural strength of LDHs dispersions. Finally, the possible stabilization mechanisms of emulsions were put forward: the decrease of particle zeta potential leads to particle adsorption at the oil-water interface and the formation of a network of particles at the interface, both of which are crucial for emulsion formation and stability; the structural strength of LDHs dispersions is responsible for emulsion stability, but is not necessary for emulsion formation.

Journal Article↗
PubMed2006

Effect of dispersion pH on the formation and stability of Pickering emulsions stabilized by layered double hydroxides particles.

Using positively charged plate-like layered double hydroxides (LDHs) particles as emulsifier, liquid paraffin-in-water emulsions stabilized solely by such particles are successfully prepared. The effects of the pH of LDHs aqueous dispersions on the formation and stability of the emulsions are investigated here. The properties of the LDHs dispersions at different pHs are described, including particle zeta potential, particle aggregation, particle contact angle, flow behavior of the dispersions and particle adsorption at a planar oil/water interface. The zeta potential decreases with increasing pH, leading to the aggregation of LDHs particles into large flocs. The structural strength of LDHs dispersions is enhanced by increasing pH and particle concentration. The three-phase contact angle of LDHs also increases with increasing pH, but the variation is very small. Visual observation and SEM images of the interfacial particle layers show that the adsorption behavior of LDHs particles at the planar oil/water interface is controlled by dispersion pH. We consider that the particle-particle (at the interface) and particle-interface electrostatic interactions are well controlled by adjusting the dispersion pH, leading to pH-tailored colloid adsorption. The formation of an adsorbed particle layer around the oil drops is crucial for the formation and stability of the emulsions. Emulsion stability improves with increasing pH and particle concentration because more particles are available to be adsorbed at the oil/water interface. The structural strength of LDHs dispersions and the gel-like structure of emulsions also influence the stability of the emulsions, but they are not necessary for the formation of emulsions. The emulsions cannot be demulsified by adjusting emulsion pH due to the irreversible adsorption of LDHs particles at the oil/water interface. TEM images of the emulsion drops show that a thick particle layer forms around the oil drops, confirming that Pickering emulsions are stabilized by the adsorbed particle layers. The thick adsorbed particle layer may be composed of a stable inner particle layer which is in direct contact with the oil phase and a relatively unstable outer particle layer surrounding the inner layer.

Journal Article↗
PubMed2006

Effects of abdominal stabilization maneuvers on the control of spine motion and stability against sudden trunk perturbations.

Much discussion exists about which is the most effective technique to improve spine stability. The purpose of this study was to evaluate the effectiveness of abdominal bracing and abdominal hollowing maneuvers to control spine motion and stability against rapid perturbations. Eleven healthy males were posteriorly loaded in different experimental conditions: resting with no knowledge of the perturbation timing; performing each of the stabilization maneuvers at 10%, 15% and 20% of internal oblique maximum voluntary contraction with no knowledge of the perturbation timing; and naturally coactivating the trunk muscles when perturbation timing was known. An EMG biofeedback system was used to control the pattern and intensity of abdominal coactivation. The muscular preactivation of seven trunk muscles (bilaterally registered), the applied force, and the torso muscular and kinematic responses to loading were measured; and the spine stability and compression were modeled. The hollowing maneuver was not effective for reducing the kinematic response to sudden perturbation. On the contrary, the bracing maneuver fostered torso cocontraction, reduced lumbar displacement, and increased trunk stability, but at the cost of increasing spinal compression. When the timing of the perturbation was known, the participants were able to stabilize the trunk while imposing smaller spine compressive loads.

Abdomen↗
PubMed2002

Conformation and stability of alpha-helical membrane proteins. 2. Influence of pH and salts on stability and unfolding of rhodopsin.

We studied the stability and pH-induced denaturation of rhodopsin and its photoproducts as a model for alpha-helical membrane proteins. The increased stability of the dark state of rhodopsin as compared to its photoproduct states allows the initiation of unfolding of the protein by light-dependent isomerization of the chromophore. We could therefore characterize the transition from the native to either acid or alkaline denatured states by light-induced Fourier transform infrared difference spectroscopy, UV-visible spectroscopy, and intrinsic tryptophan fluorescence spectroscopy. The results indicate a loss of important tertiary interactions within the protein and between the protein and the retinal chromophore in the denatured state, despite that the secondary structure of the protein is almost fully retained during the transition. We therefore propose that in this denatured state the protein adopts the conformation of a loose bundle of preserved, but only weakly interacting, transmembrane helices with a largely des-oriented and partly solvent-exposed chromophore. We further characterized the influence of salts on the stability of the rhodopsin helix bundle, which was found to follow the Hofmeister series. We found that the effect of sodium chloride may be stabilizing or destabilizing, depending on the intrinsic stability of the examined protein conformation and on salt concentration. In particular, sodium chloride is shown to counteract the formation of the denatured loose bundle state presumably by increasing the lateral pressure on the helix bundle, thereby stabilizing native-like tertiary contacts within the protein.

Animals↗
PubMed2004

The factors governing the thermal stability of frataxin orthologues: how to increase a protein's stability.

Understanding the factors governing the thermal stability of proteins and correlating them to the sequence and structure is a complex and multiple problem that can nevertheless provide important information on the molecular forces involved in protein folding. Here, we have carried out a comparative genomic study to analyze the effects that different intrinsic and environmental factors have on the thermal stability of frataxins, a family of small mitochondrial iron-binding proteins found in organisms ranging from bacteria to humans. Low expression of frataxin in humans causes Friedreich's ataxia, an autosomal recessive neurodegenerative disease. The human, yeast, and bacterial orthologues were selected as representatives of different evolutionary steps. Although sharing high sequence homology and the same three-dimensional fold, the three proteins have a large variability in their thermal stabilities. Whereas bacterial and human frataxins are thermally stable, well-behaved proteins, under the same conditions yeast frataxin exists in solution as an unstable species with apprechable tracts in a conformational exchange. By designing suitable mutants, we show and justify structurally that the length of the C-terminus is an important intrinsic factor that directly correlates with the thermal stabilities of the three proteins. Thermal stability is also gained by the addition of Fe(2+). This effect, however, is not uniform for the three orthologues nor highly specific for iron: a similar albeit weaker stabilization is observed with other mono- and divalent cations. We discuss the implications that our findings have for the role of frataxins as iron-binding proteins.

Amino Acid Sequence↗
PubMed2002

A relative organolithium stability scale derived from tin-lithium exchange equilibria. Substituent effects on the stability of alpha-oxy- and alpha-aminoorganolithium compounds.

Quantitative thermodynamic stability scales of organolithium compounds can be derived from measurements of tin-lithium exchange equilibria. A DeltaG(eq) scale of alpha-oxy- and alpha-aminoorganolithium compounds was established, and quantitative stabilization effects of O-alkyl, O-alkoxyalkyl, O-carbamoyl, N-carbamoyl, and O-carbonyl groups of the alpha-carbanion are presented. It has been found that an alpha-oxycarbanion is far better stabilized by a carbonyl group as the O-substituent than by an alkyl or alkoxyalkyl group, while the anion-stabilizing effects of the different O-carbonyl substituents are comparable. An N-carbamoyl group was found to have a somewhat higher stabilizing effect than its O-carbamoyl counterpart. NMR data are presented that show that benzylic N- or O-substituted carbanions have highly planarized structures where the negative charge is highly delocalized. The stability data obtained from the tin-lithium exchanges can be easily converted into "effective pK" data that are useful for predicting the acid-base behavior of this type of organolithium species.

Journal Article↗
PubMed2004

Factors controlling the formation and stability of air bubbles stabilized by partially hydrophobic silica nanoparticles.

Air bubbles have been formed using partially hydrophobic silica nanoparticles as the stabilizer. The particles were of primary particle size 20 nm, chemically treated to different degrees with dichlorodimethylsilane to render them partially hydrophobic. Above a certain bubble size range (typically 80-microm diameter), the bubbles seemed to be almost indefinitely stable, while for any size above 20 microm their stability against disproportionation is far better than bubbles stabilized by any protein film investigated in previous studies. A possible theoretical justification for this observation is presented. Bubbles could be formed by shaking water with the particles, but a much higher volume fraction of bubbles was obtained by pressurizing the aqueous phase to 5 atm overnight followed by suddenly releasing the pressure to nucleate bubbles within the silica dispersion. Sonicating the silica dispersion before nucleation also gave more bubbles, which were also found to be more stable. There appeared to be an optimum degree of surface hydrophobicity that gave maximum foamability and foam stability, where around 20-33% of the silanol groups on the silica surface had been converted to dimethylsilane groups. However, a sharp increase in stability occurred when between 1.8 and 2 mol dm(-3) NaCl was also included in the aqueous phase. The change in stability due to inclusion of salt can be rationalized in terms of changes occurring in the value of the particle contact angle. The effects of increasing sonication and an optimum surface chemical treatment can be explained by the need to make the particles sufficiently hydrophobic so that they adsorb strongly enough, while at the same time minimizing their tendency to aggregate in the bulk aqueous phase, which hinders their adsorption. Furthermore, confocal laser scanning microscopy of the bubble dispersions suggests that a large volume fraction of stable bubbles is only formed when the particles adsorbed to the bubbles are also part of a spanning silica particle network in the bulk aqueous solution, forming a weak gel with a finite yield stress.

Journal Article↗
PubMed2000

Cyclic nucleotide regulation of Na+/glucose cotransporter (SGLT1) mRNA stability. Interaction of a nucleocytoplasmic protein with a regulatory domain in the 3'-untranslated region critical for stabilization.

Expression of the Na(+)-coupled glucose cotransporter SGLT1 is regulated post-transcriptionally at the level of mRNA stability. We have previously demonstrated that cAMP-dependent stabilization of the SGLT1 message was correlated with the protein phosphorylation-dependent binding of cytoplasmic proteins to a uridine-rich sequence (URE) in the 3'-untranslated region (UTR). In the present study, the regulatory role of the URE was demonstrated by inserting it into the 3'-UTR of a beta-globin reporter minigene under the control of the tetracycline-regulated promoter. The resultant chimeric globin/SGLT1 mRNA expressed after transfection into LLC-PK1 cells exhibited a decreased half-life compared with the beta-globin control, indicating that the URE serves a destabilizing function. Activation of protein kinase A stabilized the chimeric message but not the beta-globin control, indicating the presence of a regulatory stabilizing sequence within the URE. A 38-kDa nucleocytoplasmic protein was identified that recognized a 12-nucleotide binding site within the URE. A mutation in this binding site that prevented protein binding assayed in vitro by UV cross-linking also prevented protein kinase A-dependent stabilization of the chimeric message assayed in vivo. These findings identify the interaction between a 38-kDa nucleocytoplasmic protein and a regulatory uridine-rich sequence in the 3'-UTR as critical for cAMP-mediated SGLT1 message stabilization.

1-Methyl-3-isobutylxanthine↗
PubMed2006

Delay-dependent stability analysis and stabilization for discrete-time fuzzy systems with state delay: a fuzzy Lyapunov-Krasovskii functional approach.

This correspondence studies stability analysis and stabilization for discrete-time Takagi and Sugeno fuzzy systems with state delay. First, a new fuzzy Lyapunov-Krasovskii functional (LKF) is constructed to derive a delay-dependent stability condition for open-loop fuzzy systems. Then, a delay-dependent stabilization approach based on a nonparallel distributed compensation scheme is provided for closed-loop fuzzy systems. Both state feedback and observer-based control cases are considered. The proposed stability and stabilization conditions are represented in terms of linear matrix inequalities (LMIs), which can be solved efficiently by using existing LMI optimization techniques. Finally, two numerical examples are given to illustrate the effectiveness of the proposed method.

Algorithms↗
PubMed2003

Stability of the G-matrix in a population experiencing pleiotropic mutation, stabilizing selection, and genetic drift.

Quantitative genetics theory provides a framework that predicts the effects of selection on a phenotype consisting of a suite of complex traits. However, the ability of existing theory to reconstruct the history of selection or to predict the future trajectory of evolution depends upon the evolutionary dynamics of the genetic variance-covariance matrix (G-matrix). Thus, the central focus of the emerging field of comparative quantitative genetics is the evolution of the G-matrix. Existing analytical theory reveals little about the dynamics of G, because the problem is too complex to be mathematically tractable. As a first step toward a predictive theory of G-matrix evolution, our goal was to use stochastic computer models to investigate factors that might contribute to the stability of G over evolutionary time. We were concerned with the relatively simple case of two quantitative traits in a population experiencing stabilizing selection, pleiotropic mutation, and random genetic drift. Our results show that G-matrix stability is enhanced by strong correlational selection and large effective population size. In addition, the nature of mutations at pleiotropic loci can dramatically influence stability of G. In particular, when a mutation at a single locus simultaneously changes the value of the two traits (due to pleiotropy) and these effects are correlated, mutation can generate extreme stability of G. Thus, the central message of our study is that the empirical question regarding G-matrix stability is not necessarily a general question of whether G is stable across various taxonomic levels. Rather, we should expect the G-matrix to be extremely stable for some suites of characters and unstable for others over similar spans of evolutionary time.

Analysis of Variance↗
PubMed1999

An mRNA stability complex functions with poly(A)-binding protein to stabilize mRNA in vitro.

The stable globin mRNAs provide an ideal system for studying the mechanism governing mammalian mRNA turnover. alpha-Globin mRNA stability is dictated by sequences in the 3' untranslated region (3'UTR) which form a specific ribonucleoprotein complex (alpha-complex) whose presence correlates with mRNA stability. One of the major protein components within this complex is a family of two polycytidylate-binding proteins, alphaCP1 and alphaCP2. Using an in vitro-transcribed and polyadenylated alpha-globin 3'UTR, we have devised an in vitro mRNA decay assay which reproduces the alpha-complex-dependent mRNA stability observed in cells. Incubation of the RNA with erythroleukemia K562 cytosolic extract results in deadenylation with distinct intermediates containing a periodicity of approximately 30 nucleotides, which is consistent with the binding of poly(A)-binding protein (PABP) monomers. Disruption of the alpha-complex by sequestration of alphaCP1 and alphaCP2 enhances deadenylation and decay of the mRNA, while reconstitution of the alpha-complex stabilizes the mRNA. Similarly, PABP is also essential for the stability of mRNA in vitro, since rapid deadenylation resulted upon its depletion. An RNA-dependent interaction between alphaCP1 and alphaCP2 with PABP suggests that the alpha-complex can directly interact with PABP. Therefore, the alpha-complex is an mRNA stability complex in vitro which could function at least in part by interacting with PABP.

3' Untranslated Regions↗
PubMed1997

Stabilized bubbles in the body: pressure-radius relationships and the limits to stabilization.

We previously outlined the fundamental principles that govern behavior of stabilized bubbles, such as the microbubbles being put forward as ultrasound contrast agents. Our present goals are to develop the idea that there are limits to the stabilization and to provide a conceptual framework for comparison of bubbles stabilized by different mechanisms. Gases diffuse in or out of stabilized bubbles in a limited and reversible manner in response to changes in the environment, but strong growth influences will cause the bubbles to cross a threshold into uncontrolled growth. Also, bubbles stabilized by mechanical structures will be destroyed if outside influences bring them below a critical small size. The in vivo behavior of different kinds of stabilized bubbles can be compared by using plots of bubble radius as a function of forces that affect diffusion of gases in or out of the bubble. The two ends of the plot are the limits for unstabilized growth and destruction; these and the curve's slope predict the bubble's practical usefulness for ultrasonic imaging or O2 carriage to tissues.

Excipients↗
PubMed1996

Landmark stability: studies exploring whether the perceived stability of the environment influences spatial representation.

To investigate whether spatial learning complies with associative learning theories or with theories of cognitive mapping, rats were trained in three experiments exploring the effect of variations in spatial predictive relationships. In experiment 1, it was found that making one of two landmarks the sole spatial predictor of reward, by varying the spatial relationship between reward and other cues, reduced the control over search exerted by that landmark compared with that observed when the landmark and context cues were both reliable predictors of reward location. This requirement for landmark stability rather than predictive power appears to contradict results obtained in conventional conditioning paradigms. Discrimination learning was unaffected, suggesting a dissociation between discrimination and spatial learning with respect to the influence of geometric stability. Further experiments used arrays of both single and multiple landmarks. Experiment 2 revealed that the stability of a single landmark improved accuracy of search, but also showed that local stability between a pair of landmarks that moved around the arena together was sufficient to support spatial learning. Experiment 3 examined landmark stability using fixed directional cues in the absence of vestibular disorientation. This also revealed a relative advantage of stable landmarks, but animals presented with a landmark that moved from trial to trial did show some evidence of learning. Parametric manipulation of landmark stability offers an intriguing way of influencing the process of spatial representation and thus understanding better the processes through which egocentric representations of perceived space are transformed into allocentric representations of the real world.

Journal Article↗
PubMed1993

Intrinsic stability and extrinsic stabilization of creatinase from Pseudomonas putida.

Creatinase (creatine amidinohydrolase, EC 3.5.3.3), a homodimer of 45 kDa subunit molecular mass, shows only limited functional stability, and is inaccessible to reconstitution after preceding deactivation, denaturation and dissociation. The enzyme has been characterized regarding its native and denatured states. Studying its unfolding characteristics in the presence of "extrinsic factors", such as DTE, BSA and glycerol, it was possible to define solvent conditions where the stability of the enzyme is significantly improved. Apart from protecting essential thiol groups and charge screening effects, the stabilization is caused mainly by preferential solvation. In the presence of 20% (w/v) glycerol, the kinetic analysis of the time course of denaturation indicates that a partially active folding intermediate, rather than the whole molecule, is involved in the stabilization. The mixed solvent improves the thermal stability, as well as the stability toward GdmCl and urea.

Circular Dichroism↗
PubMed2003

Kinetic analysis of enhanced thermal stability of an alkaline protease with engineered twin disulfide bridges and calcium-dependent stability.

The thermal stability of a cysteine-free alkaline protease (Alp) secreted by the eukaryote Aspergillus oryzae was improved both by the introduction of engineered twin disulfide bridges (Cys-69/Cys-101 and Cys-169/Cys-200), newly constructed as part of this study, and by the addition of calcium ions. We performed an extensive kinetic analysis of the increased thermal stability of the mutants as well as the role of calcium dependence. The thermodynamic activation parameters for irreversible thermal inactivation, the activation free energy (deltaG), the activation enthalpy (deltaH), and the activation entropy (deltaS) were determined from absolute reaction rate theory. The values of deltaH and deltaS were significantly and concomitantly increased as a result of introducing the twin disulfide bridges, for which the increase in the value of deltaH outweighed that of deltaS, resulting in significant increases in the value of deltaG. The enhancement of the thermal stability obtained by introducing the twin disulfide bridges is an example of the so-called low-temperature stabilization of enzymes. The stabilizing effect of calcium ions on wild-type Alp is similar to the results we obtained by introducing the engineered twin disulfide bridges.

Aspergillus oryzae↗
PubMed2005

Stabilizing peptide fusion for solving the stability and solubility problems of therapeutic proteins.

PURPOSE: Protein aggregation is a major stability problem of therapeutic proteins. We investigated whether a novel stabilizing peptide [acidic tail of synuclein (ATS) peptide] could be generally used to make a more stable and soluble form of therapeutic proteins, particularly those having solubility or aggregation problems. METHODS: We produced ATS fusion proteins by fusing the stabilizing peptide to three representative therapeutic proteins, and then compared the stress-induced aggregation profiles, thermostability, and solubility of them. We also compared the in vivo stability of these ATS fusion proteins by studying their pharmacokinetics in rats. RESULTS: The human growth hormone-ATS (hGH-ATS) and granulocyte colony-stimulating factor-ATS (G-CSF-ATS) fusion proteins were fully functional as determined by cell proliferation assay, and the ATS fusion proteins seemed to be very resistant to agitation, freeze/thaw, and heat stresses. The introduction of the ATS peptide significantly increased the storage and thermal stabilities of hGH and G-CSF. The human leptin-ATS fusion protein also seemed to be very resistant to aggregation induced by agitation, freeze/thaw, and heat stresses. Furthermore, the ATS peptide greatly increased the solubility of the fusion proteins. Finally, pharmacokinetic studies in rats revealed that the ATS fusion proteins are also more stable in vivo. CONCLUSION: Our data demonstrate that a more stable and soluble form of therapeutic proteins can be produced by fusing the ATS peptide.

Animals↗
PubMed2005

Oxidative stability of bovine serum albumin- and sodium caseinate-stabilized emulsions depends on metal availability.

Proteins often stabilize food emulsions and are also able to promote or delay lipid oxidation in complex systems. The purpose of this work was to investigate the relationship between metal ion availability and oxidative stability of oil-in-water emulsions stabilized by bovine serum albumin (BSA) or sodium caseinate (NaCas). Emulsions with similar and stable droplet size distributions were prepared with stripped sunflower oil (30 vol %) and protein solutions (20 g L(-)(1); pH = 6.5). In the absence of the water-soluble metal chelator EDTA, oxygen uptake, conjugated dienes, and volatile compounds developed faster in NaCas-stabilized emulsions than in those prepared with BSA. This effect is attributed to the chelating properties of NaCas and to electrostatic interactions that attract some metal ions at the interface where they could initiate lipid oxidation. When EDTA (100 muM) was present, oxidation was delayed to a greater extent in emulsions made with NaCas than in BSA stabilized emulsions. These conditions probably enabled NaCas to exert free-radical-scavenging activity.

Caseins↗
PubMed1994

Physical stability and protein stability of freeze-dried cakes during storage at elevated temperatures.

The relationship between physical stability of freeze-dried cakes and protein stability during storage was studied using beta-galactosidase as a model protein and inositol as an excipient. Amorphous samples freeze-dried from solutions containing the enzyme and various concentrations of inositol in sodium phosphate buffer (50 mM, pH 7.4) were stored for 7 days over P2O5 at 40 to 70 degrees C. Structural collapse and inositol crystallization were observed in some of the samples, depending on the formulation and storage temperature. The physical stability of freeze-dried samples was also studied by differential scanning calorimeter (DSC). Inositol showed a protein-stabilizing effect when its amorphous form was retained during storage, regardless of structural collapse. However, crystallization of inositol during storage removed its stabilizing effect. Addition of water-soluble polymers such as dextran, Ficoll and carboxymethyl cellulose sodium salt (CMC-Na) preserved activity of the enzyme by preventing inositol crystallization.

Aspergillus oryzae↗