Temperature Of Maximum Stability Research Articles

First principles prediction of the Al-Li phase diagram including configurational and vibrational entropic contributions

The whole Al-Li phase diagram is predicted from first principles calculations and statistical mechanics including the effect of configurational and vibrational entropy. The formation enthalpy of different configurations at different temperatures was accurately predicted by means of cluster expansions that were fitted from first principles calculations. The vibrational entropic contribution of each configuration was determined from the bond length vs. bond stiffness relationships for each type of bond and the Gibbs free energy of the different phases was obtained as a function of temperature from Monte Carlo simulations. The predicted phase diagram was in excellent agreement with the currently accepted experimental one in terms of the stable (AlLi, Al2Li3, AlLi2, Al4Li9) and metastable (Al3Li) phases, of the phase boundaries between them and of the maximum stability temperature of line compounds. In addition, it provided accurate information about the gap between Al3Li and AlLi solvus lines. Finally, the influence of the vibrational entropy on the correct prediction of the phase diagram is discussed. Overall, the methodology shows that accurate phase diagrams of alloys of technological interest can be predicted from first principles calculations.

Study of the nickel-phosphorus coating structural-phase states

The results of modeling a composite nickel-phosphorus coating obtained by chemical deposition are presented. The state of structural-phase disordering of the coating surface is considered. The analysis of possible structural states in the surface layers of the coating is carried out. A graph of Me atoms volume changes in phosphorus-containing compounds MexPy(Ме – Fe, Cr, Ni) and the maximum stability temperature NixPyis plotted depending on the composition. The idealized images of the projections of tetragonal and trigonal phosphorus-containing phases structures are constructed.

Antibacterial polyester fabrics via diffusion process using active bio-based agents from essential oils

Antimicrobial products such as silver nanoparticles and QACs - Quaternary ammonium compounds, used to produce antibacterial textiles, are being targeted due to environmental concerns. Most of the active antibacterial molecules from essential oils are small molecules with a solubility parameter (δ) close to that of polyester –PET-poly(ethylene terephtalate) (δ=21.9 MPa1/2 at 129.4 °C), and can therefore be potentially used to functionalize polyester fabrics using diffusion process. The aim of this paper was to explore the diffusion process to produce antibacterial polyester textiles using active agent(s) from essential oils.Diffusion process was explored at high temperature (130 °C) and pressure, to functionalize polyester fabric (PET) with five different bio-based antimicrobial agents isolated from essential oils. Thymol, geraniol, cinnamaldehyde, ortho-vanillin and para-vanillin, were selected based on their solubility parameter (in the range of 19 to 25 MPa1/2), and small their molecular size (molecular weight<155 g/mol).Both quantitative and qualitative tests were used to analyze antibacterial activity of the functionalized polyester fabrics. Quantitative absorption method (ISO20743) showed that most of the functionalized polyester textiles exhibit antibacterial activity towards gram positive (Staphylococcus aureus) and gram negative (Klebsiella pneumoniae) bacteria, though no inhibition zone was observed with the qualitative agar diffusion method. Spectrophotometric analysis of the functionalized fabrics confirmed the diffusion of the active agents inside the PET fibers in between macromolecular chains, as PET fibers became pale yellow.Antibacterial polyester fabrics functionalized with O-vanillin and cinnamaldehyde exhibited good wash durability.Thermogravimetric analysis showed that most of the active agents (except thymol), have maximum stability temperature (<5% weight loss) higher than the diffusion temperature (130 °C) used. Some degradation products may be formed, which may also diffuse inside the PET fiber, but further studies need to be considered in future.The toxicity issue is also discussed with the view of using the antibacterial polyester textiles for outdoor applications.

Enthalpic stabilization of an SH3 domain by D2 O.

The stability of a protein is vital for its biological function, and proper folding is partially driven by intermolecular interactions between protein and water. In many studies, H2 O is replaced by D2 O because H2 O interferes with the protein signal. Even this small perturbation, however, affects protein stability. Studies in isotopic waters also might provide insight into the role of solvation and hydrogen bonding in protein folding. Here, we report a complete thermodynamic analysis of the reversible, two-state, thermal unfolding of the metastable, 7-kDa N-terminal src-homology 3 domain of the Drosophila signal transduction protein drk in H2 O and D2 O using one-dimensional 19 F NMR spectroscopy. The stabilizing effect of D2 O compared with H2 O is enthalpic and has a small to insignificant effect on the temperature of maximum stability, the entropy, and the heat capacity of unfolding. We also provide a concise summary of the literature about the effects of D2 O on protein stability and integrate our results into this body of data.

Protein Stability in Reverse Micelles.

The N-terminal SH3 domain of the Drosophila signal transduction protein drk was encapsulated in reverse micelles. Both the temperature of maximum stability and the melting temperature decreased on encapsulation. Dissecting the temperature-dependent stability into enthalpic and entropic contributions reveals a stabilizing enthalpic and a destabilizing entropic contribution. These results do not match the expectations of hard-core excluded volume theory, nor can they be wholly explained by interactions between the head groups in the reverse micelle and the test protein. We suggest that geometric constraints imposed by the reverse micelles need to be considered.

Protein Stability in Reverse Micelles

AbstractThe N‐terminal SH3 domain of the Drosophila signal transduction protein drk was encapsulated in reverse micelles. Both the temperature of maximum stability and the melting temperature decreased on encapsulation. Dissecting the temperature‐dependent stability into enthalpic and entropic contributions reveals a stabilizing enthalpic and a destabilizing entropic contribution. These results do not match the expectations of hard‐core excluded volume theory, nor can they be wholly explained by interactions between the head groups in the reverse micelle and the test protein. We suggest that geometric constraints imposed by the reverse micelles need to be considered.

Signatures of protein thermal denaturation and local hydrophobicity in domain specific hydration behavior: a comparative molecular dynamics study.

We investigate, using atomistic molecular dynamics simulations, the association of surface hydration accompanying local unfolding in the mesophilic protein Yfh1 under a series of thermal conditions spanning its cold and heat denaturation temperatures. The results are benchmarked against the thermally stable protein, Ubq, and behavior at the maximum stability temperature. Local unfolding in Yfh1, predominantly in the beta sheet regions, is in qualitative agreement with recent solution NMR studies; the corresponding Ubq unfolding is not observed. Interestingly, all domains, except for the beta sheet domains of Yfh1, show increased effective surface hydrophobicity with increase in temperature, as reflected by the density fluctuations of the hydration layer. Velocity autocorrelation functions (VACF) of oxygen atoms of water within the hydration layers and the corresponding vibrational density of states (VDOS) are used to characterize alteration in dynamical behavior accompanying the temperature dependent local unfolding. Enhanced caging effects accompanying transverse oscillations of the water molecules are found to occur with the increase in temperature preferentially for the beta sheet domains of Yfh1. Helical domains of both proteins exhibit similar trends in VDOS with changes in temperature. This work demonstrates the existence of key signatures of the local onset of protein thermal denaturation in solvent dynamical behavior.

The non-uniform early structural response of globular proteins to cold denaturing conditions: a case study with Yfh1.

The mechanism of cold denaturation in proteins is often incompletely understood due to limitations in accessing the denatured states at extremely low temperatures. Using atomistic molecular dynamics simulations, we have compared early (nanosecond timescale) structural and solvation properties of yeast frataxin (Yfh1) at its temperature of maximum stability, 292 K (Ts), and the experimentally observed temperature of complete unfolding, 268 K (Tc). Within the simulated timescales, discernible "global" level structural loss at Tc is correlated with a distinct increase in surface hydration. However, the hydration and the unfolding events do not occur uniformly over the entire protein surface, but are sensitive to local structural propensity and hydrophobicity. Calculated infrared absorption spectra in the amide-I region of the whole protein show a distinct red shift at Tc in comparison to Ts. Domain specific calculations of IR spectra indicate that the red shift primarily arises from the beta strands. This is commensurate with a marked increase in solvent accessible surface area per residue for the beta-sheets at Tc. Detailed analyses of structure and dynamics of hydration water around the hydrophobic residues of the beta-sheets show a more bulk water like behavior at Tc due to preferential disruption of the hydrophobic effects around these domains. Our results indicate that in this protein, the surface exposed beta-sheet domains are more susceptible to cold denaturing conditions, in qualitative agreement with solution NMR experimental results.

Smooth change, mechanistic fluctuation: thermodynamic system drift in protein evolution.

Smooth change, mechanistic fluctuation: thermodynamic system drift in protein evolution.

Scheme of thermal compression of hydrogen (TCH) using MmNi4.25Al0.75 recovered with ethyl alcohol and handled under non protective atmospheres

A MmNi4.25Al0.75 intermetallic was obtained by low energy mechanical alloying and low temperature heating at 600 °C for 24 h under Ar. The intermetallic was recovered from milling chamber using ethyl alcohol, dried, stored and handled under air at room conditions. Structure was characterized by XRD. A maximum stability temperature of 160 °C was obtained from non-isothermal DSC measurement under air. The kinetics of oxidation at 200 °C was analyzed. A maximum reaction degree (α = 0.1) was obtained after 2500 s of treatment. The hydrogen sorption properties of samples were studied by volumetric measurements. Hydrogen maximum mass percent capacity (mass %) was reached in less than 300 s. The thermodynamic sorption properties were measured. Values of ΔHf = −29 ± 2 kJ mol−1 and ΔSf = 197 ± 10 J mol−1 K−1 were obtained for absorption process and ΔHd = 28 ± 2 kJ mol−1 and ΔSd = 189 + 10 J mol−1 K−1 were obtained for desorption process. From these results, a one-stage of thermal compression of hydrogen is proposed with a standard compression ratio (Rc) of 5.71 in the 25–80 °C range.

Protein Heat Capacity: An Anomaly that Maybe Never Was

Protein unfolding in aqueous solution is usually accompanied by an increase in heat capacity (ΔCp), and this has long been regarded as somewhat anomalous. However, neither the absolute heat capacities (Cp) of folded globular proteins nor the heat capacity increments upon unfolding (ΔCp) are unusual in comparison to values observed for order−disorder (melting) transitions in other organic substances. The consequences for protein stability, including cold denaturation, enthalpy−entropy compensation, and the temperature of maximum stability, may seem counterintuitive but should not be unexpected. Nevertheless, while perhaps not so anomalous as once thought, quantitative interpretation of protein heat capacity and related effects remains a theoretical challenge.

Thermodynamic Characterization of the Folding Equilibrium of the Human Nedd4-WW4 Domain: At the Frontiers of Cooperative Folding

WW domains are the smallest naturally independent beta-sheet protein structures available to date and constitute attractive model systems for investigating the determinants of beta-sheet folding and stability. Nonetheless, their small size and low cooperativity pose a difficult challenge for a quantitative analysis of the folding equilibrium. We describe here a comprehensive thermodynamic characterization of the conformational equilibrium of the fourth WW domain from the human ubiquitin ligase Nedd4 (hNedd4-WW4) using a combination of calorimetric and spectroscopic techniques with several denaturing agents (temperature, pH, and chemical denaturants). Our results reveal that even though the experimental data can be described in terms of a two-state equilibrium, spectral data together with anomalous values for some thermodynamic parameters (a strikingly low temperature of maximum stability, a higher than expected native-state heat capacity, and a small specific enthalpy of unfolding) could be indicative of more complex types of equilibria, such as one-state downhill folding or alternative native conformations. Moreover, double-perturbation experiments reveal some features that, in spite of the apparent linear correlation between the thermodynamic parameters, seem to be indicative of a complex conformational equilibrium in the presence of urea. In summary, the data presented here point toward the existence of a low-energy barrier between the different macrostates of hNedd4-WW4, placing it at the frontier of cooperative folding.

Protein Cold Denaturation as Seen From the Solvent

Unlike most ordered molecular systems, globular proteins exhibit a temperature of maximum stability, implying that the structure can be disrupted by cooling. This cold denaturation phenomenon is usually linked to the temperature-dependent hydrophobic driving force for protein folding. Yet, despite the key role played by protein-water interactions, hydration changes during cold denaturation have not been investigated experimentally. Here, we use water-(17)O spin relaxation to monitor the hydration dynamics of the proteins BPTI, ubiquitin, apomyoglobin, and beta-lactoglobulin in aqueous solution from room temperature down to -35 degrees C. To access this temperature range without ice formation, we contained the protein solution in nonperturbing picoliter emulsion droplets. Among the four proteins, only the destabilized apomyoglobin was observed to cold denature. Ubiquitin was found to be thermodynamically stable at least down to -32 degrees C, whereas beta-lactoglobulin is expected to be unstable below -5 degrees C but remains kinetically trapped in the native state. When destabilized by 4 M urea, beta-lactoglobulin cold denatures at 10 degrees C, as found previously by other methods. As seen from the solvent, the cold-denatured states of apomyoglobin in water and beta-lactoglobulin in 4 M urea are relatively compact and are better described as solvent-penetrated than as unfolded. This finding challenges the popular analogy between cold denaturation and the anomalous low-temperature increase in aqueous solubility of nonpolar molecules. Our results also suggest that the reported cold denaturation at -20 degrees C of ubiquitin encapsulated in reverse micelles is caused by the low water content rather than by the low temperature.

Monte Carlo simulation study of melittin: Protein folding and temperature dependence

The tetramerization of melittin, a 26-amino-acid peptide, is considered as a model for protein folding. The Monte Carlo simulation was used to study the folding arrangement of melittin, and the results are compared with the experiment. An acceptance rate of 50% for new configurations is achieved by using ranges of ±0.001 A for the translations and ±15°C for the rotations. Around 311 K, the folded structure of the protein has the greatest stability; the range from −40 to −80 shows the best ϕ angles for melittin. The final optimized structure of melittin strongly depends on the temperature. The melittin tetramer is found to have a temperature of maximum stability ranging from 35.5 to 43°C.

A Thermodynamic Comparison of HPr Proteins from Extremophilic Organisms

A thermodynamic stability study of five histidine-containing phosphocarrier protein (HPr) homologues derived from organisms inhabiting diverse environments is described. These HPr homologues are from Bacillus subtilis (Bs), Streptococcus thermophilus (St), Bacillus staerothermophilus (Bst), Bacillus halodurans (Bh), and Oceanobacillus iheyensis (Oi). Analyses of solvent and thermal denaturation experiments provide the cardinal thermodynamic parameters, like deltaG, deltaH, deltaS, T(m), and deltaC(p), that characterize the conformational stability for each homologue. The homologue from Bacillus staerothermophilus (BstHPr) was established as the most thermostable homologue and also the homologue with highest deltaG at all temperatures. A good correlation between habitat temperature of the organism and thermal stability of the protein is also seen. Stability curves (deltaG vs T) for every homologue are also reported; these reveal very similar deltaC(p) and temperature of maximum stability (T(S)) values for all HPr homologues. Stability curves show that the higher thermal stability of some homologues is not a result of change in curvature of the curve or a shift to higher temperature, but rather a displacement of the stability curves to higher deltaG values. Stability curves also allowed estimation of deltaG at habitat temperature of the organisms, and we find good agreement between homologues. Electrostatic contributions to stability of each homologue were investigated by measuring stability as a function of varying pH and NaCl concentration, and our results suggest that most HPr homologues share similar electrostatic contributions to stability.

Conformational stability and thermodynamic characterization of the lipoic acid bearing domain of human mitochondrial branched chain alpha-ketoacid dehydrogenase.

The lipoic acid bearing domain (hbLBD) of human mitochondrial branched chain alpha-ketoacid dehydrogenase (BCKD) plays important role of substrate channeling in oxidative decarboxylation of the branched chain alpha-ketoacids. Recently hbLBD has been found to follow two-step folding mechanism without detectable presence of stable or kinetic intermediates. The present study describes the conformational stability underlying the folding of this small beta-barrel domain. Thermal denaturation in presence of urea and isothermal urea denaturation titrations are used to evaluate various thermodynamic parameters defining the equilibrium unfolding. The linear extrapolation model successfully describes the two-step; native state <-->denatured state unfolding transition of hbLBD. The average temperature of maximum stability of hbLBD is estimated as 295.6 +/- 0.9 K. Cold denaturation of hbLBD is also predicted and discussed.

Rapid Cooperative Two-state Folding of a Miniature α–β Protein and Design of a Thermostable Variant

There is a great deal of interest in developing small stably folded miniature proteins. A limited number of these molecules have been described, however they typically have not been characterized in depth. In particular, almost no detailed studies of the thermodynamics and folding kinetics of these proteins have been reported. Here we describe detailed studies of the thermodynamics and kinetics of folding of a 39 residue mixed α–β protein (NTL9 1–39) derived from the N-terminal domain of the ribosomal protein L9. The protein folds cooperatively and rapidly in a two-state fashion to a native state typical of those found for normal globular proteins. At pH 5.4 in 20 mM sodium acetate, 100 mM NaCl the temperature of maximum stability is 6 °C, the t m is 65.3 °C, Δ H°( t m) is between 24.6 kcal mol −1 and 26.3 kcal mol −1, and Δ C p° is 0.38 kcal mol −1 deg −1. The thermodynamic parameters are in the range expected on the basis of per residue values determined from databases of globular proteins. H/ 2H exchange measurements reveal a set of amides that exchange via global unfolding, exactly as expected for a normal cooperatively folded globular protein. Kinetic measurements show that folding is two-state folding. The folding rate is 640 s −1 and the value of Δ G° calculated from the folding and unfolding rates is in excellent agreement with the equilibrium value. A designed thermostable variant, generated by mutating K12 to M, was characterized and found to have a t m of 82 °C. Equilibrium and kinetic measurements demonstrate that its folding is cooperative and two-state.

Hydrothermal synthesis of pumpellyite-okhotskite series minerals

¶Hydrothermal experiments to synthesize pumpellyite group minerals of the pumpellyite–okhotskite series and to investigate their stability have been carried out at 200, 300 and 400 MPa P fluid and 250–500 °C by using cold-seal pressure vessels and solid buffers of MnO2–Mn2O3, Cu2O–CuO and Cu2O–Cu buffer assemblages. Okhotskite and pumpellyite rich in the okhotskite component crystallized from an oxide mixture starting material of Ca4MgMn3+ 3Al2Si6O24.5-oxide+excess H2O at P fluid of 200, 300 and 400 MPa and temperatures of 300 and 400 °C. However, a single phase of okhotskite was not produced, and associated piemontite, hausmannite, wollastonite, clinopyroxene, corundum, braunite–neltnerite solid solution and alleghanyite also formed. Mn-pumpellyite of the okhotskite–pumpellyite join occurs as aggregates of needle crystals, rounded grains or flaky crystals. Chemical compositions are variable and range from pumpellyite-(Mn2+) to okhotskite: 31–36 SiO2, 13–21 Al2O3, 12–25 total Mn2O3, 0.6–4 MgO and 20–24 wt.% CaO. Reconnaissance experiments using a starting material of synthetic Ca2Mn3+Al2Si3O12(OH)-piemontite at 300 MPa and temperatures of 250, 300, 400 and 500 °C indicate that Mn-rich pumpellyite can crystallize from piemontite at lower temperatures than the stability field of piemontite. The Mn-rich pumpellyite was accompanied by garnet, wollastonite and alleghanyite. The chemical compositions of the Mn-pumpellyites are 32–36 SiO2, 18–27 Al2O3, 8–18 total Mn2O3 and 20–23 wt.% CaO. This study shows that the stability fields of piemontite, piemontite+Mn-pumpellyite, and Mn-pumpellyite range in this order with decreasing temperature under high fO2 conditions. The maximum stability temperature of Mn-rich pumpellyite lies between 400 and 500 °C at 200–400 MPa in high fO2 conditions.

Thermodynamic characterization of the human acidic fibroblast growth factor: evidence for cold denaturation.

The thermodynamic parameters characterizing the conformational stability of the human acidic fibroblast growth factor (hFGF-1) have been determined by isothermal urea denaturation and thermal denaturation at fixed concentrations of urea using fluorescence and far-UV CD circular dichroism (CD) spectroscopy. The equilibrium unfolding transitions at pH 7.0 are adequately described by a two-state (native <--> unfolded state) mechanism. The stability of the protein is pH-dependent, and the protein unfolds completely below pH 3.0 (at 25 degrees C). hFGF-1 is shown to undergo a two-state transition only in a narrow pH range (pH 7.0-8.0). Under acidic (pH <6.0) and basic (pH >8.0) conditions, hFGF-1 is found to unfold noncooperatively, involving the accumulation of intermediates. The average temperature of maximum stability is determined to be 295.2 K. The heat capacity change (DeltaC(p)()) for the unfolding of hFGF-1 is estimated to be 2.1 +/- 0.5 kcal.mol(-1).K(-1). Temperature denaturation experiments in the absence and presence of urea show that hFGF-1 has a tendency to undergo cold denaturation. Two-dimensional (1)H-(15)N HSQC spectra of hFGF-1 acquired at subzero temperatures clearly show that hFGF-1 unfolds under low-temperature conditions. The significance of the noncooperative unfolding under acidic conditions and the cold denaturation process observed in hFGF-1 are discussed in detail.

The reversible two-state unfolding of a monocot mannose-binding lectin from garlic bulbs reveals the dominant role of the dimeric interface in its stabilization.

Allium sativum agglutinin (ASAI) is a heterodimeric mannose-specific bulb lectin possessing two polypeptide chains of molecular mass 11.5 and 12.5 kDa. The thermal unfolding of ASAI, characterized by differential scanning calorimetry and circular dichroism, shows it to be highly reversible and can be defined as a two-state process in which the folded dimer is converted directly to the unfolded monomers (A2 if 2U). Its conformational stability has been determined as a function of temperature, GdnCl concentration, and pH using a combination of thermal and isothermal GdnCl-induced unfolding monitored by DSC, far-UV CD, and fluorescence, respectively. Analyses of these data yielded the heat capacity change upon unfolding (DeltaC(p) and also the temperature dependence of the thermodynamic parameters, namely, DeltaG, DeltaH, and DeltaS. The fit of the stability curve to the modified Gibbs-Helmholtz equation provides an estimate of the thermodynamic parameters DeltaH(g), DeltaS(g), and DeltaC(p) as 174.1 kcal x mol(-1), 0.512 kcal x mol(-1) x K(-1), and 3.41 kcal x mol(-1) x K(-1), respectively, at T(g) = 339.4 K. Also, the free energy of unfolding, DeltaG(s), at its temperature of maximum stability (T(s) = 293 K) is 13.13 kcal x mol(-1). Unlike most oligomeric proteins studied so far, the lectin shows excellent agreement between the experimentally determined DeltaC(p) (3.2 +/- 0.28 kcal x mol(-1) x K(-1)) and those evaluated from a calculation of its accessible surface area. This in turn suggests that the protein attains a completely unfolded state irrespective of the method of denaturation. The absence of any folding intermediates suggests the quaternary interactions to be the major contributor to the conformational stability of the protein, which correlates well with its X-ray structure. The small DeltaC(p) for the unfolding of ASAI reflects a relatively small, buried hydrophobic core in the folded dimeric protein.