Simultaneous determination of structural and thermodynamic effects of carbohydrate solutes on the thermal stability of ribonuclease A

This communication describes a new technique for the study of the effects of carbohydrates on the thermal stability of proteins. This approach combines capillary electrophoresis (CE) and protein charge ladders, collections of proteins that differ incrementally in number of chemically modified charged groups, to provide information on both the thermodynamics (i.e., the free energy, DeltaGN-D, of denaturation), and structural changes (i.e., the effective hydrodynamic radius, RH, of proteins in both the native and denatured states) associated with stability. This information, obtained in a single set of electrophoresis experiments, allows a simple microscopic interpretation of the effects of carbohydrate solutes on protein stability. We use this technique to show that the stabilization of ribonuclease A at pH 8.4 by sucrose and fructose can be explained entirely by the contribution these solutes make to the entropy of formation of the protein-solution interface. There is no need, in this case, to refer to quasichemical concepts such as preferential hydration, binding, or exchange of solutes with water at specific sites on the protein to account for the stabilizing effects observed.     

Effect of structure and thermodynamic stability on the response of lanthanide stannate pyrochlores to ion beam irradiation

The lanthanide stannates, Ln2Sn2O7, Ln=La-Lu and Y, have the isometric pyrochlore structure, A2B2O7, and their structural properties have been refined by Rietveld analysis of powder neutron and synchrotron X-ray diffraction data. In this study, the enthalpies of formation of selected stannate pyrochlores, Ln=La, Nd, Sm, Eu, Dy, and Yb, were measured by high-temperature oxide melt solution calorimetry. Their radiation response was determined by 1 MeV Kr2+ ion irradiation combined with in situ TEM observation over the temperature range of 25 to 1000 K. The enthalpy of formation from binary oxides of stannate pyrochlores became more endothermic (from -145 to -40 kJ/mol) as the size of the lanthanide in the A-site decreases. A more exothermic trend of the enthalpy of formation was observed in stannate pyrochlores with larger lanthanide ions, particularly La, possibly as a result of increased covalency in the Sn-O bond. In contrast to lanthanide titanate pyrochlores, Ln2Ti2O7, that are generally susceptible to radiation-induced amorphization and zirconate pyrochlores, Ln2Zr2O7, that are generally resistant to radiation-induced amorphization, the lanthanide stannate pyrochlores show a much greater variation in their response to ion irradiation. La, Nd, and Gd stannates experience the radiation-induced transformation to the aperiodic state, and the critical amorphization temperatures are approximately 960, 700, and 350 K, respectively. Y and Er stannate pyrochlores cannot be amorphized by ion beam irradiation, even at 25 K, and instead disorder to a defect fluorite structure. Comparison of the calorimetric and ion irradiation data for titanate, zirconate, and stannate pyrochlores reveals a strong correlation among subtle changes in crystal structure with changing composition, the energetics of the disordering process, and the temperature above which the material can no longer be amorphized. In summary, as the structure approaches the ideal, ordered pyrochlore structure, radiation-induced amorphization is more easily attained. This is consistent with an increasingly exothermic trend in the enthalpies of formation of pyrochlores from the oxides, that is, the greater the thermochemical stability of the pyrochlore structure, the more likely it will be amorphized upon radiation damage rather than recover to a disordered fluorite structure.     

Effect of dextran polymer on glycocalyx structure and cell electrophoretic mobility

We determined the electrophoretic mobility of human red blood cells suspended in physiological dextran solution for polymer samples with different polydispersity indexes. The electrophoretic mobility of cells is shown to depend mainly on the physical and hydrodynamical characteristics of the porous glycocalyx coating the membrane bilayer. We then propose a new model for the observed increases in effective mobility when cells are exposed to neutral polymers. The extension of glycoproteins under the action of weakly adsorbed chains is shown to reduce the frictional interaction of the surface coat on the flowing liquid. Such a structural rearrangement of the outer membrane improves the penetration of the electro-osmotic flow within the glycocalyx and increases the effective cell mobility.     

A thermodynamic model for the shape and stability of twinned nanostructures

Although thermodynamically metastable, planar defects are often observed in many faceted nanomaterials including nanocrystals, nanorods, and nanowires, even after annealing. These planar defects include contact twins and (intrinsic or extrinsic) stacking faults, and are usually neglected by most analytical models. For example, many bulk metals have the face-centered cubic structure, but small nanocrystals and nanorods of the same material often exhibit various structural and morphological modifications such as single or multiple symmetric twinning, as well as 5-fold cyclic twinning resulting in decahedral and truncated decahedral nanostructures. Presented here is a general analytical model for the investigation of nanomaterials of arbitrary shape, and with any configuration of planar defects. The model is tested for the case of twinning in unsupported gold nanocrystals and nanorods, and is shown to give results in excellent agreement with experimental and computational studies reported in the literature.     

Effect of structure on the thermal stability of polyimides

Polyimides with different proportions of m-phenylene and p-phenylene (or p,p′-biphenylene) were prepared by polymerizing different molar ratios of m-phenylene diamine and p-phenylene diamine (or p,p′-diaminobiphenyl) with pyromellitic dianhydride in dimethylformamide at 0°C. Chemical cyclodehydration of polyamic acids resulted in the corresponding polyimides. Polymers were characterized by infrared (IR), viscosity, and density measurements. Viscosity and density of polymers decreased with an increase on m-phenylene groups in the backbone. The thermal and thermooxidative stabilities were investigated by dynamic thermogravimetry. Stability decreased when m-phenylene groups were introduced in the backbone.     

Effect of hydrogen bonding of solvent on the thermodynamic stability of cadmium ethylenediamine complexes

Changes in the stability of the cadmium(ii) ethylenediamine complexes in mixed water—DMSO solvents were studied by pH-metry and calorimetry. Complex cations [Cd(en)]²⁺, [Cd(en)₂]²⁺, and [Cd(en)₃]²⁺ are formed in aqueous solutions, and the [Cd(en)₄]²⁺ complex with a partially dentate ligand is stable in DMSO. An increase in the DMSO content in a solvent increases the stability of the complexes. The maximum increase in logK is observed for coordinatively saturated compounds. The thermodynamics of complexation is discussed from the viewpoint of solvation approach. Principal differences in the influence of aqueous-alcohol and aqueous-aprotic solvents on the stability of the metal amino complexes were revealed. Protolytic solvents exert a destabilizing effect on the multiligand complexes, because the coordination sphere is involved in H bonding.     

Effect of thioxopeptide bonds on alpha-helix structure and stability

Thioxoamide (thioamide) bonds are nearly isosteric substitutions for amides but have altered hydrogen-bonding and photophysical properties. They are thus well-suited backbone modifications for physicochemical studies on peptides and proteins. The effect of thioxoamides on protein structure and stability has not been subject to detailed experimental investigations up to date. We used alanine-based model peptides to test the influence of single thioxoamide bonds on alpha-helix structure and stability. The results from circular dichroism measurements show that thioxoamides are strongly helix-destabilizing. The effect of an oxo-to-thioxoamide backbone substitution is of similar magnitude as an alanine-to-glycine substitution resulting in a helix destabilization of about 7 kJ/mol. NMR characterization of a helical peptide with a thioxopeptide bond near the N-terminus indicates that the thioxopeptide moiety is tolerated in helical structures. The thioxoamide group is engaged in an i, i+4 hydrogen bond, arguing against the formation of a 3(10)-helical structure as suggested for the N-termini of alpha-helices in general and for thioxopeptides in particular.     

Effect of dextran molecular weight on resonance light-scattering of quantum dots modified with dextran and concanavalin A

The reversible aggregation between dextran-conjugated CdSe quantum dots(Dex-CdSe-QDs) and concanavalin A(Con A) was explored based on the specific affinity of polysaccharide for Con A by resonance light-scattering technique.In this study,the aggregation of Dex-CdSe-QDs induced by Con A and sequential dissociation by glucose was determined over a wide range of dextran molecular mass(10-500 kDa).The results demonstrate that the sensitivity of lectin-dextran-modified-QDs interactions increase with dextran molecular mass,and an optimum dextran molecular mass is 500 kDa.     

Gaseous salts of oxygen-containing acids: Thermal stability,structure, and thermodynamic properties

Experimental data and results of quantum-chemical calculations on the structure and thermodynamic properties of gaseous salts of oxygen-containing acids were systematized. A criterion of thermal stability of these compounds was offered and regularities in the atomization enthalpies were established.     

Thermal vacancy effects on the thermodynamic properties and stability of fullerites

We study the influence of thermal vacancies on equilibrium thermodynamic properties of the high-temperature phase of fullerites taking into account the strong anharmonicity of the lattice vibrations. Treating a crystal with point defects as a quasi-multicomponent system and using the correlative method of the unsymmetrized self-consistent field and the Girifalco interaction potential for the molecular subsystem, we have obtained the vacancy formation parameters for the C(60) fullerite. We also take into account the divacancies. The influence of the lattice defects on the specific heats of fullerites is negligible, since a dominant contribution to them is given by intramolecular degrees of freedom. We have calculated the contributions of vacancies to various thermodynamic properties, the volume thermal expansion coefficient, the isothermal bulk modulus, and the components of the isothermal elastic tensor, depending on the temperature and pressure. Near the estimated triple point, these contributions run to more than 10% and increase still further at a metastable region. We also discuss the influence of defects on the thermodynamic stability of fullerites.     

On the thermodynamic and kinetic stability of nonequilibrium systems

A criterion of evolution of nonequilibrium systems in the region of stable states was formulated.     

On the thermodynamic stability of hydrogen clathrate hydrates

The cage occupancy of hydrogen clathrate hydrate has been examined by grand canonical Monte Carlo (GCMC) simulations for wide ranges of temperature and pressure. The simulations are carried out with a fixed number of water molecules and a fixed chemical potential of the guest species so that hydrogen molecules can be created or annihilated in the clathrate. Two types of the GCMC simulations are performed; in one the volume of the clathrate is fixed and in the other it is allowed to adjust itself under a preset pressure so as to take account of compression by a hydrostatic pressure and expansion due to multiple cage occupancy. It is found that the smaller cage in structure II is practically incapable of accommodating more than a single guest molecule even at pressures as high as 500 MPa, which agrees with the recent experimental investigations. The larger cage is found to encapsulate at most 4 hydrogen molecules, but its occupancy is dependent significantly on the pressure of hydrogen.     

Effect of formic acid addition on water cluster stability and structure

Computational chemistry simulations were performed to determine the effect that the addition of a single formic acid molecule has on the structure and stability of protonated water clusters. Previous experimental studies showed that addition of formic acid to protonated pure water results in higher intensities of large-sized clusters when compared to pure water and methanol-water mixed clusters. For larger, protonated clusters, molecular dynamics simulations were performed on H(+)(H(2)O)(n), H(+)(H(2)O)(n)CH(3)OH, and H(+)(H(2)O)(n)CHOOH clusters, 19-28 molecules in size, using a reactive force field (ReaxFF). Based on these computations, formic acid-water clusters were found to have significantly higher binding energies per molecule. Addition of formic acid to a water cluster was found to alter the structure of the hydrogen-bonding network, creating selective sites within the cluster, enabling the formation of new hydrogen bonds, and increasing both the stability of the cluster and its rate of growth.     

The effect of tricarbonylchromium on the thermodynamic stability of stable carbocations

The pK_R+'s for the ionization equilibria between a number of p,p′-substituted diphenylmethanols mono- and bis-complexed with the Cr(CO)₃ group and the corresponding carbocations in aqueous sulfuric acid were determined by spectrophotometric methods. The same pK_R+'s were measured for two ferrocenyltricarbonylchromiumarylmethanols in aqueous trifluoroacetic acid. The introduction of the Cr(CO)₃ group can make the complexed cation either more or less stable than the corresponding free ion, depending upon the structure. The effect of the complexation is explained in terms of electronic and conformational effects.     

Effect of structural defects on the electronic structure and stability of cubic molybdenum carbide

Institute of Chemistry, Ural Branch, Academy of Sciences of the USSR. Translated from Zhurnal Strukturnoi Khimii, Vol. 30, No. 2, pp. 19–24, March–April, 1989.     

On the thermodynamic stability and structural transition of clathrate hydrates

Gas mixtures of methane and ethane form structure II clathrate hydrates despite the fact that each of pure methane and pure ethane gases forms the structure I hydrate. Optimization of the interaction potential parameters for methane and ethane is attempted so as to reproduce the dissociation pressures of each simple hydrate containing either methane or ethane alone. An account for the structural transitions between type I and type II hydrates upon changing the mole fraction of the gas mixture is given on the basis of the van der Waals and Platteeuw theory with these optimized potentials. Cage occupancies of the two kinds of hydrates are also calculated as functions of the mole fraction at the dissociation pressure and at a fixed pressure well above the dissociation pressure.     

Energetics and thermodynamic stability of the mixed valence ytterbium germanides

The results of an experimental study concerning the thermodynamic stability of the Yb germanides, described as intermediate valence compounds, complemented by a computational investigation for the Yb3Ge5 compound are reported. These compounds belong to the rare earth (RE) tetrelides (tetrel = Si, Ge, i.e., group 14 elements), a class of intermetallic materials showing unusual and promising physical properties (giant magnetocaloric effect, magnetostriction, and magnetoresistence). The high-temperature decomposition reactions of the Yb-Ge intermediate phases were studied experimentally by means of the KEMS (Knudsen effusion mass spectrometry) and KEWL (Knudsen effusion weight loss) techniques. From the reaction enthalpies derived by measuring the Yb(g) decomposition pressures as a function of temperature, the heats of formation of five out of six of the intermediate phases in the Yb-Ge system were calculated. From the computational side, the stability of the Yb3Ge5(s) compound has been investigated by DFT-LCAO-B3LYP (density functional theory-linear combination of atomic orbitals-hybrid b3lyp exchange-correlation functional) first principles calculations deriving its equilibrium geometry and the enthalpy of formation at 0 K in relation to the intermediate valence state of Yb in the lattice.