Insights into the energetics and mechanism underlying the interaction of tetraethylammonium bromide with proteins

Calorimetry has been employed to investigate the quantitative energetic aspects and mechanism underlying protein–tetraethylammonium bromide (TEAB) interactions. Differential scanning calorimetry and UV–Visible spectroscopy have been used to study the thermal unfolding of three proteins of different structure and function (bovine serum albumin, α-lactalbumin, and bovine pancreatic ribonuclease A). The mode of interaction has been studied by using isothermal titration calorimetry, which demonstrates the absence of appreciable specific binding of TEAB to the protein. This suggests the involvement of solvent mediated effects and, possibly weak non-specific binding. The thermal unfolding transitions were found to be calorimetrically reversible for α-lactalbumin and bovine pancreatic ribonuclease A and partially reversible in the case of bovine serum albumin. The results indicate protein destabilization promoted by the TEAB interaction. The preferential interaction parameters of TEAB with α-lactalbumin and ribonuclease A confirm that an increased interaction of the hydrophobic groups of the TEAB with that of the protein upon denaturation is responsible for the reduced thermal stability of the protein. The decrease in the thermal stability of proteins in the presence of TEAB is well supported by a red shift in the intrinsic fluorescence of these proteins leading to conformational change thereby shifting the native ? denatured equilibrium towards right. The forces responsible for the thermal denaturation of the proteins of different structure and function in the presence of TEAB are discussed.     

Thermochemical behavior of crown ethers in the mixtures of water with organic solvents. Part VIII. Hydrophobic hydration and preferential solvation of 1,4-dioxane in {(1 − x)amide + xH2O} at T = 298.15 K

The enthalpies of solution of 1,4-dioxane in {(1 − x)F + xH₂O}, {(1 − x)NMF + xH₂O}, and {(1 − x)DMF + xH₂O} have been measured within the whole mole fraction range at T = 298.15 K. Based on the obtained data, the effect of substituting methyl groups at the nitrogen atom in formamide on the preferential solvation of 1,4-dioxane has been analyzed. A simple model has been proposed to describe the influence of structural and energetic properties of the mixed solvent on the energetic effect of hydrophobic hydration and preferential solvation of 1,4-dioxane by the components of the examined mixture.     

Solubility of gases in fluoroorganic alcohols. Part II. Solubilities of noble gases in (water + 1,1,1,3,3,3-hexafluoropropan-2-ol) at 298.15 K and 101.33 kPa

Solubilities of noble gases He, Ne, Ar, Kr and Xe in mixtures of {water + 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP)} at 298.15 K and 101.33 kPa partial pressure of gas are reported. A polynomial dependence of the solubilities on the mole fraction of the binary liquid mixture is found. The Henry’s constants at the vapor pressure of the mixture, the standard changes in the Gibbs free energy for the solution process and for the solvation process, and the so-called excess Henry’s constant are calculated. The results have been compared with those obtained by scaled particle theory (SPT).     

Solution thermodynamics of pyrazinamide,isoniazid, and p-aminobenzoic acid in buffers and octanol

The solubility values of pyrazinamide, isoniazid, and p-aminobenzoic acid in buffers (рН 2.0 and 7.4) and octanol were measured in the temperature range of 293.15 to 313.15 K. The dissolution Gibbs energy, enthalpy, and entropy were calculated. The dissolving process was endothermic and enthalpy-determined. The activity coefficients of the compounds at infinite dilution were determined based on the solubility data and thermophysical parameters. A positive deviation from the ideality was observed in all the solutions. A common tendency of the solubility increase with a decrease in the activity coefficients at T = 298.15 K was revealed for the investigated solute-solvent systems. The excess thermodynamic solubility functions were calculated from the temperature dependences of the activity coefficients. The solvation processes were found to have a considerable influence on the solubility of the substances in solutions studied.     

Enthalpies of solvation of ethylene oxide oligomers CH3O(CH2CH2O)nCH3 (n = 1 to 4) in different H-bonding solvents: Methanol,chloroform, and water. Group contribution method as applied to the polar oligomers

The enthalpies of solution and solvation of ethylene oxide oligomers CH₃O(CH₂CH₂O)_nCH₃ (n = 1 to 4) in methanol and chloroform have been determined from calorimetric measurements at T = 298.15 K. The enthalpic coefficients of pairwise solute–solute interaction for methanol solutions have been calculated. The enthalpic characteristics of the oligomers in methanol, chloroform, water and tetrachloromethane have been compared. The hydrogen bonding of the oligomers with chloroform and water molecules is exhibited in the values of solvation enthalpy and coefficient of solute–solute interaction. This effect is not observed for methanol solvent. The thermochemical data evidence an existence of multi-centred hydrogen bonds in associates of polyethers with the solvent molecules. Enthalpies of hydrogen bonding of the oligomers with chloroform and water have been estimated. The additivity scheme has been developed to describe the enthalpies of solvation of ethylene oxide oligomers, unbranched monoethers and n-alkanes in chloroform, methanol, water, and tetrachloromethane. The correction parameters for contribution of repeated polar groups and correction term for methoxy-compounds have been introduced. The obtained group contributions permit to describe the enthalpies of solvation of unbranched monoethers and ethylene oxide oligomers in the solvents with standard deviation up to 0.6 kJ · mol⁻¹. The values of group contributions and corrections are strongly influenced by solvent properties.     

Clarification of the thermally-induced pretransition of ribonuclease A in solution by principal component analysis and two-dimensional correlation infrared spectroscopy

Thermal denaturation of ribonuclease A (RNase A) in D₂O solution is studied by Fourier transform infrared (FT-IR) spectroscopy. Sample–sample two-dimensional correlation (SS 2D) spectroscopy and principal component analysis (PCA) are applied to these spectral data to reveal the thermal kinetics of RNase A. The second scores plot of PCA constructed from temperature-dependent original IR spectra illustrates a pretransition at 46 °C as well as a clear main transition at 66 °C. The latter is revealed by the SS 2D correlation spectra and the first score of PCA because of their illustration of the main denaturation event of RNase A, while the former cannot. Therefore, the present study demonstrates the great potential of PCA in revealing subtle phase transition of proteins in aqueous solutions.     

Isothermal titration calorimetric and spectroscopic studies on (alcohol + salt) induced partially folded state of α-lactalbumin and its binding with 8-anilino-1-naphthalenesulfonic acid

Interaction of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) and isopropanol in the presence of equimolar quantities of guanidine thiocyanate (GndSCN) with bovine α-lactalbumin (α-LA) has been investigated by using a combination of isothermal titration calorimetry, circular dichroism, fluorescence, and ultra-violet spectroscopies at in 20 · 10^?3 mol · dm^?3 phosphate buffer pH 7.0. All the thermal unfolding transitions, in the presence of both the (alcohol + salt) mixtures were found to be reversible as judged by the same values of absorbance observed at different temperatures during cooling after the completion of thermal unfolding. In the presence of the 0.25 mol · dm^?3 (HFIP + GndSCN) equimolar mixture and 0.85 mol · dm^?3 (isopropanol + GndSCN) equimolar mixture, α-lactalbumin was observed to be in the partially folded state with significant loss of native tertiary structure. Intrinsic fluorescence results, acrylamide and potassium iodide quenching, 8-anilino-1-naphthalenesulfonic acid (ANS) binding, and energy transfer results also corroborate the presence of partially folded states of α-lactalbumin. Apart from the generation of the partially folded states, it was also observed that destabilizing action of GndSCN is reduced in the presence of isopropanol compared to that in HFIP. Isothermal titration calorimetry has been used to characterize the energetics of ANS binding to the partially folded states of the protein. ITC results indicate that ANS binds to these partially folded states at pH 7.0 due to the presence of two sequentially binding sites on the protein under the solvent conditions employed. For example, ANS binds to the 0.25 mol · dm^?3 (HFIP + GndSCN) induced partially folded state with affinity constants K₁ = (858 ± 220), K₂ = (1.12 ± 0.25) · 10³; enthalpies of binding ΔH₁ = (4.4 ± 1.0) kJ · mol^?1, ΔH₂ = (2.1 ± 0.2) kJ · mol^?1; and entropies of binding ΔS₁ = 70 J · K^?1 · mol^?1 and ΔS₂ = 65 J · K^?1 · mol^?1, respectively at these two sequential binding sites. In light of the fluorescence results, possible binding sites where ANS can bind to the protein have also been suggested.     

Thermodynamic aspects of solubility,solvation and partitioning processes of some sulfonamides

The thermodynamic aspects of sublimation processes of three sulfonamides with the general structures C₆H₅–SO₂NH–C₆H₄–R (R = 4-NO₂) and 4-NH₂–C₆H₄–SO₂NH–C₆H₄–R (R = 4-NO₂; 4-CN) were studied by investigating the temperature dependence of vapor pressure using the transpiration method. These data together with those obtained earlier for C₆H₅–SO₂NH–C₆H₄–R (R = 4-Cl) and 4-NH₂–C₆H₄–SO₂NH–C₆H₄–R (R = 4-Cl; 4-OMe; 4-C₂H₅) were analyzed and compared. A correlation was derived between sublimation Gibbs free energies and the sum of H-bond acceptor factors of the molecules. Solubility processes of the compounds in water, phosphate buffer with pH 7.4 and n-octanol (as phases modeling various drug delivery pathways) were investigated and corresponding thermodynamic functions were calculated as well. Thermodynamic characteristics of the sulfonamides solvation were evaluated. Also in this case a correlation between solubility/solvation Gibbs free energy values and the sum of H-bond acceptor factors was observed. For the sulfonamides with various substituents at para-position the processes of transfer from one solvent (water or buffer) to n-octanol were studied by a diagram method combined with analysis of enthalpic and entropic terms. Distinguishing between enthalpy and entropy, as is possible through the present approach, leads to the insight that the contribution of these terms is different for different molecules (entropy- or enthalpy-determined). Thus, in contrast to the interpretation of only the Gibbs free energy of transfer (extensively used for pharmaceuticals in the form of the partition coefficient, log P), the analysis of thermodynamic functions of the transfer process provides additional mechanistic information. This may be important for further evaluation of the physiological distribution of drug molecules and may provide a better understanding of biopharmaceutical properties of drugs.     

The heat capacities and thermodynamic functions of some derivatives of ferrocene

The heat capacities of benzoylferrocene (BOF), C₅H₅FeC₅H₄COC₆H₅, and benzylferrocene (BF), C₅H₅FeC₅H₄CH₂C₆H₅, have been measured by the low-temperature adiabatic calorimetry in the temperature range from 6 K to 372 K. The purity benzylferrocene and thermodynamic properties – the triple point temperature and the enthalpy of fusion have been obtained. The ideal gas thermodynamic functions (changes of the entropy, enthalpy, and Gibbs free energy) of BOF and BF were derived at T = 298.15 K using the heat capacities and previously determined data on the saturation vapours pressures and the enthalpies of sublimation. The ideal gas enthalpy of formation and absolute entropy of BOF at T = 298.15 K have been obtained from quantum chemical calculations, where as the thermodynamic properties of BF have been estimated by empirical method of group equations. A good agreement between experimental and theoretical values provides an additional check of the reliability of the experimental data.     

Caractérisation par spectrométrie de masse MALDI–TOF de poly(isosorbide-éther)s cycliques et linéaires obtenus sous irradiation microondes

Characterisation of cyclic and linear poly(isosorbide-ether)s obtained under microwave irradiation by MALDI–TOF mass spectrometry. We studied the influence of the alkyl chain length and of the leaving group on the fraction of cyclic or linear poly(isosorbide-ether)s. A survey by MALDI–TOF MS showed that cyclic chains (C) are predominant when some short aliphatic chains are used (m = 4 or 6), whereas with longer chains (m = 8, 10 or 12), we noted that cyclic products were in a minority, whatever the leaving group (bromide or mesyl). Important non-thermal microwave effects have been demonstrated, notably a methanol-insoluble fraction of polyether considerably larger than the one obtained by conventional heating. These poly(isosorbide-ether)s have been characterized by RMN, MALDI–TOF, SEC, elementary analysis, and DSC. The survey of the thermal behaviour by DSC showed that the temperatures of fusion increase with the aliphatic chain's length (m = 12; T_f = 37 °C).     

Liquid–liquid equilibria for ternary systems of (water + carboxylic acid + 1-octanol) at 293.15 K: modeling phase equilibria using a solvatochromic approach

Liquid–liquid equilibrium data of the solubility (binodal) curves and tie-line end compositions are presented for mixtures of [water (1) + formic acid or propanoic acid or levulinic (4-oxopentanoic) acid or valeric (pentanoic) acid or caproic (hexanoic) acid (2) + 1-octanol (3)] at 293.15 K and 101.3 ± 0.7 kPa. A log-basis approach SERLAS (solvation energy relation for liquid associated system) has been proposed to estimate the properties and liquid–liquid equilibria (LLE) of tertiary associated systems containing proton-donating and -accepting components capable of a physical interaction through hydrogen bonding. The model combines the solvatochromic parameters with the thermodynamic factors derived from the UNIFAC-Dortmund model. The reliability of the model has been analyzed against the LLE data with respect to the distribution ratio and separation factor. The tie-lines were also correlated using the UNIFAC-original model. The proposed model, reflecting the simultaneous impact of hydrogen bonding, solubility and thermodynamic factors, yields a mean error of 27.9% for all the systems considered.     

Solvation of multivalent cations in methanol – Apparent molar volumes,expansibilities, and isentropic compressibilities

Densities of the solutions of gallium, yttrium, lanthanum, gadolinium, and lutetium trifluoromethanesulfonates in methanol have been determined at temperatures ranging from (283.15 to 313.15) K. The sound velocities in these systems have been measured at T = 298.15 K. From obtained data the standard partial molar volumes and their temperature derivatives as well as standard partial molar adiabatic compressibilities for ions in solutions have been estimated. The results have been discussed on the basis of the classical model of ion in solution and compared with our earlier findings for divalent cations. The importance of the numbers of solvent molecules in first solvation spheres of ions has been revealed.     

A comparative study for treatment of white liquor by different applications of Fenton process

In this paper, the treatability of white liquor by conventional (CFP), modified (MFP) and electro-Fenton oxidation processes (EFP) was investigated depending on the COD parameter. Based on the experimental results, up to 62.4%, 58.4% and 54.9% COD removals by the CFP, MFP and EFP were achieved, respectively. It was observed that adjustment of initial pH to acidic values is not required in the CFP. The optimal operational conditions were found to be [Fe²⁺] = 500 mg/L, [H₂O₂] = 1000 mg/L at pH 7.3 (original pH) in the CFP, [Fe⁰] = 1250 mg/L, [H₂O₂] = 1000 mg/L at pH 3 in the MFP, and I = 1.0 A, [H₂O₂] = 1500 mg/L at pH 3 in the EFP, respectively. As a result, the CFP has been determined as a more efficient alternative treatment method.     

Heat capacities and thermodynamic functions of hexagonal ice from T = 0.5 K to T = 38 K

The heat capacity of water in the form of hexagonal ice was measured between T = 0.5 K and T = 38 K using a semi-adiabatic calorimetric method. Since heat capacity data below T = 2 K have never been measured for water, this study presents the lowest measured values of the specific heat of water to date. Fits of the data were used to generate thermodynamic functions of water at smoothed temperatures between 0.5 K and 38 K. Both our experimental heat capacities and calculated enthalpy increments agree well with previously published values and thus supplement other studies well.     

Low-temperature thermodynamic properties of Ir(C5H7O2)3: Connection of entropy with the molecule volume for tris-acetylacetonates of metals

The heat capacity of Ir(C₅H₇O₂)₃ has been measured by the adiabatic method within the temperature range (5 to 305) K. The thermodynamic functions (entropy, enthalpy, and reduced Gibbs free energy) at 298.15 K have been calculated using the obtained experimental heat capacity data. A connection has been found between the entropy and the volume of the elementary crystalline cell for β-acetylacetonates of some metals. The reasons for this interdependence are discussed. The values of entropies at T = 298.15 K have been calculated for all the metal acetylacetonates on which there are structural data.     

Buffer standards for the physiological pH of the zwitterionic compound of 3-(N-morpholino)propanesulfonic acid (MOPS) from T = (278.15 to 328.15) K

This paper reports the pH values of five NaCl-free buffer solutions and 11 buffer compositions containing NaCl at I = 0.16 mol · kg⁻¹. Conventional pa_H values are reported for 16 buffer solutions with and without NaCl salt. The operational pH values have been calculated for five buffer solutions and are recommended as pH standards at T = (298.15 and 310.15) K after correcting the liquid junction potentials. For buffer solutions with the composition m₁ = 0.04 mol · kg⁻¹, m₂ = 0.08 mol · kg⁻¹, m₃ = 0.08 mol · kg⁻¹ at I = 0.16 mol · kg⁻¹, the pH at 310.15 K is 7.269, which is close to 7.407, the pH of blood serum. It is recommended as a pH standard for biological specimens.     

Phase equilibria for ternary liquid systems of (water + tetrahydrofuran + nonprotic aromatic solvent) at T = 298.2 K

(Liquid + liquid) equilibrium (LLE) data of the solubility curves and tie-line end compositions are presented for mixtures of {water (1) + tetrahydrofuran (2) + xylene or chlorobenzene or benzyl ether (3)} at T = 298.2 K and P = (101.3 ± 0.7) kPa. Among the studied C6 ring-containing aromatic solvents, xylene gives the largest distribution ratio and separation factors for extraction of tetrahydrofuran. A solvation energy relation (SERLAS) has been used to estimate the (liquid + liquid) equilibria of associated systems containing a nonprotic solvent. The tie-lines were also predicted using the UNIFAC-original model. The reliability of both models has been analyzed against the LLE data with respect to the distribution ratio and separation factor. SERLAS matches LLE data accurately, yielding a mean error of 9.9% for all the systems considered.     

Tetrakis(4-sulfonatophenyl)porphyrin fluorescence as reporter of human serum albumin structural changes induced by guanidine hydrochloride

The interaction of the drug carrier protein human serum albumin (HSA) with the ionic, free base porphyrin tetrakis(4-sulfonatophenyl)porphyrin (TSPP) was investigated under chemical denaturation conditions using guanidine hydrochloride (Gdn-HCl) in aqueous solution at pH 7 and 2.5. Protein stability was studied by fluorescence spectroscopy using intrinsic tryptophan fluorescence, whereas far-UV circular dichroism gave information regarding conformational changes. Steady-state and time-resolved fluorescence as well as extinction and induced visible CD of TSPP were also monitored in the presence of the denaturant.The addition of 1.0 M Gdn-HCl inhibited the FRET process between the sole tryptophan residue of HSA and the porphyrin as inferred by an increase in the intrinsic fluorescence of the former together with a drop in the fluorescence of the latter. Simultaneously, an induced bisignate CD band was detected in the Soret region of TSPP extinction following the changes in the monomer ? aggregate equilibrium of TSPP. The features in the extinction spectra pointed to the formation of J-aggregates at pH = 2.5 and were confirmed by fluorescence lifetime measurements. At pH = 7, no TSPP dimers were detected in the absence of the protein or in the presence of native HSA. However, H-dimers or higher aggregates of TSPP associated to HSA were induced at concentrations of Gdn-HCl below 2 M.The main unfolding transition probed by HSA intrinsic fluorescence took place between 2 and 3 M Gdn-HCl at pH = 7, whereas at pH = 2.5 it was detected only above 2.8 M Gdn-HCl, coinciding with TSPP release into solution which occurs at high denaturant concentration for both pH studied. The results suggest that the chemical unfolding of HSA is a multistep process. The free base porphyrin contributes to an increase in the protein stability, particularly important under acidic conditions, where the protein is known to be in an expanded form in the absence of TSPP.The analysis of TSPP fluorescence fluctuations in the autocorrelation functions obtained using fluorescence correlation spectroscopy (FCS) in the presence of HSA at different denaturant concentrations showed that the porphyrin only interacts with the native form of the protein.Both fluorescence and circular dichroism data confirmed that in the noncovalent complex HSA–TSPP the free base porphyrin can act as a reporter of the protein structural changes induced either by pH or chemical denaturation.