Aase Hvidt

Excess partial molar enthalpies, entropies, Gibbs energies, and volumes in aqueous dimethylsulfoxide

The excess partial molar enthalpies, the vapor pressures, and the densities of dimethylsulfoxide (DMSO)−H2O mixtures were measured and the excess partial molar Gibbs energies and the partial molar volumes were calculated for DMSO and for H2O. The values of the excess partial molar Gibbs energies for both DMSO and H2O are negative over the entire composition range. The results for the water-rich region indicated that the presence of DMSO enhances the hydrogen bond network of H2O. Unlike monohydric alcohols, however, the solute-solute interaction is repulsive in terms of the Gibbs energy. This was a result of the fact that the repulsion among solutes in terms of enthalpy surpassed the attraction in terms of entropy. The data in the DMSO-rich region suggest that DMSO molecules form clusters which protect H2O molecules from exposure to the nonpolar alkyl groups of DMSO.

Densities of aqueous mixtures of non-electrolytes

Densities at 25 °C of aqueous mixtures of urea, formamide, N -methylacetamide, dimethylacetamide, acetyl-alanine-methylamide, acetone, and butanone have been measured, and specific volumes, partial volumes, and excess volumes of the mixtures are evaluated. An inflection point of the specific volume, corresponding to a maximum of the partial volume of water, is observed for mixtures of the substances containing aliphatic groups. The excess volumes are all negative, but it is tentatively concluded that the contribution to the excess volumes due to interactions between the aliphatic groups and water is positive . This effect is discussed with particular reference to protein denaturation. The interaction between aliphatic groups and water is further discussed in an appendix to the paper. It is suggested that more than 15 water molecules may interact co-operatively with the non-polar groups, and that the probability that a given group takes part in this interaction is less than one-half.

Excess volumes of propanol-water mixtures at 5, 15, and 25°C

This paper reports on equipment for the automatic measurement of the density of binary mixtures as a function of concentration. The equipment is applied to mixtures of 1-propanol and water at 5, 15, and 25°C. An equilibrium mixture model, in which an alcohol-water solution is regarded as an ideal associated mixture, is fitted to the experimental data. Thermodynamic parameters characterizing the association of water to nonpolar alkyl groups (hydrophobic solvation) and to polar hydroxyl groups (hydrophilic solvation) are estimated.

Different Views on the Stability of Protein Conformations and Hydrophobic Effects

Apparently contradictory statements about the thermodynamics of aqueous protein solutions and of hydrophobic effect are quoted and discussed. Some credibility is found in the divergent points of view and it is pointed out that they focus attention on different aspects of the complicated conditions in aqueous solutions, some of which are more important than others for the stability of protein conformations.The importance of characteristics of solvent water is emphasized, in particular (1) the strong mutual cohesion of water molecules, and (2) structural changes of water induced by (nonpolar) solute molecules. It is stressed that consideration of only one of these effects and an inexpedient choice of standard states are origins of confusion in the literature about aqueous systems. A simple approach to hydrophobic effects considering both of the above mentioned effects, is proposed.

Densities and derived volume functions of binary mixtures: (an ethylene glycol derivative + water) at 298.15 K

Abstract Densities at 298.15 K, closely distributed over the entire mole-fraction range, have been measured of { x HOCH 2 (CH 2 OCH 2 ) n − 1 CH 2 OH + (1 − x )H 2 O} and { x CH 3 OCH 2 (CH 2 OCH 2 ) n − 1 CH 2 OCH 3 + (1 − x )H 2 O} for n = 2, 3, and 4. The measurements were made by means of a vibrational-tube densimeter. The mole-fraction dependence of the volume of the mixtures is illustrated by plots of (a), the apparent molar volume of the non-aqueous component and (b), the excess volume divided by the amount of substance of that component, both as functions of x . The mole-fraction dependence of the volume is typical of aqueous mixtures of amphiphilic molecules.

Enzymes immobilized as crystals. Hydrogen isotope exchange of crystalline lysozyme

Immobilization of enzymes by crystallization and subsequent cross-linking provide structures characterized by a regular three-dimensional molecular arrangement and a high packing density. Compared to randomly immobilized enzymes, such structures permit more detailed analysis of the variations in kinetic properties arising from the three-dimensional network or perturbations of the molecular conformations. To obtain information on the effect of crystallization on the dynamic properties of the folded peptide chain in an enzyme molecule, hydrogen exchange rates were measured for both dissolved and crystalline lysozyme over a wide range of pH. Using this method, which reflects molecular oscillations between closely related conformations, no differences were detected between lysozyme in crystalline and dissolved state.

ICE-nucleating activity in the freeze-tolerant tardigrade Adorybiotus coronifer

Abstract 1. 1. The ice-nucleating activity in the cold-hardy tardigrade Adorybiotus coronifer was studied by differential scanning calorimetry. A crystallization temperature of − 6.7°C of intact animals indicates the presence of ice-nucleating agents (INA). Such compounds have not previously been described in this phylum. 2. 2. The ice-nucleating activity in body fluid from A. coronifer is reduced by 50% following ca 7 × 10 3 times dilution. Heating to temperatures above 68°C induces an abrupt decrease in the activity, suggesting that the nucleators are proteinaceous. 3. 3. Gel filtration of homogenized animals indicates that the ice-nucleating activity is confined to molecules with a molecular weight above 200,000.

Heat capacity of aqueous solutions of monohydric alcohols at subzero temperatures

Abstract Differential scanning calorimetry was used to measure the heat capacity of binary aqueous solutions of methanol, ethanol or 1-propanol over the whole composition range, and at temperatures ranging from 20°C down to the freezing temperature of the supercooled mixture (−20 to −35°C). The apparent specific heat capacity of the alcohols was derived from the data. The heat capacity depends strongly on the temperature and composition of the mixtures. At alcohol weight fractions below 0.3, the apparent heat capacity of the alcohol is larger than that of the pure alcohol at 20°C, but at −30°C it is lower. The measurements are discussed with reference to their possible relevance in attempts to understand protein stability at low temperature.

Volumetric properties of aqueous solutions of alkali halides

The density of aqueous solutions of LiF, NaF, or KF, at room temperature, is measured as a function of the concentration. The measurements, together with corresponding data available on other alkali halides, are used to illustrate the concentration dependence of the apparent molar volume of the electrolytes in aqueous solution.The data is in accordance with the conception that an essential, negative contribution to the volume is due to a progressive hydration of the ions upon dilution (i.e. with increasing water concentration).The peculiar behaviour of the Li+-salts, compared with the other alkali halides, is associated with the small size of the Li+-ion compared with a water molecule.

Transition of the mixing scheme in the water-rich region of aqueous 2-butoxyethanol : heat capacities and their temperature derivatives

Abstract Weak jump anomalies were observed in the temperature derivatives of the heat capacities of aqueous 2-butoxyethanol. The derivative, (∂ C p /∂ T ), is proportional to the third derivative of the Gibbs free energy with respect to T . The loci of these anomalies in the composition-temperature field fall on the boundary between two regions previously characterized by differences in the mixing scheme. Other third derivatives of free energy involving at least one composition differentiation showed peak anomalies at the same boundary.