Enthalpic Pairwise Interaction Coefficients Research Articles

Excess enthalpies of N-acetylglycineamide and N-acetyl- l-leucineamide in concentrated aqueous solutions of tetramethylurea

A comparison has been made among the values of the enthalpic pairwise interaction coefficients of two N-acetylamides of the amino acids glycine (NAGA) and leucine (NALA) in concentrated aqueous solutions of tetramethylurea (TMU) and urea, in pure water, and in pure liquid amides. The second virial coefficients of the excess enthalpies are found to be negative for both the model peptidic molecules in 4M TMU, as in liquid amides, dimethylformamide (DMF) and fused N-methylacetamide (NMA), substances assumed to mimic the core of globular proteins. This is the reverse of what was found for concentrated aqueous solutions of urea (U), where all the enthalpic second virial coefficients were positive. This suggests a completely different mechanism for the denaturation of protein in the presence of urea and TMU, due to different protein-denaturant interactions.

Enthalpies of interaction of N-acetyl amides of sarcosine and N-methyl-L-alanine dissolved in N,N-dimethylformamide at 25�C

Enthalpies of dilution of N-acetyl amides of sarcosine and N-methyl-L-alanine dissolved in N,N-dimethylformamide have been measured calorimetrically at 25°C. The enthalpic pairwise interaction coefficients calculated there from are negative, indicating a energetically favorable interaction. The results were used to make a comparison with other peptides with regard to the methylation of amide groups. Substituting a primary amide hydrogen by a methyl group gives a smaller positive change of the pairwise interaction coefficient than substituting a secondary amide hydrogen.

Thermodynamic behaviour of some uncharged organic molecules in concentrated aqueous urea solutions and other polar solvents

A comparison has been made between the values of the enthalpic pairwise interaction coefficients of several organic molecules (peptides, amides and alcohols) in water, in concentrated aqueous solutions of urea, in N, N-dimethylformamide and in liquid N-methyl-acetamide. The second virial coefficients of the excess enthalpies are found to be positive for all the systems studied in water-urea mixtures. A preliminary analysis, carried out using the Savage and Wood group additivity approach, suggests that, in concentrated aqueous solutions of urea, this arises from the peptide-peptide or hydroxyl-hydroxyl, and the apolar-apolar group contributions, all being positive, and these overwhelm the negative contributions from the polar-apolar group interactions. A remarkable feature is the inversion of the signs of the peptide-peptide, hydroxyl-hydroxyl, peptide-methylene and hydroxyl-methylene, group interactions when compared with those which prevail in pure water. This suggests a completely different solvation and interaction mechanism in concentrated urea solutions. The enthalpic contributions from apolar-apolar group interactions in the mixed solvent, in turn, are higher than the values found in water. Some comments are made on the behaviour of some of the above solutes, in the liquid amide solvents N, N-dimethylformamide and N-methylacetamide.

Enthalpies of interaction of some N-acetyl amino acid amides in aqueous urea solutions at 298.15 K

Enthalpies of dilution of the N-acetyl amino acid amides of glycine, L-alanine, L-valine, L-leucine, L-proline, L-phenylalanine, sarcosine and N-methyl-L-alanine, and the N-acetyl-N′-methyl amino acid amides of glycine, L-alanine, L-leucine, L-proline, sarcosine, and N-methyl-L-alanine dissolved in aqueous 8 mol dm–3 urea have been measured calorimetrically at 298.15 K.The results were used to calculate enthalpic interaction coefficients employing a concentration expansion of the enthalpies of dilution. The enthalpic pairwise interaction coefficients obtained are more positive in 8 mol dm–3 urea than in water in most cases. It is proposed that aqueous urea influences the interactions between peptides in two ways. First, urea solvates the amide groups of the peptides and, secondly, urea reduces the hydrophobic interaction of the apolar side chains of the peptides. The consequences for the ability of aqueous urea to denature globular proteins are discussed.

The enthalpy of dilution of aqueous cellobiose solutions at 298.15 K

Enthalpy of dilution data for aqueous solutions of cellobiose in water at 25°C are reported and the results obtained are compared with some recent free energy data (T.M. Herrington et al., J. Chem. Soc., Faraday Trans. 1, 79 (1983) 845). The free energetic and enthalpic pairwise interaction coefficients obtained from the experimental results have been transposed to the McMillan—Mayer (MM) state to reflect better molecular events. Application of a simplified model for the potential of mean force between solute species leads to the conclusion that there are solute—solute attractions occurring in solution but that these are weak and sensitive to temperature.