Solvation thermodynamics of xenon in n-alkanes, n-alcohols and water

Experimental thermodynamic data for the solvation of xenon in n-alkanes, water and n-alcohols, from different sources, are analysed by means of a general theory of solvation. The standard solvation Gibbs energy change is given by the sum of the work spent to create a cavity suitable to host xenon and the energy gained to turn on xenon-solvent attractive interactions. The latter contribution is larger in magnitude than the former in both n-alkanes and n-alcohols; the reverse holds in water. This finding is due to the fact that liquid water is characterized by the largest work of cavity creation, because of the smallness of its molecules. Since the two contributions to the solvation Gibbs energy change are both larger in magnitude in n-alcohols with respect to n-alkanes of the same number of heavy atoms, the xenon solubility is not so different between the two classes of solvents. The H-bonds play an indirect role, being important to determine the density of the liquid. It is also shown that there is no clear correlation between the cohesive energy density of the liquid and the solubility of xenon in the liquid itself. The present study confirms that the effective molecular size of solvent molecules is the principal factor in controlling the solvation Gibbs energy changes.

Sequential hydration of small protonated peptides

The sequential addition of water molecules to a series of small protonated peptides was studied by equilibrium experiments using electrospray ionization combined with drift cell techniques. The experimental data were compared to theoretical structures of selected hydrated species obtained by molecular mechanics simulations. The sequential water binding energies were measured to be of the order of 7-15 kcal/mol, with the largest values for the first water molecule adding to either a small nonarginine containing peptide (e.g., protonated dialanine) or to a larger peptide in a high charge state (e.g., triply protonated neurotensin). General trends are (a) that the first water molecules are more strongly bound than the following water molecules, (b) that very small peptides (2-3 residues) bind the first few water molecules more strongly than larger peptides, (c) that the first few water molecules bind more strongly to higher charge states than to lower charge states, and (d) that water binds less strongly to a protonated guanidino group (arginine containing peptides) than to a protonated amino group. Experimental differential entropies of hydration were found to be of the order of -20 cal/mol/K although values vary from system to system. At constant experimental conditions the number of water molecules adding to any peptide ion is strongly dependent on the peptide charge state (with higher charge states adding proportionally more water molecules) and only weakly dependent on the choice of peptide. For small peptides molecular mechanics calculations indicate that the first few water molecules add preferentially to the site of protonation until a complete solvation shell is formed around the charge. Subsequent water molecules add either to water molecules of the first solvation shell or add to charge remote functional groups of the peptide. In larger peptides, charge remote sites generally compete more effectively with charge proximate sites even for the first few water molecules.