Hydrophobicity of amino-acid molecules. Solvation of amino-acid hydrobromides in mixtures of water and NN-dimethylformamide

Abstract

Enthalpies of solution of 8 amino-acid hydrobromides are measured calorimetrically in NN-dimethylformamide + water mixtures at 298.15 K over the whole composition range. A deviation from linear behaviour at high water content for the alkyl-substituted 2-amino acids is considered to result from hydrophobic hydration. This effect becomes more pronounced when it is considered with respect to glycine–HBr. The solvation of the alkyl groups is described by a hydrophobic hydration model involving two adjustable parameters, i.e. the enthalpic effect of the hydrophobic hydration and a measure of the number of water molecules in a clathrate-like structure around an alkyl group. Trends in the enthalpic contributions of the alkyl groups parallel those obtained for other compounds and the commonly used but more arbitrary measures of hydrophobicity based on the transfer properties of amino acids.