SOLUTION NONIDEALITY RELATED TO SOLUTE MOLECULAR CHARACTERISTICS OF AMINO-ACIDS

By measuring the freezing-point depression for dilute, aqueous solutio ns of all water-soluble amino acids, we test the hypothesis that nonid eality in aqueous solutions is due to solute-induced water structuring near hydrophobic surfaces and solute-induced water destructuring in t he dipolar electric fields generated by the solute. Nonideality is exp ressed with a single solute/solvent interaction parameter I, calculate d from experimental measure of Delta T. A related parameter, I-n, give s a method of directly relating solute characteristics to solute-induc ed water structuring or destructuring. I-n-values correlate directly w ith hydrophobic surface area and inversely with dipolar strength. By c omparing the nonideality of amino acids with progressively larger hydr ophobic side chains, structuring is shown to increase with hydrophobic surface area at a rate of one perturbed water molecule per 8.8 square angstroms, implying monolayer coverage. Destructuring is attributed t o dielectric realignment as described by the Debye-Huckel theory, but with a constant separation of charges in the amino-carboxyl dipole. By using dimers and trimers of glycine and alanine, this destructuring i s shown to increase with increasing dipole strength using increased se paration of fixed dipolar charges. The capacity to predict nonideal so lution behavior on the basis of amino acid characteristics will permit prediction of free energy of transfer to water, which may help predic t the energetics of folding and unfolding of proteins based on the cha racteristics of constituent amino acids.