Characterization of the hydrophobic properties of amino acids on the basis of their partition and distribution coefficients in the 1-octanol-water system

The hydrophobic properties of amino acid side chains were characterized on the basis of the partition process in the 1-octanol-water system. The partition coefficients were calculated from the published data and the distribution coefficients were determined experimentally on the basis of a double partition process utilizing the fact that the amino acids pass almost completely into the aqueous phase in the partition process. When the volumes of water and 1-octanol are suitably selected, this fact permits avoidance of the difficulties associated with the determination of amino acids in 1-octanol, where their solubilities are very low. Our scale is the only complete experimental scale based on the partition process of amino acids in the 1-octanol-water system. It follows from comparison of the calculated and the experimental data with the values published for the distribution coefficients of N-acetyl amides of amino acids that the best agreement was achieved for hydrophobic amino acids, while greater differences were observed for hydrophilic amino acids. These differences, expressed as the logarithm of the distribution coefficients, correspond to an average of 0.08 for nonpolar amino acids and 0.30 for acidic and basic amino acids: expressed as relative deviations, these values correspond to 2-10% for nonpolar amino acids, and 5-30% for charged amino acids.