Comparison of conventional partitioning systems used for studying the hydrophobicity of polar organic compounds

An analysis of published partition coefficients measured in various two-phase systems for homologous series of fatty acids, aliphatic alcohols and amines was performed in order to clarify the physical meaning of the constants in the so-called solvent regression equation: ln K"i = a"i ln K&...
Ausführliche Beschreibung

An analysis of published partition coefficients measured in various two-phase systems for homologous series of fatty acids, aliphatic alcohols and amines was performed in order to clarify the physical meaning of the constants in the so-called solvent regression equation: ln K"i = a"i ln K"0 + b"i, where K"i and K"0 are the partition coefficients for a given solute determined in the partitioning system in question and in that chosen as the reference, respectively, and a"i and b"i are constants. The difference in the relative hydrophobicities of two phases of a given partitioning system is expressed in terms of the free energy of transfer of a CH"2 group from the non-aqueous to the aqueous phase of the system (Δg"i^C^H^2). It is shown that a"i is related to the Δg"i^C^H^2 and Δg"0^C^H^2 values by the equation a"i = Δg"i^C^H^2/Δg"0^C^H^2. The term b"i appears to account for the specific solute-water (and/or solute-solvent) interactions characteristic of a polar group contained by the solute molecule.The analysis of literature data indicates that the difference in the relative hydrophobicities between any two groups depends on the choice of partitioning system. The physical meaning of the relative hydrophobicity of a chemical compound is discussed, and it is concluded that an aqueous two-phase system should be used as the reference. The aqueous two-phase polymeric system Ficoll-dextran is proposed for this purpose and its advantages are briefly considered. Ausführliche Beschreibung