pH dependence of the hydrophobicity of β-blocker amine compounds measured by counter-current chromatography

The octanol–water partition coefficients (Poct) of 17 antiadrenergic β-blocker compounds were determined by counter-current chromatography (CCC). Since CCC uses a biphasic liquid system, the octanol–water liquid system was used with essentially an octanol stationary phase and aqueous buffer mobile phase. The Poct coefficients were obtained directly without any extrapolation. The measured Poct values were in the 0.0015–4070 range (−2.8<log Poct<3.6). Since the β-blocking agents are ionizable compounds, the Poct values obtained were strongly dependent on the aqueous-phase pH. The apparent Poct coefficients of the β-blockers were determined at three different pH values (∼3, 7 and 11) using 0.01 M ammonium phosphate buffers saturated with octanol. A model allowed us to obtain the molecular and ionic Poct value using the solute pKa with these three experimental octanol–water coefficients. Often, the Poct coefficients of the molecular forms obtained with the CCC method differ significantly from computed literature values and/or experimental values obtained by extrapolation. Relationships between biological properties and hydrophobicity were also examined.