Evaluation of Liposome−Water Partitioning of Organic Acids and Bases. 2. Comparison of Experimental Determination Methods

The experimental determination of liposome−water partition coefficients, K lipw, of the neutral and charged species of ionogenic compounds is rather tedious because apparent distribution ratios have to be measured as a function of pH and ionic strength. The partition coefficients of the single species have to be extrapolated from the experimental data using an appropriate partition model. A number of new assay methods have been developed in the past years to facilitate and speed up the experimental procedure for the determination of K lipw . Here we compare three proposed assay methods with the well-established equilibrium dialysis method, and we evaluate their applicability for ionogenic compounds. The potentiometric method was initially developed for the determination of octanol−water partitioning but was recently extended to liposome−water partitioning. The potentiometric method described here was found to yield satisfactory estimates of the K lipw -values of both neutral and charged species, although manifestation of electrostatic saturation phenomena cannot be fully ruled out due to the relatively high concentrations of compounds that have to be used. Two further methods based on immobilization of lipid material on a solid support, which were initially developed for the estimation of K lipw of neutral compounds, were tested here for their applicability to charged organic species. IAM (immobilized artificial membrane) chromatography was found not to be suitable for prediction of membrane−water partitioning of ionic compounds because the charges on the chromatographic support material are not sufficiently shielded. A model including free silica and free amine sites explained the experimental data very well. A new type of immobilized liposomes in the form of noncovalently coated large porous particles (TRANSIL) gave promising results with K lipw -values that agree well with those determined with the traditional equilibrium dialysis method. The advent of these alternative methods for the determination of K lipw of ionogenic compounds will facilitate the application of K lipw as a descriptor for biological systems in quantitative structure activity relationships in environmental chemistry and toxicology.