Hydrophobicity parameters determined by reversed-phase liquid chromatography .12. Comparison of capacity factors and octane/methanol-water partition coefficients for monosubstituted pyrazines, and effect of octanol added to both partitioning systems

Partition coefficients (P) of pyrazine and its COOMe derivative (strong hydrogen bond acceptor), were measured for the octane/aqueous methanol partitioning system (PO/M-W) at different methanol (MeOH) concentrations and the dependence of log PO/M-W on the methanol concentration was compared to the corresponding change in log k' (k': capacity factor) obtained by reversed-phase liquid chromatography with aqueous methanol, Next, in order to make the chromatographic system approximate more closely to the octanol/water partitioning system, a small quantity of octanol was added to the eluents and also to the octane/aqueous methanol partitioning system and further comparisons were made, It was found that the octanol effect was minimum at 50% MeOH, suggesting that the chromatographic system with eluents containing about 50% MeOH has properties more similar to the octanol/water partitioning system than with eluents of other compositions. This confirms our previous result that the log k' parameter obtained at 50% MeOH yields a better correlation with log P-oct (P-oct: octanol/water partition coefficient) than those obtained at other mobile phase compositions in the range from 5 to 70% MeOH.