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 (stron g hydrogen bond acceptor), were measured for the octane/aqueous methan ol partitioning system (PO/M-W) at different methanol (MeOH) concentra tions and the dependence of log PO/M-W on the methanol concentration w as compared to the corresponding change in log k' (k': capacity factor ) obtained by reversed-phase liquid chromatography with aqueous methan ol, 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 metha nol partitioning system and further comparisons were made, It was foun d that the octanol effect was minimum at 50% MeOH, suggesting that the chromatographic system with eluents containing about 50% MeOH has pro perties more similar to the octanol/water partitioning system than wit h eluents of other compositions. This confirms our previous result tha t the log k' parameter obtained at 50% MeOH yields a better correlatio n with log P-oct (P-oct: octanol/water partition coefficient) than tho se obtained at other mobile phase compositions in the range from 5 to 70% MeOH.