Determination of n-octanol/water partition coefficients of weak ionizable solutes by RP-HPLC with neutral model compounds

The utilization of neutral compounds as model compounds is put forward for determination of the n-octanol/water partition coefficient (Kow) of highly hydrophobic, weak acidic compounds by reversed-phase high performance liquid chromatography (RP-HPLC). It is based on a linear relationship between the logarithm of apparent n-octanol/water partition coefficient (log Kow″), expressing hydrophobicity of acidic solutes more accurately, and the logarithm of RP-HPLC retention factor of the solutes corresponding to the neat aqueous fraction of mobile phase (log kw). The availability of neutral model compounds was theoretically tested for this novel protocol. Moreover, a high consistency of linear log Kow″–log kw correlations was demonstrated between a mixed training set of neutral and acidic model compounds, and a training set of neutral model compounds. It is proved in theory that for a certain set of compounds investigated, all derived linear relationships between log Kow″ and log kw have a unit slope and the same intercept, regardless of mobile phase pH. This model was applied to measure log Kow of lipophilic aristolochic acid I (AA I) and aristolochic acid II (AA II). Log Kow values for AA I and AA II are 4.45±0.07 and 3.99±0.06, respectively. To the best of our knowledge, this is the first report on experimental log Kow data for AAs. The proposed strategy solves the problem of lacking suitable acidic model compounds with reliable experimental Kow in determining Kow of lipophilic acidic solutes by RP-HPLC.