Estimating the hydrophobicity extent of molecular fragments using reversed‐phase liquid chromatography

A fast HPLC method was developed to study the hydrophobicity extent of pharmaceutically relevant molecular fragments. By this strategy, the reduced amount of sample available for physico-chemical evaluations in early-phase drug discovery programs does not represent a limiting factor. The sixteen acid fragments investigated were previously synthesized also determining potentiometrically their experimental log D values. For four fragments it was not possible to determine such property since their values were outside of the instrumental working range (2 < pKa < 12). An RP-HPLC method was therefore optimized. For each scrutinizedmethod, some derived chromatographic indiceswere calculated, and Pearson’s correlation coefficient (r) allowed to select the so-called “phi0 index” as the best correlating with the log D. The swPH was fixed at 3.5 and a modification of some variables [organic modifier (methanol vs. ACN), stationary phase (octyl vs. octadecyl), presence/absence of the additives n-octanol, n-butylamine, and n-octylamine], allowed to select the best correlation conditions, producing a r = 0.94 (p < 0.001). Importantly, the phi0 index enabled the estimation of log D values for four fragments which were unattainable by potentiometric titration. Moreover, a series of molecular descriptors were calculated to identify the chemical characteristics of the fragments explaining the obtained phi0. The number of hydrogen bond donors and the index of cohesive interaction correlated with the experimental data.