Counter-current chromatographic estimation of hydrophobicity of Z-(cis) and E-(trans) enalapril and kinetics of cis/trans isomerization

The kinetics of Z-(cis)/E-(trans) isomerization of enalapril was investigated by reversed phase high-performance liquid chromatography (RP-HPLC) using a monolith ODS column under a series of different temperature and pH conditions. At a neutral pH 7, the rate (kobs) of Z-(cis)/E-(trans) isomerization of enalapril at 4 °C (9.4 × 10−3 min−1) is much lower than at 23 °C (1.8 × 10−1 min−1), while the fractional concentration of Z-(cis) isomer is always higher than that of E-(trans) isomer in the pH range 2–7. The fractional concentration of the E-(trans) isomer becomes a maximum (about 40%) in the pH range 3–6, where enalapril exists as a zwitterion. The hydrophobicity (log PO/W) of both isomers was estimated by high-speed counter-current chromatography (HSCCC). Normal phase HSCCC separation using a tert-butyl methyl ether–acetonitrile–20 mM potassium phosphate buffer (pH 5) two-phase solvent system (2:2:3, v/v/v) at 4 °C was effective in partially separating the isomers, and the partition coefficient (K) of each isomer was directly calculated from the retention volume (VR). The log PO/W values of Z-(cis) and E-(trans) isomers were −0.46 and −0.65, respectively.