Study of the cis-trans isomerization of enalapril by reversed-phase liquid chromatography

Enalapril is a dipeptidic angiotensin converting enzyme inhibitor. It exists as a mixture of two conformers in solution with respect to the peptide bond involving the proline amino group. The RPLC of such products may yield peak splitting or multiple peaks as a result of the slow kinetics of the conformation change. In this study, the influence of the flow-rate, pH, temperature, organic modifier and counter ion on the peak shape and the separation of the cis and trans conformers are examined qualitatively by HPLC. It appears that decrease of relaxation time for isomerization with concomitant improvement in peak shape is favoured by a decrease in pH and flow-rate, increase of temperature, choice of organic solvent (nature, amount) and cationic counter ion concentration in the mobile phase. The elution order of the isomers was dependent on the nature of the organic modifier whereas the separation selectivity was improved by an increase of pH or the addition of a negatively charged counter ion. In addition, an NMR investigation on enalapril is described.