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Investigation of the Rotational Isomerism of Quinapril and Quinaprilat by UPLC–DAD and Elucidation of the Conformational Equilibrium by NMR

Quinapril and quinaprilat are two angiotensin-converting enzyme inhibitors (ACEIs) characterized by a peak broadening and splitting when they were analyzed by ultra-performance liquid chromatography (UPLC). This phenomenon is due to the existence of the two isomers cis and trans around the amide band. In order to confirm the existence of the two conformers and allow identification of the predominant form, NMR studies of quinapril, involving 1H, 13C, 1H-1HCOSY, Impact-HMBC, JMOD, HSQC, and ROESY have been conducted in this work. The analyses allowed us to identify the predominant form of quinapril; the conformer trans is the predominant form (75 %). In addition, this study highlights the important benefits of UPLC to separate quinapril and quinaprilat isomers due to its high resolving power. The effect of various operating conditions on the retention peak, namely, splitting and band broadening of quinaprilat and quinapril, has been qualitatively examined in this study. Several practical experimental conditions have been tested, allowing both the elution of the two ACEIs as single peaks, while keeping at the same time an acceptable separation. The effect of various factors on the conformational s-cis–s-trans equilibrium of quinapril and quinaprilat, namely, the composition of the mobile phase, column temperature, flow rate, pH, and type and amount of organic modifier was investigated by UPLC–DAD (diode array detector) with a BEH C18 column (100 mm, 2.1 mm internal diameter × 1.7 µm particle diameter). Several deconvolution models were used to model overlapped peaks and to determine resolution. Results obtained showed that a mobile phase consisting of ammonium buffer (10 mM; pH 8) and acetonitrile allows the separation of the quinapril and quinaprilat conformers. Maximum resolution was obtained for a composition of mobile phase (55/45) and (65/35) (ammonium buffer/acetonitrile, v/v) for quinapril and quinaprilat, respectively at 45°C and flow rates of 0.4 and 0.5 mL min–1.