The effects of micelles of differently charged surfactants on the equilibrium between (Z)- and (E)-diastereomers of five ACE inhibitors in aqueous media

Abstract

The (Z)/(E) equilibria in water solutions of five ACE inhibitors (captopril, enalapril, lisinopril, perindopril, and ramipril) in presence and in absence of surfactants (anionic sodium dodecyl sulfate, cationic cetyltrimethyl ammonium bromide, and non-ionic 4-octylphenol polyethoxylate) were analyzed by reversed-phase high-performance liquid chromatography. The results showed that the (Z)/(E) equilibrium of lisinopril and captopril was the least sensitive to the effect of the examined surfactants. 4-Octylphenol polyethoxylate expressed the weakest effect on isomerization of the ACE inhibitors, while the (Z)/(E) equilibria of enalapril, ramipril, and perindopril were the most sensitive to the presence of sodium dodecyl sulfate and cetyltrimethyl ammonium bromide. In addition, the response of two structurally very close compounds, such as enalapril and ramipril to the presence of sodium dodecyl sulfate and cetyltrimethyl ammonium bromide was opposite, and these two ACE inhibitors had different chromatographic behavior. To provide a better insight into the reasons for the differences in order of elution of (Z)- and (E)-diastereomers and the possible molecular mechanism underlying their retention, (Z)- and (E)-diastereomers of enalapril and ramipril were subjected to a theoretical study optimized at the B3LYP/6-31G (d,p) level of density functional theory. The Connolly solvent-excluded volume was taken as the most significant parameter that can be connected to differences in chromatographic behavior of the (Z)- and (E)-diastereomers of enalapril and ramipril. [GRAPHICS]