Cis-Trans Isomerization of Proline Dipeptides during Liquid Chromatography: Kinetic Analysis of the Elution Profile

Abstract

A proline dipeptide, Leu-Pro, was eluted on a reversed-phase liquid chromatographic column with UV detection. Since this peptide undergoes cis-trans isomerization, various chromatographic profiles were produced. With the column kept at a low temperature, two peaks were produced, corresponding to the cis- and the trans-form. With elevation of column temperature or with increase of retention time, the peak area of the non-isomerized molecules was decreasing while the area corresponding to the isomerized molecules was increasing in a complementary manner. With further temperature elevation or with further retention time increase, only a broad peak of the isomerized molecules was produced. We demonstrate that the isomerization velocity could be estimated from the chromatograms and that the peak height of the isomerized molecules was proportional to the square root of retention time. Three other X-Pro dipeptides (Ala-Pro, Val- Pro, Phe-Pro) showed similar chromatographic behavior that was characteristic for reversible isomerization between two forms.