Cis−trans isomerization of proline-like oxazolidines and thiazolidines, denoted pseudo-prolines, is investigated spectrophotometrically with a chymotrypsin coupled assay and by 1H NMR. A series of peptides carrying substituents of varying stereochemistry at the 2-C position of ΨPro was prepared for evaluating kinetic and thermodynamic data pertaining to the isomerization of the imidic bond. ΨPro are shown to exhibit an enhanced proline effect, allowing the cis content along the imide bond to be tailored between 5 and about 100%. These Pro surrogates may serve as β-turn type VI mimetics and can be used to introduce specifically cis-imide bonds into peptides and proteins. ΨPro derived from Ser and Thr show a marked difference in the rate of isomerization about the imide bond compared to Cys derived thiazolidines. As with Pro itself, the cis content for the (S) epimers of oxazolidines and thiazolidines can be raised from about 40−60% using LiCl/TFE as a solvent, allowing more accurate measurements of the iso...