Cis-trans isomerization and puckering of pseudoproline dipeptides

We report here the results on N-acetyl-N'-methylamides of oxazolidine and thiazolidine (Ac-Oxa-NHMe and Ac-Thz-NHMe) calculated using the ab initio and density functional computations with the reaction field theory at the HF, MP2, and B3LYP levels of theory with the 6-31+G(d) basis set. The displacement of the gamma-CH2 group in proline ring by oxygen and sulfur atoms has affected the structure of proline, cis-trans equilibrium, and rotational barrier. The up-puckered structure is found to be prevalent for the trans conformers of pseudoproline amides. The higher cis populations of pseudoproline amides can be interpreted due to the longer distance between the acetyl methyl group and the 5-methylene group of the ring for the trans conformer of pseudoproline amides than that of the proline amide. The changes in charge of the prolyl nitrogen and the decrease in electron overlap of the C-N bond for transition state (TS) structures seem to play a role in lowering rotational barriers of Oxa and Thz amides compared to that of the proline (Pro) an-tide. The calculated preferences for cis conformers in the order of Oxa > Thz > Pro amides and for trans-to-cis rotational barriers in the order of Pro > Oxa > Thz amides in water are consistent with experimental results on pseudoproline-containing peptides. The pertinent distance between the prolyl nitrogen and the N-H amide group to form a hydrogen bond might indicate that this intramolecular hydrogen bond could contribute in stabilizing the TS Structures of pseudoproline and proline amides and play a role in prolyl isomerization.