Cis-trans isomerization of an X-Pro peptide bond, with its two transition states.

A peptide bond is an amide linkage. One very important fact about peptide bonds is that the bond obtains partial double bond character from resonance with the carbonyl group. The lack of free rotation around peptide bonds limits the ways that the polypeptide chain can fold. The partial double bond character makes the amide group planar and means that peptide bonds could technically either be cis and trans, but synthesis is stereospecific for trans except in the case of peptide bonds with proline, where the isomerism varies at a 50:50 ratio.