Conformational isomers of a Phenylalanine-Proline (Phe-Pro) peptide bond. The displayed structure is 2H50, obtained from the Protein Data Bank. The zoomed isomers are indicative of the differences in the three dimensional structure of a Phe-Pro peptide bond in cis and in trans conformation respectively. In the cis conformation, the alpha carbons are locked on the same side of the peptide bond, whereas in the trans conformation they lie on opposite sides relative to the bond. Moreover in the lower part of the figure, a schematic representation of the energy difference between the two conformations is shown.