سوالات میان ترم 2018 خارج از کشور

? Why are peptide bonds uncommon or rare in proteins

Two configurations are possible for a planar peptide bond. In the trans configuration, the two a-carbon atoms are on opposite sides of the peptide bond. In the cis configuration, these groups are on the same side of the peptide bond. Almost all peptide bonds in proteins are trans. This preference for trans over cis can be explained by the fact that steric clashes between groups attached to the a-carbon atoms hinder the formation of the cis form but do not arise in the trans configuration (Figure 2.20).