Chapter 25.4, Problem 26CC

Interpretation Introduction

Interpretation:

For the dipeptide Phe-Phe, s-cis conformation needs to be drawn and the source of large steric interaction needs to be identified.

Concept introduction:

The peptide bonds are simply amide linkages.  These amide linkages have a resonance structure where the $\text{C-N}$ bond has got some double-bond character.  Due to this there is a restricted rotation in the amide bond as we see in normal amides.  This restricted rotation gives rise to two possible conformations known as s-trans and s-cis.

Please remember that s-trans conformation is more stable than s-cis conformation due to less steric interactions.

To draw and identify: the s-cis conformation for Phe-Phe and to identify the source of steric interactions