Chapter 25.4, Problem 25CC

Interpretation Introduction

Interpretation:

For the dipeptide Phe-Leu, s-trans conformation needs to be drawn and both N-terminus and C-terminus need 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-trans conformation for Phe-Leu and identify the N-terminus and C-terminus.