Which of the following statements about the peptide bond is correct?

Options

A. The `-CO-NH-` group lies in `-CO-NH-` has a partial double character and is shorter than usual carbon-nitrogen bonds
B. The carbon-nitrogen bond in -`CO-NH-` has a partial double bond character and is shorter than usual carbon-nitrogen bonds.
C. The rotation about `C-N` bond of peptide is somewhat restricted,
D. All are correct

Answer

D

Explanation

The peptide bond shows resonance as shown below

Due to the partial double bond character of the `C - N` bond in peptide linkage, the amide part (i.e., `(- CO - NH-`) is planar and rigid, thus no free rotation about this bond is possible.However, the free rotation of a peptide chain can only occur around the bond, joining the nearly planar amide groups to the `alpha`-carbons:

The angles shown as `Phi` and `Psi` in the figure are known as Ramachandran angles after the name of the Indian Biophysicist, `G.N.A` Ramachandran. Note that `C = O` and the `-NH` groups of the peptide bond are trans to each other.