Question

Function of planar peptide bond in ternary structure of protein

Answer 1

If the amine and carboxylic acid functional groups in amino acids join together to form amide bonds, a chain of amino acid units, called a peptide, is formed. A simple tetrapeptide structure is shown in the following diagram. By convention, the amino acid component retaining a free amine group is drawn at the left end (the N-terminus) of the peptide chain, and the amino acid retaining a free carboxylic acid is drawn on the right (the C-terminus). As expected, the free amine and carboxylic acid functions on a peptide chain form a zwitterionic structure at their isoelectric pH. 
By clicking the "Grow Peptide" button, an animation showing the assembly of this peptide will be displayed. The "Show Structure" button displays some bond angles and lengths that are characteristic of these compounds.

Answer 2

Interestingly, peptide bonds have a second resonance form, as demonstrated below. This means that the peptide bond (the C=O. and N-H) all reside in a single plane. Thus, there is no rotation around the bond.