Answer
a) The shorter the chemical bonds the stronger and higher order a bond is.
b) There is no rotation around the C-N peptide bond because of the partial double bond character.
Work Step by Step
a) The shorter the chemical bonds the stronger and higher order a bond is.
The C-N bond in a peptide bond is shorter than usual because of its partial double-bond characteristic due to resonance that occurs with amides. The C-N bond length in a peptide is smaller than a single bond and greater than a double bond.
b) There is no rotation around the C-N peptide bond because of the partial double bond character. The two alpha-carbons are always trans to each other because of the steric hinderance of the functional groups.
An exception to this is proline which is found in the cis conformation just as much as the trans conformation. This is because of the cyclic characteristic of proline which is equivalently energetically favorable at both the trans and cis conformation.
