Question

In x=ray studies of crystalline peptides, Linus Pauling and Robert Corey found that the C-N bond in the peptide link is intermediate in length (1.32 Å) between a typical C-N single bond (1.49 Å) and a C=N double bond (1.27 Å). They also found that the peptide bond is planar (all four atoms attached to the C-N group are located in the same plane) and that the two α-carbon atoms attached to the C-N are always trans to each other (on opposite sides of the peptide bond).

(a) What does the length of the C-N bond in the peptide linkage indicate about its strength and its bond order? (i.e. is it a single, double or triple bond)?

(b) What do the observations of Pauling ad Corey tell us about the ease of rotation about the peptide bond?

Answer 1

Peptide bond is formed by condensation of two amino acids.

CO-NH bond is peptide bond.

The observation has shown that the bond between C and N is neither single nor double but is inbetween the both. It means that CN has partial double bond character. This partial double bond character is shown by resonating structures of peptide bond shown below

Nitrogen has two unshareunshared electrons pair (lone pair) which is in resonance with Oxygen. Oxygen being electronegative prefer negative charge on it. This shows that CN has partial double bond character.

B. Free rotation is possible in carbon carbon single bond very effectively. But in case of double bond rotation is hindered due to pii electron cloud. Here in case of CN bond. Due to the partial double bond character the rotation is restricted.

Hence, from observation we concluded that rotation around CN bond is restricted due to partial double bond character of CN.