Biomolecules

[figure] Which statement is incorrect about peptide bond?

1. C-N bond length in proteins is longer than usual bond length of N-C bond.
2. Spectroscopic analysis shows planar structure of [figure] group.
3. C-N bond length in proteins is smaller than usual bond length of C-N bond.
4. None of the above.

Answer

(1) C-N bond length in proteins is longer than usual bond length of N-C bond.

Solution

In the peptide bond the lone pair on N is delocalised into the C=O group by resonance:

- O \|C- NH- - O |C= NH-

so the C–N bond gets partial double-bond character. Hence the peptide C–N bond (about 132 pm) is shorter than a normal C–N single bond (about 147 pm), which makes (3) correct and (1) incorrect.

Because of this partial double-bond character the –CO–NH– group is planar, as shown by spectroscopic (X-ray) analysis, so (2) is also correct.

Answer: (1)