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[figure] Which statement is incorrect about peptide bond?
- C-N bond length in proteins is longer than usual bond length of N-C bond.
- Spectroscopic analysis shows planar structure of [figure] group.
- C-N bond length in proteins is smaller than usual bond length of C-N bond.
- 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 (resonance):
- O \|C- NH- - O |C= NH-
The C–N bond gets partial double-bond character, so the peptide C–N bond (about 132 pm) is shorter than a normal C–N single bond (about 147 pm). Hence (3) is correct and (1) is incorrect.
Because of this partial double-bond character, the –CO–NH– group is planar, as shown by spectroscopic (X-ray) analysis. Hence (2) is correct.
Answer: (1)
AIPMT 2001