Kaplan MCAT Sample Question

Organic Chemistry

All of the following statements concerning peptide bonds are true EXCEPT:

  1. their formation involves a reaction between an amine group and a carboxyl group.
  2. they are the primary bonds found in proteins.
  3. they have partial double-bond character.
  4. their formation involves hydration reactions.

The answer is:

D. their formation involves hydration reactions.

Why?
Formation of a peptide bond, which is the primary covalent bond found in proteins (B), involves a condensation reaction between the amine group of one amino acid and the carboxyl group of an adjacent amino acid (A). As a result of the carbonyl group present at the bond, the double bond resonates between C=O and C=N. This resonance gives the peptide bond a partial double-bond character (C) and limits the rotation about the bond. We can see that choices (A), (B), and (C) are all characteristics of the peptide bond. Choice (D) is false because the formation of the peptide bond is a condensation reaction (involving the loss of water) rather than a hydration reaction (which involves the addition of water).

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