AQA A-Level Chemistry Paper 3, 2019: Question 29

📋 What this question tests

This multiple-choice question tests your foundational organic chemistry knowledge regarding:

Recognition of the amide / peptide linkage ( -CONH- ) formed between amino acids.
Distinguishing between different condensation linkages: ester, anhydride, imide, and peptide links.
Understanding protein structure and condensation polymerisation.

Option D shows a carbonyl group directly bonded to an N-H unit:

-C(=O)-NH-

This is the secondary amide functional group, commonly referred to as a peptide link or peptide bond in biochemistry.

Proteins are condensation polymers formed from 2-amino acids.
The carboxylic acid group ( -COOH ) of one amino acid reacts with the amine group ( -NH₂ ) of another.
A molecule of water ( H₂O ) is eliminated:
R-COOH + R'-NH₂ → R-CONH-R' + H₂O
The resulting bond between the carbonyl carbon and the nitrogen atom is the peptide bond.

A: -C(=O)-O-C-
This is an ester link, found in polyesters (such as Terylene) and lipids (triglycerides), not proteins.
B: -C(=O)-NH-C(=O)-
This is an imide linkage (a nitrogen atom bonded to two carbonyl groups). In proteins, the nitrogen atom is bonded to a carbonyl carbon on one side and an α-carbon ( -CHR- ) on the other, not two carbonyls.
C: -C(=O)-O-C(=O)-
This is an acid anhydride linkage. It contains no nitrogen and is not found in proteins.

Look for Nitrogen: Proteins must contain nitrogen. This immediately eliminates options A and C (both contain only oxygen between carbons).
Count the Carbonyls: Each standard amino acid backbone only has one carbonyl group per repeating unit. Therefore, two carbonyl groups joined to a single nitrogen in the backbone (as in B) is chemically incorrect for a protein.
This leaves D as the definitive answer within 10 seconds.

Mark Scheme Reference: Question 29 → D [1 mark].

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