AQA A-Level Chemistry Paper 3, June 2025: Question 29

1 mark · Easy difficulty · Multiple Choice

Identify the structure of the dipeptide formed from serine and alanine.

Practise this question

Question

Question 29 asks: 'What is the structure of a dipeptide formed from the two amino acids shown?' Above the options, two amino acid skeletal structures are displayed: serine (2-amino-3-hydroxypropanoic acid) and alanine (2-aminopropanoic acid). Four multiple-choice options, labeled A to D, depict different dipeptide structures featuring variations in the side chains attached to the peptide backbone.
Question text

29 What is the structure of a dipeptide formed from the two amino acids shown?

[1 mark]

A

B

C

D

Mark scheme

Show the mark scheme Mark scheme for question 29 shows the correct answer as C, worth 1 mark for AO2. Beside it is a diagram of the dipeptide formed.

29 C 1 (AO2)

How to answer it

Dipeptide Formation from Amino Acids

What this question tests

Core Chemistry Concepts:

  • Recognition of α-amino acid backbone structures: H₂N–CH(R)–COOH .
  • Condensation reaction between an amine group ( –NH₂ ) and a carboxylic acid group ( –COOH ) to form a secondary amide (peptide) link ( –CONH– ) with loss of H₂O .
  • Tracking side chains (R-groups) accurately: serine ( R = –CH₂OH ) and alanine ( R = –CH₃ ).
Question 29 • Multiple Choice • 1 Mark

Identifying the Correct Dipeptide Condensation Product

AQA A-Level Chemistry • Organic & Biological Chemistry

📐 Step-by-Step Structural Analysis

  1. Identify Amino Acid 1 (left reactant):
    Formula: HO–CH₂–CH(NH₂)–COOH (Serine / 2-amino-3-hydroxypropanoic acid).
    • Central α-carbon has: –NH₂ , –H , –COOH , and a hydroxymethyl side chain ( –CH₂OH ).
  2. Identify Amino Acid 2 (right reactant):
    Formula: CH₃–CH(NH₂)–COOH (Alanine / 2-aminopropanoic acid).
    • Central α-carbon has: –NH₂ , –H , –COOH , and a methyl side chain ( –CH₃ ).
  3. Form the peptide link between Serine and Alanine:
    React the –COOH of serine with the –NH₂ of alanine:
    HO–CH₂–CH(NH₂)–C(=O)OH + H–NH–CH(CH₃)–COOH → HO–CH₂–CH(NH₂)–C(=O)–NH–CH(CH₃)–COOH + H₂O
  4. Match to multiple choice options:
    Structure C correctly retains the –CH₂OH group on the N-terminal residue and the –CH₃ group on the C-terminal residue.

✅ Correct Answer

C

Option C clearly shows:

  • Left residue: Serine residue, preserving the primary alcohol side-chain carbon ( HO–CH₂–CH(NH₂)– ).
  • Central link: Correct amide / peptide linkage ( –C(=O)–NH– ).
  • Right residue: Alanine residue, preserving the intact methyl group ( –CH(CH₃)–COOH ).

💡 Key Knowledge

  • Condensation Mechanism: An OH group leaves the carboxylic acid and an H atom leaves the amine, releasing a molecule of water ( H₂O ).
  • Peptide Bond Geometry: Always check that the amide bond links carbonyl carbon to nitrogen ( –C(=O)–NH– ), not to an oxygen or side-chain group.
  • Carbon Counting: Never lose an aliphatic carbon (e.g. confusing –CH₂OH with a direct secondary alcohol –CH(OH)– ).

🧠 Exam Technique & Elimination

  • Eliminate A and D immediately: Look at the left side of A and D. The alcohol is drawn directly on the α-carbon as –CH(OH)– instead of –CH₂OH . That removes one carbon from the backbone!
  • Eliminate B: The right-hand residue in B is glycine ( –NH–CH₂–COOH ), meaning the methyl group ( –CH₃ ) of alanine has gone missing.
  • Verification: Only C has the correct formula matching the starting components: 6 carbons in total (3 from serine + 3 from alanine).

❌ Common Traps & Errors

  • Missing the side-chain methylene group: Mistaking the serine side chain ( –CH₂–OH ) for a hydroxyl group attached directly to the main chain ( –CH(OH)– ), leading to options A or D.
  • Ignoring the alanine methyl group: Selecting B because the left side looks familiar, without noticing that alanine's –CH₃ side chain was omitted.
  • Rushing without carbon counts: A fast check of the total carbon count (3 + 3 = 6 carbons) would instantly rule out A (5 carbons) and B (5 carbons).
Mark Scheme Breakdown:
• 1 Mark (AO2): Selecting option C.
Examiner Note: In multiple-choice questions involving peptides, examiners often create distractors simply by removing or shifting a single –CH₂– unit. Always verify the total number of carbon atoms in both residues before finalizing your choice.

Topics

Organic Chemistry · 3.3.13 Amino Acids, Proteins and DNA

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.