AQA A-Level Chemistry Paper 3, June 2025: Question 33

1 mark · Medium difficulty · Multiple Choice

Identify which two labelled R group interactions in a protein's tertiary structure represent an intermolecular force and a covalent bond, respectively.

Practise this question

Question

A diagram shows a folded polypeptide chain represented as a string of connected circles, showing part of the tertiary structure of a protein. Four interactions between amino acid R groups are indicated with dashed lines labelled T, U, V, and W: T is between two Phe residues, U is between Asp and Ser residues, V is between two Cys residues, and W is between Lys and Asp residues. The question asks which two R group interactions are an intermolecular force and a covalent bond, with four multiple-choice options: A (T and U), B (V and W), C (U and V), and D (U and W).
Question text

33 The diagram shows part of the tertiary structure for a protein.

Some of the amino acids in this structure are shown using the first three letters of

their names.

The general formula of an amino acid is RCH(NH2)COOH

Interactions between amino acid R groups are shown at positions T, U, V and W.

Which two R group interactions are a type of intermolecular force and a

covalent bond?

Use the Data Booklet to help you answer this question.

[1 mark]

A T and U

B V and W

C U and V

D U and W

Mark scheme

Show the mark scheme Mark scheme table showing question number 33, correct answer C ('U and V'), 1 mark, assessment objective AO2.

33 C 1 (AO2) U and V

How to answer it

Proteins: Bonding & Tertiary Structure Interactions

📋 What this question tests

This question tests your ability to interpret amino acid structures from the Data Booklet and deduce the precise types of bonding and attractive forces maintaining a protein's tertiary structure. Specifically, you must distinguish between intermolecular forces (van der Waals, hydrogen bonds), ionic bonds (salt bridges), and covalent bonds (disulfide bridges).

Question 33 Analysis

Identifying R Group Interactions in Polypeptide Chains

✅ Correct Answer

C: U and V

  • U is a hydrogen bond (a type of intermolecular force) between the side chains of Aspartate and Serine.
  • V is a disulfide bridge (a covalent bond) between two Cysteine residues.
Mark Scheme: 1 mark (AO2) for selecting option C.

💡 Data Booklet Reference & Breakdown

Looking up the R groups of the given amino acids:

Site Residues Involved R Group Chemistry Interaction Type
T Phe – Phe Non-polar aromatic rings: –CH₂–C₆H₅ Van der Waals forces (IMF)
U Asp – Ser –CH₂COOH with –CH₂OH Hydrogen bonding (IMF)
V Cys – Cys Thiol groups: –CH₂SH oxidized to –S–S– Disulfide bridge (Covalent bond)
W Lys – Asp Basic amine –NH₃⁺ with carboxylate –COO⁻ Ionic bond / Salt bridge

🧠 Exam Technique: Systematic Elimination

  1. Find the covalent bond first: In protein tertiary structures, the only common covalent cross-link is a disulfide bridge (formed by the oxidation of two –SH groups on cysteine). Therefore, the correct pair must contain V.
  2. Eliminate non-V options: This immediately rules out A (T and U) and D (U and W).
  3. Evaluate the remaining options (B vs C):
    • Option B gives V and W. Interaction W is between a protonated amino group (Lysine) and a carboxylate group (Aspartate)—this is an ionic bond, not an intermolecular force.
    • Option C gives U and V. Interaction U involves hydrogen bonding between –COOH (Asp) and –OH (Ser), which is classified as an intermolecular force.

❌ Common Misconceptions & Traps

  • Confusing Ionic Bonds with IMFs: Students often incorrectly label the interaction between Lys ( –NH₃⁺ ) and Asp ( –COO⁻ ) as a dipole-dipole force or hydrogen bond. It is a full electrostatic/ionic attraction (salt bridge).
  • Forgetting Disulfide Bridges are Covalent: Unlike hydrogen bonds and van der Waals forces, a disulfide bond ( –S–S– ) is a strong, permanent covalent bond formed by an oxidation reaction:
    2 R–SH → R–S–S–R + 2H⁺ + 2e⁻
  • Not checking the Data Booklet: Trying to memorize every 3-letter code often leads to guessing. Look up the structures on page 12 of the AQA data booklet to be 100% certain of the functional groups involved.

Topics

Organic Chemistry · Physical Chemistry · 3.3.13 Amino Acids, Proteins and DNA · 3.1.3 Bonding

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.