AQA A-Level Chemistry Paper 3, 2023: Question 3

12 marks · Medium difficulty · Practical Techniques & Data Analysis

Analysis of Endomorphin-2 This question involves completing the structure of endomorphin-2, drawing the skeletal formula of proline, identifying reagents and conditions for peptide hydrolysis, analysing apparatus for thin-layer chromatography (TLC), explaining amino acid separation on a TLC plate, naming a suitable developing agent, and determining the Rf value for tyrosine (Tyr).

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AQA A-Level Chemistry Paper 3, 2023: Question 3
Question text

03 Endomorphin-2 is a peptide with the amino acid sequence shown.

Tyr–Pro–Phe–Phe–NH2

Each amino acid is represented by a three-letter abbreviation.

Tyr = tyrosine Pro = proline Phe = phenylalanine

Figure 2 shows part of the structure of endomorphin-2, showing the Tyr–Pro–Phe–

part of the molecule.

Figure 2

03.1 The –NH2 at the end of the amino acid sequence of endomorphin-2 shows that the

terminal functional group is an amide, not an acid.

Complete the structure of endomorphin-2 in Figure 2.

[2 marks]

03.2 Use the structure in Figure 2 to draw the skeletal formula of proline, Pro.9

[1 mark]

A student hydrolyses a sample of endomorphin-2 to break it down into its constituent

amino acids.

The student analyses the resulting mixture by thin-layer chromatography, TLC.

03.3 State a reagent and the conditions needed for the hydrolysis.

[2 marks]

Reagent

Conditions

03.4 Figure 3 shows the apparatus used for the TLC.

*08* Figure 3

There is a piece of the apparatus missing from Figure 3. This omission will result in

an inaccurate chromatogram.

Identify the missing piece of the apparatus.

State and explain why this piece of the apparatus is needed.

[3 marks]

Missing piece

Explanation

03.5 State why the amino acids separate on the TLC plate.

[1 mark]

When the solvent has risen up the TLC plate, the student removes the plate from the

*09* beaker and sprays it with a developing agent.

Figure 4 shows the result.

Figure 4

03.6 Name a suitable developing agent.

State why the developing agent is needed.

[2 marks]

Name

Why needed

03.7 Determine the Rf value for Tyr.

[1 mark]

Rf

Mark scheme

Show the mark scheme Mark scheme for AQA A-Level Chemistry Paper 3, 2023: Question 3

Question Answers Additional comments/Guidelines Mark

3.1 M1 if Phe drawn with COOH or CONH2

M2 ALLOW if no Phe drawn i.e. if NH2 only

attached directly to C=O on diagram

Alternative form Scores M2 for ending in amide group

Scores M1 for Phe group

M1 Phe structure drawn with correct peptide link M1 H needed on N of peptide link drawn unless

M2 amide group shown on end C−CH−CH2 drawn skeletal

– A-LEVEL CHEMISTRY – –

3.2 ALLOW zwitterion

ALLOW -NH + and/or COO–

ALLOW with C shown in COOH group

ALLOW without H on N

ALLOW N–H

NOT N–

3.3 M1 (aqueous) HCl/hydrochloric acid

Name or formula of any strong acid or alkali

M2 reflux/heat ALLOW warm / hot / high temperature for heat

NOT T>200°C

IGNORE conc as condition with acid/alkali

IGNORE pressure

Alternative

M1 protease/(poly)peptidase/peptase/named

protease

IGNORE enzyme

M2 warm

NOT hot / high temperature / T>50–A-LEVEL CHEMISTRY°C – –

3.4 M1 lid/cover (on beaker) 3

Then any 2 from these 3

• prevents escape of vapour (from beaker) / evaporation of

solvent (from beaker)

• so atmosphere in beaker is saturated with solvent vapour owtte

• to reduce evaporation from the plate ALLOW (for bullet point 3) so solvent can rise up

plate

ALLOW (for bullet point 3) to avoid plate drying

out

3.5 Difference in the balance between solubility in solvent/mobile phase ALLOW difference between (relative) 1

and attraction to/retention on stationary phase affinity/attraction for solvent and stationary phase

ALLOW absorption/adsorption for retention on

stationary phase

3.6 M1 ninhydrin ALLOW iodine 2

IGNORE UV

M2 amino acids are colourless / to make the amino acids visible IGNORE stated final colour e.g. “turns the amino

acids purple” is not enough on its own

IGNORE clear

3.7 0.54 ALLOW 0.53 – 0.55 (to min two sig figs) 1

How to answer it

Step-by-Step Guide: Endomorphin-2 Peptide Question

Question (a): Complete the Structure of Endomorphin-2

Step 1: Add the Correct Functional Group

The terminal functional group is an amide , not an acid.

Answer: Add the correct -CONH2 group to the end of the molecule to complete the peptide link.

Teacher Tip: Ensure you draw the peptide bond accurately, showing the C=O and N-H groups explicitly. Missing bonds can lose marks.

Question (b): Skeletal Formula of Proline

Step 1: Identify the Structure

Proline is a cyclic amino acid with a side chain bonded to the amine group. Convert the structure into a skeletal formula, omitting H atoms on carbons.

Answer: Ensure the cyclic structure is drawn correctly with the NH group and carboxyl group present.

Teacher Tip: Practice converting structures into skeletal form. Pay attention to rings and functional groups.

Question (c): Hydrolysis of Endomorphin-2

Step 1: State the Reagent and Conditions

The hydrolysis of peptides requires:

  • Reagent: A strong acid such as HCl (aqueous).
  • Conditions: Reflux with heat.

Alternative: Use a protease enzyme under warm conditions.

Teacher Tip: Remember that hydrolysis can be done chemically (acid/alkali) or enzymatically. Mention both where appropriate.

Question (d): Missing Apparatus for TLC

Step 1: Identify the Missing Piece

The missing piece is a lid or cover for the beaker.

Step 2: Explain Its Purpose

The lid prevents the evaporation of the solvent and creates a saturated atmosphere, ensuring the solvent rises up the TLC plate evenly.

Teacher Tip: Always include a lid for TLC setups. Without it, the chromatogram will be inaccurate.

Question (e): Separation of Amino Acids on TLC

Step 1: Explain the Separation

The separation occurs due to differences in the amino acids' affinities for the mobile phase (solvent) and stationary phase (silica). Amino acids with higher affinity for the solvent travel further up the plate.

Teacher Tip: Use the terms "mobile phase" and "stationary phase" to explain chromatography.

Question (f): Suitable Developing Agent

Step 1: Name a Developing Agent

The developing agent is ninhydrin .

Step 2: Explain Its Purpose

Ninhydrin reacts with amino acids to form a coloured product, making them visible on the TLC plate.

Teacher Tip: Ninhydrin is a common reagent for amino acid detection. Practice explaining its purpose clearly.

Question (g): Rf Value for Tyr

Step 1: Use the Rf Formula

Rf = Distance moved by the spot ÷ Distance moved by the solvent front.

Answer: For Tyr, Rf = 0.54 (allow 0.53–0.55).

Teacher Tip: Always measure distances carefully from the baseline to the centre of the spot and solvent front.

Topics

Organic Chemistry · 3.3.13 Amino Acids, Proteins and DNA

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 3, 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.