ProteinsAQA A-Level Chemistry: Subtopic test

1. 1
   Glycine, H₂NCH₂COOH, and alanine, CH₃CH(NH₂)COOH, can join together to form dipeptides.

   (a)

   Which group is the peptide link?

   [1 mark]

   • A–COO–
   • B–C=N–
   • C–NH–NH–
   • D–CONH–

   (b)

   How many different dipeptides can be made from one molecule of glycine and one molecule of alanine?

   [1 mark]

   • A1
   • B2
   • C3
   • D4

   (c)

   Give the structural formula of the dipeptide in which the –COOH group of glycine has reacted with the –NH₂ group of alanine, and identify the peptide link.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A mixture of amino acids from the hydrolysis of a protein is separated by thin-layer chromatography (TLC). The plate is then sprayed with ninhydrin. The solvent front moved 8.0 cm from the baseline. The centre of spot 1 is 3.6 cm above the baseline and the centre of spot 2 is 5.2 cm above the baseline.

   (a)

   Why is the plate sprayed with ninhydrin?

   [1 mark]

   • ATo make the colourless amino acid spots visible
   • BTo increase the solubility of the amino acids
   • CTo act as the mobile phase
   • DTo hydrolyse the peptide links

   (b)

   What is the Rf value of spot 1?

   [1 mark]

   • A0.55
   • B2.22
   • C0.45
   • D4.4

   (c)

   Calculate the Rf value of spot 2 and explain how it can be used to identify the amino acid.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Keratin, the protein in hair, contains regions that are coiled into an α-helix. Its shape is also held by sulfur–sulfur bonds between cysteine residues.

   (a)

   Describe what is meant by the primary structure and the secondary structure of a protein such as keratin.

   [3 marks]

   (b)

   Explain how the tertiary structure of keratin is maintained.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A chemist is studying a tripeptide made from glycine, H₂NCH₂COOH, alanine, CH₃CH(NH₂)COOH and serine, HOCH₂CH(NH₂)COOH. The amino acids are joined in the order glycine, alanine, serine, with the –COOH group of each reacting with the –NH₂ group of the next.

   (a)

   Give the structural formula of the tripeptide and state how many peptide links and how many molecules of water are involved in its formation. State how the tripeptide is completely hydrolysed and give the equation for the hydrolysis, using the names of the products.

   [6 marks]

   (b)

   The products of hydrolysis are spotted on a TLC plate. The solvent front moves 9.0 cm. The centres of three spots are at 2.7 cm, 3.8 cm and 4.5 cm above the baseline. Under the same conditions the Rf values of glycine, alanine and serine are 0.30, 0.42 and 0.50. Describe how the spots are located, calculate the Rf values and explain what the results show.

   [6 marks]

   Total for question 4: 12 marks

End of questions