Amino acids and proteinsEdexcel International A Level Chemistry: Subtopic test

1. 1
   Alanine, CH₃CH(NH₂)COOH, is one of the amino acids found in proteins. A student dissolves alanine in water and then changes the pH of the solution by adding acid or alkali.

   (a)

   Which species is the main form of alanine in a neutral aqueous solution?

   [1 mark]

   • ACH₃CH(NH₂)COOH
   • BCH₃CH(NH₃⁺)COOH
   • CCH₃CH(NH₃⁺)COO⁻
   • DCH₃CH(NH₂)COO⁻

   (b)

   Excess dilute hydrochloric acid is added to the alanine solution. Which species is now the main form of alanine?

   [1 mark]

   • ACH₃CH(NH₂)COO⁻
   • BCH₃CH(NH₃⁺)COOH
   • CCH₃CH(NH₃⁺)COO⁻
   • DCH₃CH(NH₂)COOH

   (c)

   Alanine (Mr = 89) is a solid with a high melting temperature, whereas butanoic acid (Mr = 88) is a liquid at room temperature. Suggest why.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Most amino acids found in proteins are chiral. A student has a solution of one pure enantiomer of alanine, CH₃CH(NH₂)COOH, and a solution of glycine, H₂NCH₂COOH, of the same concentration. Each is placed in turn in a polarimeter using monochromatic light.

   (a)

   Why does the glycine solution have no effect on plane-polarised light?

   [1 mark]

   • AGlycine has no amine group
   • BGlycine exists as a racemic mixture
   • CGlycine has a zwitterion form
   • DThe central carbon atom has two hydrogen atoms attached, so it is not chiral

   (b)

   The student mixes equal amounts of both enantiomers of alanine in solution (a racemic mixture). What is the effect on the plane of polarisation of monochromatic light?

   [1 mark]

   • ANo overall rotation
   • BRotation twice as large as for one enantiomer
   • CRotation half as large as for one enantiomer
   • DRotation in a direction that depends on the volume used

   (c)

   Explain why alanine shows optical isomerism.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Glycine, H₂NCH₂COOH, and alanine, CH₃CH(NH₂)COOH, can react together to form peptides.

   (a)

   Give the structural formula of the dipeptide formed when the carboxylic acid group of glycine reacts with the amine group of alanine. Name the type of reaction and the small molecule released.

   [3 marks]

   (b)

   A mixture of glycine and alanine can form four different dipeptides. Name or write them, explain why two of them are different from each other although they contain the same amino acids, and explain why a dipeptide can be extended into a polypeptide.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A technician is given three unlabelled white solids: sodium carbonate, ammonium chloride and glycine. All three dissolve in water, and the technician must identify each using chemical tests.

   (a)

   Describe chemical tests that would allow sodium carbonate and ammonium chloride to be identified, and explain how glycine would be identified.

   [6 marks]

   (b)

   Explain how glycine behaves as both an acid and a base, using equations, and why a solution of glycine does not rotate plane-polarised light.

   [6 marks]

   Total for question 4: 12 marks

End of questions