Amino acids and proteinsEdexcel A-Level Chemistry: Subtopic test

1. 1
   Alanine, CH₃CH(NH₂)COOH, is a 2-amino acid. In the solid state and in neutral aqueous solution it exists mainly as a zwitterion.

   (a)

   Which formula represents the zwitterion form of alanine?

   [1 mark]

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

   (b)

   Which statement about 2-amino acids is explained by their zwitterion structure?

   [1 mark]

   • AThey have low melting temperatures, like other molecules of similar size
   • BThey are crystalline solids with high melting temperatures and dissolve in water
   • CThey are insoluble in water but dissolve in hexane
   • DTheir solutions are strongly acidic, with pH below 1

   (c)

   Alanine can act as both an acid and a base. Give the formula of the main species formed when alanine reacts with (i) excess dilute hydrochloric acid and (ii) excess aqueous sodium hydroxide.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   Alanine is a 2-amino acid whose central carbon atom is bonded to four different groups. A solution containing only one enantiomer of alanine rotates the plane of plane-polarised monochromatic light. Glycine, H₂NCH₂COOH, does not rotate the plane of polarised light.

   (a)

   Why does glycine not rotate the plane of plane-polarised light?

   [1 mark]

   • AGlycine has no carboxylic acid group
   • BGlycine is not a 2-amino acid
   • CIts central carbon atom is bonded to two identical hydrogen atoms, so it is not chiral
   • DGlycine is too soluble in water

   (b)

   A solution contains equal amounts of the two enantiomers of alanine. What is observed when plane-polarised light passes through it?

   [1 mark]

   • AThere is no net rotation, because the two rotations cancel
   • BThe plane is rotated clockwise
   • CThe plane is rotated anticlockwise
   • DThe light is absorbed completely

   (c)

   Alanine made by a chemist in the laboratory does not rotate plane-polarised light, but alanine obtained by hydrolysing a natural protein does. Explain this difference.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   Glycine, H₂NCH₂COOH, and alanine, CH₃CH(NH₂)COOH, can join together by condensation polymerisation to form peptides and proteins. A sample of protein is hydrolysed so that the amino acids it contains can be analysed.

   (a)

   Glycine and alanine react to form a dipeptide in which the carboxyl group of glycine has reacted with the amino group of alanine. Name the type of reaction, state the small molecule lost, and give the structural formula of the dipeptide.

   [3 marks]

   (b)

   Describe how the amino acids in the protein can be released and then identified.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A student completely hydrolyses a small peptide. The hydrolysate contains glycine, alanine and valine. These 2-amino acids are colourless, crystalline solids with high melting temperatures, and they dissolve in water.

   (a)

   Explain, in terms of structure and bonding, why 2-amino acids have high melting temperatures and dissolve in water, and why they can act as both acids and bases.

   [6 marks]

   (b)

   The student spots the hydrolysate on a chromatography plate alongside reference amino acids. After the solvent has risen 8.0 cm the plate is dried and sprayed with ninhydrin. Three spots are seen at 2.4 cm, 4.0 cm and 5.6 cm from the baseline. Reference Rf values in the same solvent are: glycine 0.30, alanine 0.50, valine 0.70. Calculate the Rf values, identify the amino acids, explain why ninhydrin is used and why the spots move different distances, and evaluate whether the results show the order of amino acids in the peptide.

   [6 marks]

   Total for question 4: 12 marks

End of questions