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A2 · Practice questions · Amino acids and peptides

Charge changes with pH.

Six original Cambridge-style questions on amino acids and peptides.

Original questions All questions on this page are original work, written in the Cambridge AS & A Level style. They are not from past papers. They test the same concepts and skills the syllabus rewards.
Keep these straight

Protonate in acid, deprotonate in alkali.

01
Recall
[3 marks]

Draw (as condensed formulae) the forms of alanine, CH3CH(NH2)COOH, present (a) at low pH, (b) at its isoelectric point, (c) at high pH.

  • (a) CH3CH(NH3+)COOH. ✓
  • (b) CH3CH(NH3+)COO− (zwitterion). ✓
  • (c) CH3CH(NH2)COO−. ✓
02
Explain
[2 marks]

Glycine melts with decomposition above 230 °C, but ethanoic acid melts at 17 °C. Explain.

  • Glycine exists as zwitterions, H3N+CH2COO−, held by strong ionic attractions in a lattice. ✓
  • Ethanoic acid molecules are held only by hydrogen bonds and weaker intermolecular forces, which need far less energy to overcome. ✓
03
Recall
[3 marks]

Write the condensed formulae of the two dipeptides formed from glycine and alanine, and name the bond formed.

  • Gly-ala: H2NCH2CONHCH(CH3)COOH. ✓
  • Ala-gly: H2NCH(CH3)CONHCH2COOH. ✓
  • The –CONH– group is the peptide (amide) bond, formed by condensation with loss of H2O. ✓
04
Calculation
[2 marks]

Calculate the Mr of the tripeptide gly-gly-gly (glycine Mr 75.0), and state how many water molecules are needed to hydrolyse it completely.

  • Two peptide bonds form, so two H2O are lost: 3 × 75.0 − 2 × 18.0 = 189.0. ✓
  • Complete hydrolysis breaks 2 peptide bonds, using 2 H2O. ✓
05
Analysis
[3 marks]

A mixture of lysine (isoelectric point 9.7), glycine (6.0) and aspartic acid (2.8) is separated by electrophoresis in a buffer at pH 6.0. Predict and explain where each spot ends up.

  • Lysine: the pH is below its isoelectric point, so it is protonated and positive; it moves to the cathode. ✓
  • Glycine: at its isoelectric point it is a neutral zwitterion and stays on the start line. ✓
  • Aspartic acid: the pH is above its isoelectric point, so it is negative and moves to the anode. ✓
06
Explain
[2 marks]

State the conditions used to hydrolyse a protein to its amino acids, and explain why the products are present as ions.

  • Heat under reflux with 6 mol dm−3 HCl(aq) for several hours (or with NaOH(aq)). ✓
  • In acid the amino acids are protonated (H3N+ groups); in alkali they are present as carboxylate ions (COO−). ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.