Amino acids are the building blocks of proteins. Explain why amino acids can act as both acids and bases in aqueous solution, and state the name of the bond formed when two amino acids join together.

Edexcel A-Level Chemistry (9CH0) — 18.2 Amines, amides and amino acids · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Glycine is a simple amino acid. Its structure contains both an amine group (–NH₂) and a carboxylic acid group (–COOH) on the same molecule.

Model answer (5 marks)

The –COOH group can donate a proton (H⁺), so the amino acid acts as an acid.
The –NH₂ group can accept a proton (H⁺), so the amino acid acts as a base.
When two amino acids join, the carboxyl group of one reacts with the amino group of the other to form a peptide bond.

Examiner tips

  • Use the exact terms ‘donate a proton’ and ‘accept a proton’. Show the two functional groups and their roles. Mention the peptide bond as the linkage between amino acids.

Common mistakes

  • Confusing the amide bond with the peptide bond. Forgetting to state that the –COOH donates a proton. Using vague language like ‘can give up a hydrogen’ instead of ‘donate a proton’.

Mark scheme (5 marks)

  1. The carboxylic acid group (–COOH) can donate a proton / H⁺ ion
  2. This allows the amino acid to act as an acid
  3. The amine group (–NH₂) can accept a proton / H⁺ ion
  4. This allows the amino acid to act as a base
  5. The bond formed when two amino acids join together is called a peptide bond

Key terms in this question

amino acid

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