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Learn9701Paper 4 · A Level Structured QuestionsNitrogen compoundsAmino acids, peptides and DNA
Nitrogen compounds · Subtopic 2 of 2

Amino Acids, Peptides and DNA

α-Amino acids have NH₂ and COOH on the same carbon. In solids and at their isoelectric point they exist as zwitterions H3N+CHRCOO−\mathrm{H_3N^+CHRCOO^-} (high melting points, soluble in water). In acid they become cations; in alkali, anions. Two amino acids join by condensation, forming a peptide (amide) link –CONH– and water. Proteins are hydrolysed by hot 6 mol dm⁻³ HCl. Amino acids separate by electrophoresis (move by charge at a given pH) or chromatography. DNA: two sugar-phosphate strands held by hydrogen bonds between bases: A–T (2 H-bonds) and C–G (3). Cisplatin binds to guanine N atoms, stopping DNA replication in cancer cells.

Cambridge 9701 Paper 4Edexcel YCH11 WCH15AQA 7405 Paper 2≈ 50 minRevision 1 · updated 2026-11-01
Section 1 of 14

Learning objectives

  • Describe zwitterions and isoelectric points.
  • Describe peptide formation and hydrolysis.
  • Explain electrophoresis.
  • Describe DNA structure and how cisplatin works.
Pause and think

Glycine has a small Mr (75) but melts above 200 °C. Why?

Section 2 of 14

Core idea

Start hereTwenty letters, every protein

Every protein in your body is a chain built from the same 20 amino acids, joined by one kind of bond.

The big idea

Amino acids carry both an acidic and a basic group, so they form zwitterions. Condensation joins them by peptide links; hydrogen bonds hold DNA's two strands together.

1. Charge and pH

Glycine at different pH

Charge changes with pH.

Worked exampleMr of a dipeptide

Glycine (MrM_r 75) + alanine (MrM_r 89). MrM_r of the dipeptide?

Goal
MrM_r.
Strategy
Subtract water.
  1. 75+89=16475+89=164
    Add monomers.
  2. 164−18=146164-18=146
    One H₂O lost per peptide link.
Answer146.
✓ CheckOne link, one water ✓.
Key takeawayBase pairs A–T: two hydrogen bonds. C–G: three hydrogen bonds.

Summary

  • Zwitterions; isoelectric point.
  • Peptide link –CONH–.
  • Electrophoresis by charge.
  • DNA: A–T, C–G.
Key words
Zwitterion
Ion with both + and − charges, overall neutral.
Isoelectric point
pH at which an amino acid has no net charge.
HistoryFranklin's Photo 51. Rosalind Franklin's 1952 X-ray image of DNA showed a helix, giving Watson and Crick the key evidence for their 1953 model.
Fun factTwenty building blocks. Just 20 amino acids combine to make every protein in your body, from collagen in skin to haemoglobin in blood.
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