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9476 / 2027
Organic Chemistry

Lesson 32 of 34

Proteins are condensation polymers of alpha-amino acids

A peptide bond is an amide bond between the carbonyl carbon of one amino acid and nitrogen of another.

A-Level 9476 (2026-2027)

  • I can describe protein formation by condensation.Syllabus

    Syllabus 9476, 11.10(c). Describe protein formation by condensation.

  • I can describe protein hydrolysis.Syllabus

    Syllabus 9476, 11.10(d). Describe protein hydrolysis.

Proteins are condensation polymers of alpha-amino acids

A peptide bond is an amide bond between the carbonyl carbon of one amino acid and nitrogen of another.

An α-amino acid has its amino group on the carbon next to the COOH group: H2NCH(R)COOH. Condensation joins these monomers through -C(=O)-NH- peptide bonds. The backbone repeats -NH-CH(R)-CO- units. The R groups vary along a protein, but the peptide backbone stays the same.

The peptide bond connects carbonyl carbon to nitrogen

Two amino-acid residues are joined as H2N-CH(R)-C(=O)-NH-CH(R-prime)-COOH. The C-N bond between the central carbonyl carbon and NH is highlighted as the peptide bond.

Formation removes OH from a carboxyl group and H from an amino group as water. Sequence matters: exchanging the two residues gives a different dipeptide when R and R' differ.

To hydrolyse the peptide links in a protein, heat it with aqueous acid or aqueous alkali. In acid, the amino groups of the amino acids are protonated. In alkali, the carboxylic acid groups become carboxylate ions. For a simple amino acid with no extra acidic or basic side group, you get H3N+CH(R)COOH in acid and H2NCH(R)COO- in alkali.

Worked exampleCount hydrolysed links rather than residues

A linear peptide contains four amino-acid residues and no crosslinks. How many peptide bonds must be hydrolysed to separate all residues?

Try it first. Then show the worked stepsHide the worked steps
  1. Four residues in one linear chain require three connecting links.
  2. Each peptide bond cleavage uses the components of one water molecule in the hydrolysis balance.
  3. The protonation state of the separated amino acids then depends on the medium.
Answer

Three peptide bonds, hence three hydrolysis events. A chain of n residues has n - 1 links when it is linear and unbranched.

Check your understanding

A linear peptide of five amino-acid residues is fully hydrolysed with hot aqueous acid. How many peptide bonds break, and in what form are the amino groups at the end?

  1. Five bonds break, and the amino groups end up as -NH2
  2. Four bonds break, and the amino groups end up as -NH3+
  3. Four bonds break, and the amino groups end up as -NH2
Show the answer

Four bonds break, and the amino groups end up as -NH3+

A linear chain of n residues has n - 1 peptide links, so four. In acid, the amino groups are protonated.