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Organic Chemistry

Topic 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)

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 COOH: H2NCH(R)COOH. Condensation links these monomers through -C(=O)-NH- peptide bonds, giving a backbone of repeating -NH-CH(R)-CO- units. The R groups vary along a protein and do not replace the peptide backbone.

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.

Heat proteins with aqueous acid or aqueous alkali to hydrolyse peptide links. Acidic hydrolysis produces amino-acid forms with protonated amino groups; alkaline hydrolysis produces carboxylate forms. For a simple amino-acid residue without extra ionisable side chains, these are H3N+CH(R)COOH in sufficiently acidic solution and H2NCH(R)COO- in sufficiently alkaline solution.

Worked example

Count 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?

  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.