Amino acid analysis (revision 6)

Amino acid analysis hydrolyses a peptide into its constituent amino acids and quantifies each. It yields two things a chromatographic purity determination cannot: the residue composition, which supports identity, and an absolute peptide content, which supports quantity.^[1]

It is one of the few methods that gives content without requiring a reference standard of the peptide itself, because the standards required are the individual amino acids.^[2] This makes it particularly valuable for compounds with no marketed product and therefore no compendial standard.^[3]

The method is destructive, laborious and subject to well-characterised systematic errors, which is why it is used to qualify a standard or to establish content on a lot rather than as a routine release test.^[1]

A weighed sample is hydrolysed, conventionally in 6 M hydrochloric acid at 110 °C for 24 hours under vacuum or inert atmosphere. The hydrolysate is derivatised and the amino acids separated and quantified by chromatography against standards of each.^[1]

The molar ratios obtained are compared with those expected from the sequence. Agreement supports identity, though it does not establish sequence order — a permutation of the same residues gives the same composition, which is why composition is complementary to mass and fragmentation rather than a substitute for them.

Absolute content follows from the total recovered amino acid mass against the sample mass taken. This is the figure that makes the method valuable for quantitation, and it is unaffected by water, counterion or residual solvent, all of which contribute to the weighed mass and none of which yields amino acids.^[3]