Amino acid analysis

What happens to the peptide during amino acid analysis?

The peptide bonds are broken by acid hydrolysis, releasing every amino acid as a free molecule. Recommendations for peptides used in mass spectrometry assays, published in Clinical Chemistry, describe hydrolysis at elevated temperature, typically 110 °C but ranging from 90 to 130 °C, in concentrated acid such as 6 mol/L hydrochloric acid (Hoofnagle and colleagues, Clinical Chemistry, 2016). The free amino acids are then separated and measured, usually by chromatography with a derivatising reagent or by LC-MS, against calibrated amino acid standards.

Because the peptide no longer exists after hydrolysis, no reference standard of the same molecule is needed. The method counts the building blocks and adds them back up, which is why it is used to assign content to a peptide that has no standard.

Which amino acids survive the acid, and why does that matter?

Not all of them come through intact. Hoofnagle and colleagues name alanine, arginine, glycine, histidine, isoleucine, leucine, lysine, phenylalanine, proline and valine as the most stable under hydrolysis, with the caveat that isoleucine, leucine and valine hydrolyse slowly, and list tryptophan, methionine, cysteine, serine and threonine as degraded. Acid also converts asparagine and glutamine to aspartic and glutamic acid, so composition tables, including theirs, report each pair together as Asx and Glx.

So a good AAA report quantifies the peptide from a chosen set of stable residues, which also lets the hydrolysis be optimised for completeness. In Hoofnagle and colleagues' worked example, the molar content of a high-purity peptide agreed with the expected content to within 10%, particularly for alanine, leucine and phenylalanine, residues that are stable and fully released in 24 hours of hydrolysis.

How does AAA give net peptide content?

Net peptide content is the share of a weighed powder that is actually peptide; the rest is water, counter-ions such as acetate or trifluoroacetate, and salts. Hoofnagle and colleagues describe AAA as the typical way purified peptides are analysed for net peptide content, expressed as a percentage of the weighed sample.

Moles of peptide (from the stable amino acids) times the peptide's molecular weight gives the mass of peptide; divided by the mass of powder weighed, that is the content. The lesson on net peptide content and counterions explains why this number sits well below the HPLC purity figure without anything being wrong.

How reproducible is amino acid analysis between laboratories?

The evidence is thin. A multi-laboratory study led by the USP Biologics Department compared HPLC assay, quantitative NMR and AAA for the nonapeptide oxytocin. The HPLC assay against the same bulk material gave the lowest inter-laboratory variability; only four laboratories ran AAA, which the authors said was not enough to draw a conclusion about it. They found accuracy and precision highly dependent on the quantitation method (Li and colleagues, Journal of Pharmaceutical and Biomedical Analysis, 2019). Hoofnagle and colleagues accordingly list the questions to put to a laboratory: hydrolysis reagent, temperature and time, internal standards, and which residues were used.

Where does AAA sit in a full peptide characterisation?

Regulators treat it as a supporting method. ICH Q6B notes that amino acid composition analysis provides useful structural information for peptides and small proteins, though it is less definitive for large proteins, and that quantitative AAA can also be used to determine protein content (ICH Q6B). The European Medicines Agency's draft guideline on synthetic peptides says AAA usually complements the characterisation of synthetic peptides, lists it among typical specification tests, and accepts it as one of the options for identification alongside mass-based methods (EMA draft guideline on synthetic peptides, 2023).

• With HPLC-UV: HPLC gives purity and an amount against a reference standard; AAA gives an amount without one.
• With LC-MS: mass confirms the molecule; composition from AAA is a second, orthogonal identity strand.
• With Karl Fischer and counter-ion tests: together they account for the non-peptide share of the powder.

Does Tested.ph offer amino acid analysis?

Planned, not yet offered. The standard package measures amount by HPLC-UV against a reference standard, performed by the partner laboratory when testing opens. The content calculator shows how net peptide content changes the milligrams of peptide in a vial.