Amino Acid Analysis: How a Peptide’s Content and Composition Are Measured

What amino acid analysis measures

In the harmonized pharmacopoeial chapter’s words, the technique “can be used to quantify protein and peptides, to determine the identity of proteins or peptides based on their amino acid composition”, and also to support structural work, plan peptide mapping and detect atypical amino acids.

One analysis yields two numbers. Composition is the ratio of the amino acids to one another: a check that the building blocks match the sequence. Content is the absolute amount of peptide in the portion that was weighed, usually expressed as a mass fraction of the powder. Neither is HPLC purity, which compares detected peaks with each other, and neither is an identity test in the mass-spectrometry sense.

The chapter is a pharmaceutical text, harmonized through the Pharmacopoeial Discussion Group and signed off by the European, Japanese and United States pharmacopoeias. We use it as a detailed public description of the method, not as a claim about which rules apply to a laboratory reagent.

Step 1: hydrolysis, and the residues it damages

Counting amino acids means breaking every peptide bond first. The chapter calls acid hydrolysis “the most common method”, and its standard version uses 6 N hydrochloric acid with 0.1% to 1.0% phenol, which “prevents the halogenation of tyrosine”. The dried sample is sealed under vacuum or inert gas and held at 110 °C for 24 hours; a vapour-phase variant suits very small samples and reduces contamination from the acid. A paper in the British Journal of Nutrition on standardized methods describes the same conditions and flags tryptophan and the sulphur amino acids as unstable under them.

The chapter is blunt about the damage: “Tryptophan is destroyed”; serine and threonine are partially destroyed; methionine may oxidize; cysteine comes back as cystine, and poorly. Bonds joining isoleucine and valine to each other are only partially cleaved, and asparagine and glutamine are deamidated to aspartic and glutamic acid. That is why those pairs are reported together (Asx and Glx), and why the loss of tryptophan, asparagine and glutamine “limits quantitation to 17 amino acids”.

Gradual losses are handled with a time course, hydrolyzing portions for 24, 48 and 72 hours. Serine and threonine are extrapolated back to zero time; isoleucine and valine climb to a plateau, which is taken as the true value. Residues that standard hydrolysis cannot recover need the alternative chemistries in the table.

The sequence therefore decides how much of a molecule can be counted reliably. Angiotensin II (DRVYIHPF), the peptide behind a Canadian reference material discussed below, contains three residues the chapter lists as typically well recovered: aspartic acid, arginine and phenylalanine. A sequence rich in tryptophan, methionine or cysteine leaves fewer.

Step 2: making amino acids visible to a detector

The freed amino acids are separated by chromatography and made detectable with a reagent. About half of the established techniques separate them by ion-exchange chromatography and derivatize after the column, with ninhydrin or o-phthalaldehyde (OPA); these tolerate small amounts of salts and urea. The rest derivatize first and separate by reversed-phase HPLC: very sensitive, but open to interference from buffer salts and sometimes yielding several derivatives of one amino acid.

The reagent decides what can be seen. With ninhydrin, amino acids “give a purple color” read at 570 nm, while imino acids such as proline give a yellow colour read at 440 nm. OPA reacts with primary amines; the post-column version adds hypochlorite to catch proline, but the chapter states that the pre-column method “does not detect amino acids that exist as secondary amines”. FMOC-Cl and NBD-F react with primary and secondary amino acids alike, DABS-Cl quantifies proline at the same sensitivity as the rest, and PITC and AQC complete the chapter’s list of pre-column reagents. For a proline-rich sequence, the chemistry is not a detail.

Calibration ties peak areas to nanomoles. Standard mixtures give each amino acid a response factor, and the chapter recommends an internal standard such as norleucine to “monitor physical and chemical losses and variations”. A protein or peptide standard analyzed alongside the samples checks the whole chain, hydrolysis included.

Step 3: from amino acid amounts to peptide content

For a known sequence, the residues recovered most reliably are taken to represent the molecule: typically aspartate-asparagine, glutamate-glutamine, alanine, leucine, phenylalanine, lysine and arginine, a list laboratories adjust from experience. The nanomoles of each are divided by the number of times that residue occurs, giving independent estimates of the peptide hydrolyzed. “Typically greater than 5% variation from the mean is considered unacceptable”, the chapter notes, so outliers are dropped and the rest averaged.

Converting nanomoles into mass needs the molecular weight, which our guide to calculating a peptide’s molecular weight works through; dividing by the mass weighed then gives content. A hypothetical illustration, not a real result: the weighed portion of an angiotensin II powder would hold 95.6 nmol if it were pure peptide (its mass divided by 1,046.2 g/mol). Aspartic acid comes back at 62.1 nmol, arginine at 61.4 and phenylalanine at 56.8, each present once in the sequence. Phenylalanine sits 5.5% below the mean of 60.1 and is dropped; the other two average 61.75 nmol; and 61.75 ÷ 95.6 gives a content of 64.6%.

The same data give the composition; the chapter says agreement with the expected composition “can be used to corroborate the identity and purity” of a sample. Two cautions explain that careful verb. Related impurities, such as sequences missing a residue, hydrolyze to the same amino acids, so an uncorrected figure measures peptide-derived material rather than the intended peptide alone. Metrology laboratories therefore use peptide impurity corrected amino acid analysis (PICAA), which the BIPM’s CCQM-K115 comparison describes as requiring “correction for amino acids originating from impurities”. And hydrolysis is rarely perfect: efficiencies in that comparison were “very high (>95 %)”, yet every PICAA participant applied an efficiency correction.

A Canadian reference material shows where that rigour leads. The National Research Council Canada’s ANGII-1 certifies angiotensin II at 691 ± 9 milligrams per gram (k = 2) and trifluoroacetic acid at 249 milligrams per gram, with water as an information value of 34 milligrams per gram. NRC scientists assigned the peptide value from isotope-dilution LC-MS/MS amino acid analysis and proton qNMR, each corrected for related impurities, together with a mass-balance estimate. Even a carefully characterized peptide powder can be little more than two-thirds peptide; our explainer on net peptide content and counter-ions covers what makes up the rest.

Amino acid analysis compared with CHN, qNMR and UV

Metrology institutes deliberately compare several routes to peptide content determination (their reports call it “purity”). Using angiotensin I as a model, Josephs and colleagues set full mass balance, PICAA, elemental analysis and qNMR side by side for one calibrator material. Agreement between independent methods is persuasive; disagreement is informative.

Elemental analysis needs no hydrolysis, but it counts nitrogen from every source, so CCQM-K115 pairs it with “a correction for nitrogen originating from impurities”. In qNMR, the integral ratio “accurately reflects the molar ratio of the hydrogen nuclei” behind the signals, and the internal standard’s purity is “the source of traceability to the SI”, according to BIPM reference data. UV is quickest but leans on aromatic residues: Pace and colleagues’ equation, built from measurements on 80 folded proteins, sums 5,500 per tryptophan, 1,490 per tyrosine and 125 per cystine (M⁻¹ cm⁻¹), and its values are “less reliable for proteins that do not” contain tryptophan. A UV reading also describes a solution, not a powder. Purity is a separate question again, covered in our guide to HPLC purity testing.

What an AAA result can and can’t tell you about a vial

A well-run analysis supports two statements: how much peptide-derived material the tested sample held, and whether its amino acid ratios fit the sequence. Everything else needs other evidence.

• It cannot read residue order. A scrambled peptide built from the same amino acids gives the same answer; order needs techniques such as tandem mass spectrometry.
• It does not separate the target from its relatives unless impurities were measured and subtracted.
• It describes the portion tested. That is why the batch number on a report must match the vial; our guide to reading a certificate of analysis covers that check.
• It says nothing about what happened afterwards, or about suitability for any use. Content and composition are analytical properties, not evidence of safety or biological effect.

What’s established and what isn’t

Reading content claims when sourcing in Canada

• Look for a content figure, and its method. If a report gives peptide content, it should say whether the number came from AAA, nitrogen analysis or qNMR.
• Ask what was corrected: which residues were used, whether the sequence needed a time course or an alternative hydrolysis, and whether related impurities were subtracted.
• If no content figure is shown, ask whether content was measured and how, rather than reading it off the purity figure. Our contact page handles documentation questions.
• Use the domestic reference point if you run AAA yourself. NRC’s ANGII-1, which ships from Ottawa, is intended for “instrument calibration, method development, and validation for peptide hydrolysis experiments”.

We are a supplier ourselves, so apply the same questions to our documentation, starting with the certificates on our Lab Testing page.

Research peptides with published test reports

Janoshik test reports for the research peptides below are listed on our Lab Testing page: the place to check which tests were run and whether content was among them.

BPC-15715-amino-acid research peptideView product GHK-CuCopper tripeptideView product TB-500Thymosin beta-4 research peptideView product EpithalonLyophilized tetrapeptideView product

Frequently asked questions