Organic Chemistry · Biomolecules: Amino Acids, Peptides, and Proteins
Amino Acid Analysis of Peptides
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In 30 seconds
Amino acid analysis answers the first question about an unknown peptide: which amino acids it contains and how many of each — its composition List of amino acids and their counts in a peptide Full entry →. It does not tell the order of residues (that is sequencing, Topic 6). The classical pipeline has three stages: (1) total acid hydrolysis 6 M HCl, 110 °C, ~24 h cleavage of all peptide bonds Full entry → — heat the peptide in 6 M HCl at 110 °C for about 24 h to break every peptide bond; (2) separation — resolve the freed amino acids by ion-exchange chromatography Separation by charge on a resin column Full entry → or HPLC; (3) detection and quantitation — react each amino acid with ninhydrin Reagent that colors amino acids purple (570 nm) Full entry → (or a similar reagent) and measure the colored product, comparing with standards of known amount.
Why this matters
Composition is the first piece of analytical information about any peptide or protein, and it is used daily: newborn screening programs quantify amino acids in dried blood spots to detect metabolic disorders such as phenylketonuria; food scientists measure protein quality by amino acid composition; pharmaceutical labs verify that a peptide drug has the expected composition before release. Composition also constrains sequencing: if an analysis says a peptide contains two glycines and one alanine, the sequence The order of residues in the chain Full entry → must be one of only three possibilities (Gly–Gly–Ala, Gly–Ala–Gly, Ala–Gly–Gly). Every step of the pipeline — including its artifacts — must be understood to interpret the numbers correctly.
The college version
Core Concepts
Total acid hydrolysis and its artifacts
Heating a peptide in 6 M HCl at 110 °C for 24 h in a sealed tube cleaves all peptide bonds, giving a mixture of free amino acids. But the harsh conditions cause reproducible side reactions that the analyst must account for:
- Tryptophan is destroyed, so it appears missing from the analysis.
- Asparagine and glutamine are deamidated to aspartic acid and glutamic acid, releasing ammonium ion (NH₄⁺). Measuring the ammonia tells you the combined Asn + Gln content.
- Serine and threonine are partially destroyed — reported values need correction factors.
- Valine–isoleucine and other hindered bonds hydrolyze slowly; a second, longer hydrolysis (48–72 h) is used, and values are extrapolated.
Alternative digestions (enzymatic proteases, or base hydrolysis with its own artifacts) give gentler but less complete cleavage.
Separation by ion-exchange chromatography
The hydrolysate is loaded onto a column of sulfonated polystyrene — a strong cation-exchange resin. At low pH the amino acids are cations and bind to the resin; they are then eluted with a gradient of increasing pH and salt concentration. Because the pKa values and charges differ, amino acids elute in a reproducible order: acidic amino acids (Asp, Glu) first, then the neutral ones, then the basic ones (His, Lys, Arg) last. The elution order is calibrated against a mixture of known amino acid standards run under identical conditions, so each peak is identified by its elution time.
Detection and quantitation with ninhydrin
Ninhydrin reacts with the free α-amino group of an amino acid to give a purple product called Ruhemann's purple The colored product of ninhydrin with primary amines Full entry →, which absorbs at 570 nm; proline, a secondary amine, gives a yellow product measured at 440 nm. The absorbance is proportional to the amount of amino acid (Beer–Lambert law, constant path length), so peak areas are converted to amounts by comparison with standards of known concentration. Modern amino acid analyzers automate hydrolysis, chromatography, post-column ninhydrin reaction, and peak integration; HPLC-based methods instead derivative the amino acids before the column (for example, with phenyl isothiocyanate) for sensitive UV or fluorescence detection.
Composition versus sequence
Composition is a list: "Gly₂, Ala₁, Ser₁" means the peptide has four residues — two glycine, one alanine, one serine — but says nothing about their order. Two different peptides can share a composition (Gly–Gly–Ala–Ser versus Gly–Ala–Gly–Ser). Establishing the order is the job of sequencing methods (Topic 6), which use the composition as a constraint and a check.
Common Confusions
| Common Confusion | Correct Understanding |
|---|---|
| Amino acid analysis gives the sequence. | It gives composition (which and how many); sequencing gives the order. |
| "Tryptophan was not detected, so the peptide has no tryptophan." | Trp is destroyed by acid hydrolysis; absence from the chromatogram is an artifact, not evidence. |
| Asparagine is reported as asparagine. | Deamidation converts Asn → Asp (and Gln → Glu) during hydrolysis; the ammonia peak reveals the original amides. |
| Proline gives the purple ninhydrin color. | Proline is a secondary amine — it gives the yellow product (440 nm). |
| One 24 h hydrolysis is complete for every bond. | Val–Ile and other hindered bonds hydrolyze slowly; a second longer run is standard practice. |
| Bigger peaks mean more amino acid only if compared to each other. | Peak areas must be compared with standards of known amount, not merely with each other, for absolute quantitation. |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Amino acid analysis is like taking apart a bead necklace and counting how many beads of each color it had — without caring about the order. You boil the necklace in strong acid to pop every knot, sort the loose beads by size and charge on a special column, and then count each color by how much purple dye it makes. Two kinds of beads melt in the boiling (tryptophan), and two others come out looking like their cousins (asparagine and glutamine), so you have to correct the count.
Worked example
Example 1: Reading a composition analysis
A peptide is hydrolyzed and analyzed. The ion-exchange chromatogram, normalized to the smallest peak, gives: Gly 2.0, Ala 1.0, Asp 1.0, Ser 1.0. The peptide therefore has 5 residues: two glycine, one alanine, one aspartic acid (or asparagine — see Example 2), and one serine. Composition: (Gly₂, Ala₁, Asp₁, Ser₁). Note that this does not identify the sequence: 5!/2! = 60 different sequences share this composition.
Example 2: Correcting for hydrolysis artifacts
A peptide known to contain one tryptophan, one asparagine, and one glutamine is hydrolyzed in 6 M HCl and analyzed. Expect: tryptophan is not detected (destroyed), and the asparagine and glutamine appear as aspartic acid and glutamic acid, with two equivalents of NH₄⁺ released per peptide (one from each amide). An analyst who does not know the peptide's sequence would report "no Trp, Asp 1, Glu 1, plus 2 NH₄⁺" and should flag the ammonia signal as evidence of two amide side chains.
Example 3: Ninhydrin quantitation with dimensional analysis
A leucine standard of 50.0 nmol gives absorbance 0.400 at 570 nm. An aliquot of the unknown hydrolysate gives 0.250 under identical conditions. Because absorbance is proportional to amount (Beer–Lambert, same path length), set up the ratio with units:
nunknown = AunknownAstandard × nstandard = 0.2500.400 × 50.0 nmol = 31.3 nmol leucine in the aliquot
The result must be scaled by the aliquot fraction and the hydrolysis dilution to report the leucine content of the whole peptide sample.
Key takeaways
- Pipeline: 6 M HCl hydrolysis (110 °C, 24 h) → ion-exchange separation → ninhydrin quantitation vs standards.
- Hydrolysis artifacts: Trp destroyed; Asn/Gln → Asp/Glu + NH₄⁺; Ser/Thr partial loss; Val–Ile bonds slow (use 48–72 h runs).
- Elution order on the cation exchanger: acidic → neutral → basic amino acids.
- Ninhydrin: primary amines give purple (570 nm); proline gives yellow (440 nm).
- Absorbance ∝ amount (Beer–Lambert); always compare with standards run under identical conditions.
- Composition ≠ sequence; composition constrains but does not determine the order.
- Quantitate NH₄⁺ to recover the Asn + Gln content lost to deamidation.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
List the three stages of the classical amino acid analysis pipeline.
Show answer
Total acid hydrolysis (6 M HCl, 110 °C, ~24 h); separation by ion-exchange chromatography or HPLC; detection/quantitation with ninhydrin (or a pre-column derivative) versus standards.
What happens to tryptophan, asparagine, and glutamine during 6 M HCl hydrolysis?
Show answer
Trp is destroyed; Asn → Asp and Gln → Glu (deamidation), each releasing NH₄⁺.
In what order do acidic, neutral, and basic amino acids elute from a sulfonated-polystyrene column?
Show answer
Acidic (Asp, Glu) first, then neutral, then basic (His, Lys, Arg).
What color does ninhydrin give with a primary amine, and with proline?
Show answer
Primary amines: purple (Ruhemann's purple, 570 nm); proline: yellow (440 nm).
A standard Known compound/amount run under identical conditions Full entry → of 25.0 nmol gives A = 0.500; the unknown aliquot gives A = 0.300. How much amino acid is in the aliquot?
Show answer
n = (0.300/0.500) × 25.0 nmol = 15.0 nmol.
Why does composition alone not determine a peptide's identity?
Show answer
Two different sequences can share the same composition; composition lists only which residues and how many.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- composition
- List of amino acids and their counts in a peptide
- total acid hydrolysis
- 6 M HCl, 110 °C, ~24 h cleavage of all peptide bonds
- deamidation
- Loss of –NH₂ from Asn/Gln side chains to give Asp/Glu
- ion-exchange chromatography
- Separation by charge on a resin column
- ninhydrin
- Reagent that colors amino acids purple (570 nm)
- Ruhemann's purple
- The colored product of ninhydrin with primary amines
- standard
- Known compound/amount run under identical conditions
- sequence
- The order of residues in the chain
Sources & references
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