Amino acid analysis in peptides explained
Amino acid analysis of peptides: composition and quantification explained
Peptides consist of chains of amino acids. The sequence of these amino acids determines much of a peptide's chemical and structural properties.
In quality assessment, techniques such as HPLC and mass spectrometry are often used to evaluate purity and molecular mass. An additional analytical method is amino acid analysis, often abbreviated as AAA.
Amino acid analysis can be used to investigate:
- which amino acids are present in a sample;
- how the measured amino acid ratios correspond to the expected composition;
- how much peptide is present;
- whether the amino acid composition matches that of the theoretical peptide;
- how the net peptide content can be calculated.
The method works differently from HPLC and mass spectrometry. In classical amino acid analysis, the peptide chain is first broken down into individual amino acids. These amino acids are then separated, detected, and quantified.
Amino acid analysis can provide valuable information, but it also has limitations. During degradation, some amino acids may partially change or be lost. Therefore, the results must always be interpreted in the context of the method used.
What is amino acid analysis?
Amino acid analysis is a laboratory technique used to examine the amino acid composition of a sample.
A classical analysis usually involves the following steps:
- sample preparation;
- hydrolysis of the peptide chain;
- release of individual amino acids;
- separation of the amino acids;
- detection and quantification;
- comparison with the expected composition.
The result can be expressed as:
- amount per amino acid;
- molar ratio;
- mass concentration;
- amino acid profile;
- calculated peptide content.
What is hydrolysis?
Hydrolysis is a chemical process in which bonds are cleaved using water.
In amino acid analysis, the peptide bonds are broken down, releasing individual amino acids.
A commonly used method is acid hydrolysis.
The sample is exposed to the following for a controlled period:
- strong acid;
- elevated temperature;
- controlled conditions.
After hydrolysis, the original peptide chain no longer exists as an intact molecule.
The released amino acids can then be analyzed individually.
Why is the peptide chain broken down?
An intact peptide can contain dozens of different amino acid residues.
By breaking down the chain, the total amount of each amino acid can be measured.
Suppose a theoretical peptide contains:
- four alanine residues;
- two glycine residues;
- three leucine residues;
- one tyrosine residue.
After hydrolysis, it can be investigated whether the measured ratios approximately correspond to this theoretical composition.
This supports assessment of the amino acid composition.
What is the difference between amino acid composition and amino acid sequence?
Amino acid analysis primarily measures which amino acids are present and in what quantities.
The method does not automatically determine the exact sequence.
Two different peptides can have the same amino acid composition but a different sequence.
For example:
- Ala-Gly-Leu;
- Leu-Gly-Ala.
Both contain:
- one alanine;
- one glycine;
- one leucine.
Nevertheless, the sequence is different.
Other techniques are required for sequence analysis, such as:
- tandem mass spectrometry;
- fragment analysis;
- specific sequencing methods.
Can amino acid analysis confirm identity?
Amino acid analysis can support the expected composition.
When the measured amino acid profile deviates substantially from the theoretical composition, this may prompt further investigation.
However, a matching profile does not independently prove that the exact amino acid sequence is correct.
For a broader identity assessment, the following can be combined:
- molecular mass;
- LC-MS;
- MS/MS;
- amino acid analysis;
- chromatographic data.
How are amino acids separated?
After hydrolysis, the sample contains a mixture of free amino acids.
These must be analytically separated from one another.
Possible techniques include:
- ion-exchange chromatography;
- HPLC;
- UPLC;
- capillary electrophoresis.
The technique selected depends on:
- desired sensitivity;
- laboratory method;
- sample type;
- required accuracy.
What is derivatization?
Many amino acids have limited detectability without additional treatment.
Therefore, they can be chemically linked to a detectable group.
This is called derivatization.
Derivatization can take place:
- before chromatographic separation;
- after chromatographic separation.
The derivatives formed can be measured more effectively using, for example:
- UV detection;
- fluorescence detection;
- mass spectrometry.
Pre-column derivatization
In pre-column derivatization, amino acids are labeled before chromatographic separation.
The derivatives formed are then separated via a column.
Possible advantages include:
- high sensitivity;
- suitable fluorescence detection;
- analysis of multiple amino acids.
The reaction must proceed reproducibly.
Incomplete derivatization can affect quantification.
Post-column derivatization
In post-column derivatization, amino acids are first separated.
After the column, they react with a detection reagent.
A well-known example is analysis with ninhydrin.
The reaction products formed can then be measured optically.
How is the amount of amino acid determined?
Reference standards are used for quantification.
Standard solutions contain known amounts of amino acids.
These are measured to establish a relationship between:
- concentration;
- detector signal.
The unknown sample is then measured.
The amount of each amino acid can be calculated using the calibration data.
What is an internal standard?
An internal standard is a known substance added to samples and calibration solutions.
This can help correct for variation during:
- sample preparation;
- injection;
- derivatization;
- analysis.
The internal standard must be analytically distinguishable from the amino acids under investigation.
How can amino acid analysis determine peptide content?
When the amino acid composition of a peptide is known, selected amino acids can be used to calculate the amount of peptide.
Suppose that each peptide chain contains exactly four alanine residues.
When the total amount of alanine is measured, it can be used to estimate how many peptide chains were present.
The calculation takes into account:
- number of residues per peptide;
- molecular mass;
- measured amino acid amount;
- analytical method.
This can be used for a quantitative peptide assay.
Why are not all amino acids always used?
Not every amino acid is equally suitable for quantification.
Some amino acids can during hydrolysis:
- be partially degraded;
- undergo chemical changes;
- be released incompletely;
- be measured reliably.
Therefore, stable amino acids are often selected.
The choice depends on the peptide sequence and analytical method.
Which amino acids can change during hydrolysis?
Acid hydrolysis can have various effects.
Tryptophan
Tryptophan can be largely degraded during standard acid hydrolysis.
Therefore, modified methods are required when tryptophan must be measured specifically.
Asparagine
Asparagine can be converted into aspartate during hydrolysis.
As a result, asparagine and aspartate are not always reported separately in conventional analysis.
Glutamine
Glutamine can be converted into glutamate.
This allows glutamine and glutamate to be reported together.
Serine
Serine can be partially lost during prolonged hydrolysis.
Threonine
Threonine can also partially degrade.
Cysteine
Cysteine can undergo oxidative changes.
Reliable analysis may require prior oxidation or an adapted method.
Why is hydrolysis time important?
The peptide bonds must be sufficiently cleaved.
Hydrolysis that is too short can lead to:
- incomplete degradation;
- amino acid values that are measured too low.
Hydrolysis that is too long can lead to:
- further degradation of sensitive amino acids;
- measurement loss.
Therefore, hydrolysis time and temperature are controlled.
Sometimes multiple hydrolysis times are used to investigate loss or incomplete degradation.
What is a molar ratio?
A molar ratio describes the relative numbers of amino acids.
Suppose a peptide theoretically contains:
- two alanines;
- one glycine;
- three leucines.
The theoretical molar ratio is:
2 : 1 : 3
After analysis, measured values can be normalized.
When the ratio is close to the theoretical value, this supports the expected amino acid composition.
Small deviations can arise from:
- measurement uncertainty;
- hydrolysis loss;
- derivatization;
- integration of chromatographic peaks.
What does normalization mean?
In normalization, measured amino acid values are converted into relative ratios.
A suitable amino acid can be used as a reference.
This allows the measured profile to be compared with the theoretical composition.
Normalization is useful for composition analysis but does not automatically provide the absolute peptide amount.
Amino acid analysis versus HPLC purity
HPLC purity and amino acid analysis measure different properties.
HPLC purity usually examines:
- main chromatographic peak;
- relative amount of minor peaks.
Amino acid analysis examines:
- amino acid composition;
- amount of selected amino acids;
- possible peptide quantification.
High HPLC purity does not automatically prove that the amino acid composition is correct.
A suitable amino acid composition does not automatically prove that no peptide-related impurities are present.
The techniques complement each other.
Amino acid analysis versus mass spectrometry
Mass spectrometry examines the mass-to-charge ratio of molecules.
An intact mass measurement can support that the molecular mass matches the theoretical value.
Amino acid analysis examines the amount and ratio of amino acids after hydrolysis.
Both techniques have different strengths.
A combination can provide more information about:
- identity;
- composition;
- peptide content.
Amino acid analysis versus peptide assay
Amino acid analysis can itself be used as an assay method.
A peptide assay defines the objective:
determine the amount of peptide.
Amino acid analysis is one of the techniques that can achieve this goal.
Other possible assay methods include:
- quantitative HPLC;
- UV spectrophotometry;
- LC-MS quantification;
- mass balance.
What is the impact of counterions?
Counterions such as:
- acetate;
- chloride;
- TFA;
contribute to the total mass of lyophilized material.
Amino acid analysis primarily examines the amino acid components of the peptide.
This can help the method distinguish between:
- total material mass;
- actual peptide quantity.
Additional analyses may be needed for a complete mass balance.
What is the impact of residual moisture?
Residual moisture contributes to the total weight but not to the amino acid composition.
When the net peptide content is calculated, moisture analysis can provide additional information.
A comprehensive mass balance may include:
- peptide content;
- water content;
- counterion content;
- other components.
Can amino acid analysis demonstrate contamination?
Some abnormal amino acid signals may indicate unexpected components.
However, the method is not designed to detect every possible contaminant.
Other techniques may be needed for:
- organic impurities;
- residual solvents;
- metals;
- microbiological contamination;
- endotoxins.
What can be included on a COA?
A certificate of analysis may contain information such as:
- test: amino acid analysis;
- analytical method;
- measured amino acid ratios;
- theoretical ratios;
- calculated peptide content;
- test date;
- batch number.
Not every COA includes amino acid analysis.
When only HPLC and MS have been performed, complete amino acid quantification cannot be derived from them.
Why is batch linkage important?
An amino acid analysis must be linked to the batch tested.
Important data include:
- lot number;
- product name;
- analysis date;
- laboratory;
- analytical method.
An analysis report without a clear batch link has less value for traceability.
What does measurement uncertainty mean?
Every quantitative analysis contains measurement uncertainty.
Sources of variation in amino acid analysis may include:
- sample preparation;
- hydrolysis;
- amino acid loss;
- derivatization;
- calibration;
- chromatographic separation;
- peak integration.
Therefore, results should not be considered absolutely error-free.
Why is method validation important?
A method must be suitable for its intended purpose.
Relevant validation characteristics for quantitative amino acid analysis may include:
- accuracy;
- precision;
- linearity;
- specificity;
- measurement range;
- robustness;
- repeatability.
A method suitable for a qualitative amino acid profile is not automatically sufficient for exact peptide quantification.
Scientific limitations
Amino acid analysis has important limitations.
The method does not independently determine:
- exact amino acid sequence;
- molecular mass;
- HPLC purity;
- three-dimensional structure;
- biological activity;
- sterility;
- endotoxin level.
Additionally, some amino acids may change during hydrolysis.
For a comprehensive quality assessment, multiple analytical techniques must be combined.
Summary
Amino acid analysis examines the composition and quantity of amino acids in a peptide.
In classical analysis, the peptide chain is first broken down by hydrolysis.
The amino acids are then:
- separated;
- detected;
- quantified;
- compared with the theoretical composition.
Amino acid analysis can be used for:
- checking amino acid composition;
- assessment of molar ratios;
- quantification of peptide content;
- supporting identity.
The method has limitations because certain amino acids may change during hydrolysis.
Therefore, amino acid analysis is preferably combined with:
- HPLC;
- LC-MS;
- MS/MS;
- peptide assay;
- residual moisture analysis;
- counterion analysis.
Research disclaimer
Peptidera's research peptides are offered exclusively for Research Use Only (RUO) and are intended for laboratory research and analytical applications.
Not intended for human or animal consumption, diagnostic use, therapeutic use, or self-administration.
Peptide knowledge & quality control
Related Peptidera products:
Lyophilized research peptides with batch identification and available analytical quality data
Related internal blogs:
- PB-0230 — Peptide purity and HPLC
- PB-0231 — Mass spectrometry of peptides
- PB-0233 — Peptide assay: purity versus peptide content
- PB-0234 — Residual moisture in lyophilized peptides
- PB-0238 — Counterions in peptides
Internal link suggestions:
- Link to PB-0233 on amino acid analysis as a peptide assay
- Link to PB-0230 on the difference from HPLC purity
- Link to PB-0231 on molecular mass and sequence analysis
- Link to PB-0234 on residual moisture and net peptide content
- Link to PB-0238 on counterions and mass balance
- Link to the Peptidera page with certificates of analysis
- Link to the research peptide collection