Anyone wanting to substantiate the purity of a research peptide thinks quickly of HPLC for purity or mass spectrometry for identity. A less discussed but equally valuable technique is amino acid analysis (AAA). Where other methods say something above all about the presence and purity of a compound, AAA gives a direct, independent figure for the actual peptide content in a sample. In this article we explain what the technique involves, how it relates to common methods and why it is a useful addition to laboratory documentation.
What is amino acid analysis?
Amino acid analysis is a quantitative technique in which a peptide is fully hydrolysed into its constituent amino acids. These free amino acids are then derivatised and separated, usually via ion-exchange chromatography or reversed-phase HPLC, after which a detector (UV, fluorescence) measures the quantity of each amino acid. From the measured molar ratios of the amino acids a laboratory can calculate back how many grams of actual peptide are present in a sample, separately from salts, residual moisture or other non-peptide components.
AAA versus UV-Vis and mass spectrometry
UV-Vis spectrophotometry at 280 nm estimates the concentration via the extinction coefficient of aromatic residues, which is fast but sensitive to deviation when the secondary structure or the matrix of the sample differs from the assumption the coefficient is based on. Mass spectrometry confirms above all the identity and molecular mass of a peptide, and is less suited to establishing an absolute content. Amino acid analysis complements both: it is more labour-intensive and slower, but yields a figure that does not depend on the secondary structure of the peptide or on assumptions about the extinction coefficient. In much research documentation the techniques are therefore combined: MS for identity, UV-Vis for a quick concentration estimate, and AAA as orthogonal confirmation of the net content.
From hydrolysis to figure: the method in outline
The process usually runs in a number of steps:
- Hydrolysis: the peptide is exposed to strong acid (often 6M HCl) at elevated temperature for several hours, breaking all the peptide bonds.
- Derivatisation: the released amino acids react with a derivatisation reagent so that they become detectable by UV or fluorescence detection.
- Chromatographic separation: the derivatised amino acids are separated on the basis of their physicochemical properties.
- Quantification: the peak areas are compared with known standards to determine the molar quantity of each amino acid.
An important limitation is that some amino acids do not survive acid hydrolysis intact. Tryptophan is almost entirely destroyed, and serine and threonine are partly lost, which published methods correct for with specific conversion factors or alternative hydrolysis conditions.
Why net content is something other than gross weight
A vial containing nominally a few milligrams of lyophilisate rarely contains peptide alone. Counterions such as acetate or TFA, residual moisture and any lyoprotectants all contribute to the gross weight without being part of the peptide chain itself. The net peptide content, as determined via AAA, indicates what percentage of the gross weight actually consists of peptide. In published analytical studies this percentage varies considerably between batches and producers, which makes AAA a relevant instrument for documenting batches for research purposes.
Application in laboratory documentation
For laboratories documenting batches of research peptides, an AAA result offers a figure that can be set alongside a certificate of analysis (CoA) in support of the stated net content. Because the technique is independent of the method the original manufacturer used, it can serve as an extra checkpoint within internal quality files, especially when results from different suppliers or lot numbers are compared with one another. Published comparative studies show that combining several orthogonal techniques increases the reliability of reported contents considerably relative to a single measurement method.
Limitations to bear in mind
AAA is no panacea. The technique requires specialised equipment, is time-intensive compared with a UV-Vis measurement, and some amino acids (cysteine, tryptophan) call for adapted protocols to be quantified reliably. Moreover AAA says nothing about the sequence or any modifications of the peptide; for that, mass spectrometry and, where relevant, sequence analysis remain necessary. In practice AAA is therefore used above all as an additional, orthogonal check alongside other analytical techniques, not as a replacement for them.
The information in this article is intended solely for laboratory and research purposes. The products of Dutch Peptide Labs are not intended for human or animal consumption, diagnostic use or therapeutic application.
