Amino Acid Analysis for Peptide Quantification in the Lab

When a lab needs to know how many moles of peptide are really in a vial, it usually turns to one of the oldest techniques in protein chemistry. Amino acid analysis (AAA) breaks a peptide into its component amino acids, counts them, and works back to the amount of intact chain. It is the reference method behind most net peptide content figures and the anchor for traceable quantification. This article follows a sample through hydrolysis, derivatization and separation, explains which residues cause trouble, and uses the tetrapeptide Epithalon to show how the numbers are turned into a peptide quantity.

Why amino acid analysis is the reference method

UV absorbance depends on having suitable chromophores and a reliable extinction coefficient. Gravimetric weighing includes counterions and water. HPLC purity is relative. AAA avoids these problems because it measures an absolute molar amount of each amino acid against calibrated standards [1]. Measurement-science work has used AAA with isotope dilution mass spectrometry to assign SI-traceable concentrations to peptide standards, which were then used to quantify proteins [4]. That is why AAA is usually the method behind the net peptide content figure discussed in net peptide content vs HPLC purity.

Step 1: acid hydrolysis

The classical hydrolysis uses 6 M hydrochloric acid at about 110 C for roughly 24 hours, in a sealed tube under vacuum or inert gas to limit oxidation [1, 2]. Vapour-phase hydrolysis, in which the dried sample sits in a small vial while acid vapour does the work, reduces contamination from the acid itself. Microwave-assisted methods shorten the time considerably.

Hydrolysis is not neutral. Reviews of the method list consistent losses and conversions [1, 2]:

Residue	Behaviour under 6 M HCl, 110 C	Common workaround
Asparagine, glutamine	Converted to Asp and Glu; reported as Asx and Glx	Accept combined value, or use enzymatic hydrolysis
Tryptophan	Largely destroyed	Alkaline hydrolysis or acid with protective additives
Cysteine, cystine	Partly oxidized, variable	Performic acid oxidation to cysteic acid first
Methionine	Partly oxidized to the sulfoxide	Performic acid oxidation to the sulfone
Serine, threonine	Progressive partial destruction	Time course and extrapolation to zero time
Valine, isoleucine (adjacent)	Bonds hydrolyse slowly	Longer hydrolysis, extrapolation to infinite time

For quantification, analysts rely on residues that are both stable and fully released, typically alanine, leucine, phenylalanine, glycine, lysine and arginine, and treat the problematic ones with caution.

Step 2: derivatization and separation

Most free amino acids have weak UV absorbance, so they are derivatized to make them detectable. Two broad strategies are used:

• Post-column derivatization. Amino acids are separated by ion-exchange chromatography and then react with ninhydrin or o-phthalaldehyde as they leave the column. This is the classical analyser design and is robust to matrix effects.
• Pre-column derivatization. Amino acids are tagged before a reversed-phase separation. One widely used reagent, 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate (AQC), reacts with primary and secondary amines to form stable, fluorescent derivatives and was introduced specifically for analysing hydrolysate amino acids [3]. Phenylisothiocyanate and o-phthalaldehyde with 9-fluorenylmethyl chloroformate are other common chemistries.

Mass spectrometric detection with isotope-labelled amino acid standards adds specificity and supports the highest-accuracy measurements [4].

Step 3: from amino acids to peptide

Each amino acid is quantified against a calibration curve, often with an internal standard such as norleucine added before hydrolysis to correct for handling losses. The peptide quantity then follows from the sequence:

1. Divide each measured amount by the number of times that residue occurs in the sequence.
2. Average the values from reliable residues to give moles of peptide.
3. Multiply by the peptide's free-base molecular weight to get the mass of peptide.
4. Divide by the weighed sample mass to get net peptide content.

The ratios between residues are also a composition check. If one residue is well out of line with the others, it may point to a hydrolysis problem, a contaminant or an incorrect sequence.

Worked example: Epithalon

Epithalon is the tetrapeptide Ala-Glu-Asp-Gly, studied in cell-culture work on telomerase activity [5]. Its average molecular weight is about 390.3 Da. For AAA it is a convenient molecule: each of its four residues occurs once, and none of them is tryptophan, cysteine, methionine, serine or threonine.

Suppose 0.500 mg of powder is hydrolysed, the hydrolysate is taken up in 1.00 mL, and a 10 uL aliquot (1% of the sample) is analysed. The run reports 10.2 nmol Ala, 10.0 nmol Glx, 10.1 nmol Asx and 10.4 nmol Gly. Scaling by 100 gives 1,020, 1,000, 1,010 and 1,040 nmol for the whole sample.

• Composition check. The four values agree within about 4%, consistent with the expected 1:1:1:1 ratio.
• Moles of peptide. The average is about 1,018 nmol, or 1.02 umol.
• Mass of peptide. 1.018 x 10^-6 mol x 390.3 g/mol = 0.397 mg.
• Net peptide content. 0.397 mg / 0.500 mg = about 79%, with the remainder attributable to counterions, water and any other non-peptide material.

The arithmetic is simple; keeping track of aliquot fractions and units is where most errors occur.

Epithalon also shows a limit of the method. Because hydrolysis turns glutamine into glutamic acid and asparagine into aspartic acid, AAA would not distinguish Ala-Glu-Asp-Gly from Ala-Gln-Asn-Gly. Mass spectrometry, which sees the 1 Da difference per amide, is needed for that; see mass spectrometry and peptide identity.

Limitations to keep in mind

• Sequence and stereochemistry. AAA gives composition, not order, and standard methods do not separate D- from L-amino acids.
• Non-standard residues. Aib, naphthylalanine, ornithine and other unusual residues need their own standards or are left out of the calculation.
• Contamination. Free amino acids from dust, skin or reagents inflate glycine, serine and others at low sample loads, so blanks are essential.
• Sample size. Precision falls at low nanomole levels, where background becomes significant.

AAA values, where reported, appear on our lab reports as net peptide content alongside purity and identity data.

Key takeaways

• Amino acid analysis converts a peptide into free amino acids and quantifies them against standards, giving an absolute amount of peptide.
• Standard 6 M HCl hydrolysis converts Asn and Gln to Asx and Glx, destroys tryptophan and partly degrades Cys, Met, Ser and Thr.
• Quantification should rest on stable, fully released residues, with an internal standard and blanks.
• Residue ratios double as a composition check, but AAA cannot confirm sequence or chirality.
• AAA is the usual basis for net peptide content and for traceable peptide standards.

This article summarizes published research for informational purposes. All Ascent Sciences products are for laboratory research use only and are not for human or animal consumption.

References

1. Rutherfurd SM, Gilani GS. Amino acid analysis. Curr Protoc Protein Sci. 2009;Chapter 11:Unit 11.9. PubMed
2. Fountoulakis M, Lahm HW. Hydrolysis and amino acid composition of proteins. J Chromatogr A. 1998;826(2):109-134. PubMed
3. Cohen SA, Michaud DP. Synthesis of a fluorescent derivatizing reagent, 6-aminoquinolyl-N-hydroxysuccinimidyl carbamate, and its application for the analysis of hydrolysate amino acids via high-performance liquid chromatography. Anal Biochem. 1993;211(2):279-287. PubMed
4. Burkitt WI, Pritchard C, Arsene C, et al. Toward Système International d'Unité-traceable protein quantification: from amino acids to proteins. Anal Biochem. 2008;376(2):242-251. PubMed
5. Khavinson VKh, Bondarev IE, Butyugov AA. Epithalon peptide induces telomerase activity and telomere elongation in human somatic cells. Bull Exp Biol Med. 2003;135(6):590-592. PubMed