Short answer

A lyophilized peptide powder is a salt. Every positively charged site is balanced by a counter-ion, usually trifluoroacetate (TFA) or acetate, and the powder also holds a few percent of water. The peptide itself is therefore only part of the weighed mass, called the net peptide content, which is often between 60 % and 90 %. HPLC purity does not measure it; amino acid analysis or quantitative NMR does.

Key facts

  • Peptides purified by reversed-phase HPLC are normally TFA salts; acetate or hydrochloride salts are made by ion exchange.
  • One TFA adds 114.02 Da per charged site and one acetate adds 60.05 Da.
  • A 1000 Da peptide with two charged sites weighs 1228.05 Da as a TFA salt, so the peptide is 81.4 % of the salt before water is counted.
  • Net peptide content is measured by amino acid analysis or quantitative NMR, and water by Karl Fischer titration.
  • Residual TFA can interfere with cell-based assays; the counter-ion should therefore be stated on the COA.

Why peptides are salts

Peptides contain basic groups, such as the N-terminal amine and the side chains of lysine, arginine and histidine. During purification the mobile phase contains trifluoroacetic acid, which protonates these groups, and the trifluoroacetate anion stays with the peptide as it is dried. The powder is therefore a salt in which each charged site is paired with a counter-ion.

If the TFA is exchanged for acetate or chloride, the peptide is delivered as an acetate or hydrochloride salt instead.

A worked example

Take a peptide with a molecular weight of 1000 Da and two basic sites. As a TFA salt, each site carries one trifluoroacetic acid (114.02 Da), so the salt weighs 1000 + 2 x 114.02 = 1228.05 Da and the peptide makes up 81.4 % of it. As an acetate salt, each site carries 60.05 Da, so the salt weighs 1120.10 Da and the peptide makes up 89.3 %.

If the powder also contains 6 % water by weight, the TFA salt gives a net peptide content of 76.5 %. The same peptide therefore has different molecular weights and different net contents depending on the salt form.

Net peptide content versus HPLC purity

HPLC purity is the share of the main peptide peak among all peptide-related peaks. It ignores counter-ions, water and inorganic salts because they do not absorb at 214 nm. A peptide can be 99 % pure by HPLC and still contain only 70 % peptide by weight. The two numbers answer different questions and should never be multiplied or substituted for each other without a statement from the laboratory.

How the content is measured

Amino acid analysis hydrolyses the peptide and quantifies the released amino acids, which gives the amount of peptide present. Quantitative NMR can measure the peptide and the TFA together against an internal standard. Water is measured by Karl Fischer titration or by loss on drying. Ion chromatography and 19F NMR are used to quantify trifluoroacetate. A COA that reports a net peptide content should name the method.

Why the counter-ion matters in an experiment

Residual trifluoroacetate has been reported to affect cell proliferation assays, and methods for exchanging it for chloride or acetate are published. For experiments that are sensitive to acidity or to TFA, researchers either ask for an acetate or hydrochloride salt or measure the TFA content and correct for it.

The salt form also matters for weighing, because the mass of powder needed to obtain a given amount of peptide depends on the net content.

What to look for on the batch documents

A useful set of documents states the counter-ion, the molecular weight of the salt (not only of the free peptide), the water content and the net peptide content, together with the methods. If only the free-peptide molecular weight and an HPLC purity are given, the amount of peptide in the vial remains unknown.

Vesta lists some items explicitly as acetate salts, and the compound pages give the molecular weight of the free peptide so that it can be compared with the batch values.

Frequently asked questions

Is an acetate salt better than a TFA salt?

Neither is better in every case. Acetate is often preferred for biological assays because residual TFA can interfere with some cell systems, but the right choice depends on the experiment, the solubility of the peptide and the purification process.

Does the vial label amount refer to peptide or to powder?

It depends on the supplier. Ask whether the labelled amount is the total powder mass or the net peptide content, and check the COA for the content value.

Can I calculate net peptide content from the molecular weights?

Only as an estimate. The calculation needs the number of charged sites and the water content, and both vary between batches, so a measured value is preferable.

Primary references

  1. PubMed: Elimination and exchange of trifluoroacetate counter-ion from cationic peptides
  2. PubMed: Determination of residual trifluoroacetate in protein purification buffers and peptide preparations by ion chromatography
  3. ICH Q6B: Specifications for biotechnological and biological products

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