Peptide salt forms
Acetate, trifluoroacetate, hydrochloride, free base. The counter-ion adds weight, so 10 mg of powder is not 10 mg of peptide.
Most peptides carry positive charges at the amino groups of lysine, arginine and histidine and at the N-terminus. In a dry powder those charges are balanced by negatively charged counter-ions. The peptide plus its counter-ions is a salt. The peptide without them, or with the charges neutralised, is the free base.
The common forms
| Form | Counter-ion | Where it comes from |
|---|---|---|
| Trifluoroacetate (TFA salt) | Trifluoroacetate | Left from solid-phase synthesis and HPLC purification, both of which use trifluoroacetic acid |
| Acetate | Acetate | Made by exchanging the TFA; the most common form among approved peptide drugs |
| Hydrochloride (HCl) | Chloride | Another exchange product; used for some approved peptides |
| Free base, or the active moiety | None counted | The peptide itself; approved labels state the dose this way |
A 2020 review of counter-ions in peptides notes that synthesis by the standard Fmoc method followed by reversed-phase HPLC "lead to peptides as TFA salts", and that most approved peptide drugs are acetates. TFA is generally avoided in finished products: the review describes reports of TFA affecting cells and immune responses in laboratory studies, and calls TFA content "highly undesirable" for peptides intended for clinical studies. Exchanging it costs time and yield, so cheaper research material is often sold as the TFA salt.
Why mg of salt is not mg of peptide
The counter-ions have weight, and so does the water a freeze-dried powder holds. Take a hypothetical peptide with a molecular weight of 1,500 and three basic sites:
| Form | Added weight per molecule | Share of salt weight that is peptide |
|---|---|---|
| Free base | None | 100% |
| Triacetate (3 x acetic acid, 60.05 each) | About 180 | About 89% |
| Tri-TFA (3 x trifluoroacetic acid, 114.02 each) | About 342 | About 81% |
Water and any excess counter-ion lower the share further. That is why test reports separate purity from content. HPLC purity says what share of the peptide material is the right peptide. Net peptide content says what share of the powder's weight is peptide. A vial filled with 10 mg of powder at 80 percent net peptide content holds 8 mg of peptide, however pure that peptide is. A certificate of analysis should say whether the stated amount is powder weight or peptide content.
The reverse matters for reconstitution. If a label states peptide content and you calculate as if it were powder weight, or the other way round, every dose is off by the same percentage. The reconstitution pages explain the arithmetic.
What the FDA has said
The approved semaglutide products state the dose as semaglutide, with a molecular weight of 4113.58 g/mol on the Wegovy label. The FDA's page on unapproved GLP-1 drugs says it is aware that some compounded semaglutide products "may be the salt forms", that "these salt forms, including semaglutide sodium and semaglutide acetate, are different active ingredients" from the approved drug, and that it is not aware of "any lawful basis for their use in compounding". The compounded entry covers the wider rules.
When you add a vial to the vial tracker, you enter the amount the way the label states it, and the app works out concentration and doses remaining from that, so it helps to know which kind of milligram the label means.
Related terms: HPLC, purity testing, lyophilised, certificate of analysis. Back to the glossary.
Questions people ask
What is the difference between a peptide acetate and a TFA salt?
The counter-ion. TFA salts come straight from standard synthesis and HPLC purification; acetate salts are made by exchanging the TFA and are the most common form among approved peptide drugs.
Is 10 mg of peptide powder 10 mg of peptide?
Not necessarily. Counter-ions and water add weight. At 80 percent net peptide content, 10 mg of powder holds 8 mg of peptide.
Does HPLC purity tell me how much peptide is in the vial?
No. Purity is the share of peptide material that is the right peptide. Net peptide content, a separate measure, is the share of the powder that is peptide.
What did the FDA say about semaglutide salt forms?
That salt forms such as semaglutide sodium and semaglutide acetate are different active ingredients from the approved drug, and that it is not aware of any lawful basis for their use in compounding.
Sources
- The role of counter-ions in peptides: an overview (Sikora et al.), Pharmaceuticals, 2020
- FDA's concerns with unapproved GLP-1 drugs used for weight loss, U.S. Food and Drug Administration
- Wegovy prescribing information, Novo Nordisk
- HPLC analysis and purification of peptides (Mant et al.), Methods in Molecular Biology, 2007
