909 AMINOSAcetate vs. TFA: What a Peptide's Salt Form Means
A peptide's salt form is the counter-ion paired with its positive charges. It doesn't change the peptide's sequence, but it changes how much of the powder's weight is peptide.
- Lyophilized peptides are usually salts: each positive charge is paired with a counter-ion, most commonly trifluoroacetate (TFA) or acetate.
- The salt form normally doesn't change the HPLC purity figure; it changes how much of the powder's weight is peptide.
- For the same number of counter-ions, the same powder weight contains less peptide as a TFA salt than as an acetate salt.
- 909's certificates report content as net peptide. They don't identify or quantify the counter-ion.
1What a salt form is
Peptides supplied as lyophilized powders are rarely the bare peptide molecule alone. They are usually salts: the peptide carries positive charges, and each charge is paired with a negatively charged counter-ion.
The two most common counter-ions are trifluoroacetate (TFA) and acetate. Hydrochloride (chloride) is also used. Counter-ions pair with the peptide's positively charged groups: the free N-terminal amine (if not capped) and the side chains of arginine (R), lysine (K) and histidine (H). A peptide with more of these residues can carry more counter-ions.
Synthetic peptides are commonly cleaved from the synthesis resin with trifluoroacetic acid, and reversed-phase HPLC purification commonly uses TFA in the mobile phase. A peptide isolated this way is usually a TFA salt.
An additional ion-exchange step can replace TFA with acetate or chloride. This is an extra process step, and not every material goes through it.
2Why it changes the weight
Each counter-ion adds mass. Forming the salt adds one proton to the peptide and one counter-ion, so each counter-ion adds the mass of the whole acid: trifluoroacetic acid (CF3COOH) is about 114.02 g/mol and acetic acid (CH3COOH) about 60.05 g/mol.
Not a measurement and not the composition of any 909 lot.
The free peptide MOTS-c (sequence MRWQEMGYIFYPRKLR) has an average molecular weight of 2174.6 g/mol (PubChem). It has five basic sites (the N-terminal amine, three arginines and one lysine) and two acidic groups (the glutamic acid side chain and the C-terminal carboxyl). If both acidic groups are protonated, as under strongly acidic purification conditions, up to five counter-ions could be present; if both are ionized, they offset two of the positive charges, leaving three.
Peptide share of the weight, by hypothetical form:
These figures show a range, not what any real material contains. Actual counter-ion content depends on the process and is measured, not assumed, and lyophilized powders also hold some water.
For the same number of counter-ions, the same powder weight contains less peptide as a TFA salt than as an acetate salt.
3How this shows up on a certificate
Compares the peaks the method detects. Counter-ions are generally not integrated as impurity peaks, so the salt form normally doesn't change the purity figure.
Answers the weight question directly. On 909's KMD certificates, content is reported as mg net peptide per vial, which the certificate distinguishes from counter-ion, water and residual solvent.
Example, lot MOTS-2610A (KMD certificate 9AL-20261020-V62F-01):
It reports content as net peptide, but it doesn't identify the counter-ion, measure how much counter-ion is present, or state how the rest of the powder's mass divides between counter-ion, water and other components. The salt form of lot MOTS-2610A is not established by this certificate, and the hypothetical figures above don't describe it. A certificate reports only the tests it lists.
4Why laboratories ask about the salt form
Working from powder weight without accounting for counter-ions and water overstates the amount of peptide. A net-peptide content result avoids that.
Residual TFA has been reported to interfere with some cell-based and spectroscopic measurements, so some laboratories specify acetate or chloride forms for those methods.
Databases often list the free peptide and its salts as separate entries with different molecular weights. Check which form a molecular weight refers to. See Peptide Identifiers.
5Checklist
- Does the documentation state the salt form (TFA, acetate, chloride)?
- Is content reported as net peptide (mg of peptide) or as gross powder weight?
- Does a quoted molecular weight refer to the free peptide or to a salt?
- If your method is sensitive to TFA, is a counter-ion result reported? If not, the certificate doesn't answer that question.
?Frequently asked questions
Does a TFA salt mean lower purity?
Not by itself. Purity by HPLC area % compares detected peaks, and the counter-ion is generally not integrated as an impurity peak. The salt form affects how much of the powder's weight is peptide, which is a content question.
Why would two vials with the same label quantity contain different amounts of peptide?
If a label quantity refers to gross powder weight, counter-ions and water make up part of it. A net-peptide content result reports the peptide itself.
Do 909's certificates state the salt form?
No. They report purity, identity and content per vial as mg net peptide for the named lot. They don't identify or quantify the counter-ion.
Sources
- KMD Analytical certificate 9AL-20261020-V62F-01 (lot MOTS-2610A), original PDF
- PubChem CID 146675088 (MOTS-c)
- UniProt A0A0C5B5G6 (MOTS-c sequence)
Sources are cited for reference. Their publishers have not reviewed or endorsed this guide.
For laboratory research use only. Not for human or veterinary use. Not FDA approved.
Source certificate: lot MOTS-2610A · Documentation Center · Ver en español