TFA vs acetate peptide salts: why the counter-ion matters

Where counter-ions come from

Reverse-phase HPLC purification commonly uses trifluoroacetic acid in the solvent. The acid pairs with basic residues such as lysine, arginine and the N-terminus, so the freeze-dried product is a TFA salt unless it is exchanged.

Acetate and other forms

Exchanging TFA for acetate, or sometimes hydrochloride, is done in an additional step and typically costs more. Acetate is often preferred for assays where residual TFA could affect cells or enzymes.

Why it changes net content

Each charged site carries a counter-ion with its own mass, and peptides also retain water. The mass of peptide per vial is therefore lower than the labelled mass of powder, which is why net peptide content is reported separately from HPLC purity.

When it matters for your work

The stock solution calculator has a peptide-content field for exactly this adjustment.

• Cell-based assays can be sensitive to TFA.
• Concentration calculations should use net content, not the nominal mass.
• Comparisons between suppliers are only fair if the salt form is the same.

What to look for on a CoA

The salt form and net peptide content (often from amino acid analysis or nitrogen content) should be stated. If they are missing, ask.