Synthetic peptides are almost universally produced as TFA salts because trifluoroacetic acid is excellent at liberating the peptide from the resin during synthesis and purification. But TFA is a PFAS (perfluoroalkyl substance), is hydrophilic, and can constitute 10–45% of the peptide's final mass. In cell assays, residual TFA can cause unexpected cytotoxicity, inhibiting the very cells you're trying to study. This article explains why and what to do about it.
Peptides are made by solid-phase peptide synthesis (SPPS), a process in which the growing chain is anchored to a resin bead. At the end of synthesis, a cleavage cocktail containing trifluoroacetic acid (TFA) is used to break the bond between the peptide and the resin, releasing the free peptide. TFA has ideal properties for this step: it is a strong acid, highly polar, and a good solvent for both the resin and the peptide. The crude peptide is isolated as a TFA salt — the peptide combined with the TFA counter-ion (TFA⁻).
After purification by HPLC, the peptide is still in TFA salt form. When lyophilized (freeze-dried), the TFA remains, sometimes comprising 10–45% of the final dried mass, depending on how completely the TFA is removed during purification.
TFA is not inert in biological systems. In cell-based assays (viability assays, proliferation assays, gene-expression assays), residual TFA can:
A 2025 consensus paper from ETH Zurich documented that peptides containing 10–20% TFA routinely show reduced or absent biological activity in cell assays compared to the same peptide in chloride or acetate salt form.
Your Certificate of Analysis (COA) should list residual TFA or TFA counter-ion content. Typical values:
If your COA doesn't specify TFA content, contact the supplier — it should be quantified.
Residual TFA can be detected by:
If your assay is sensitive to TFA (which most cell assays are), request a TFA-exchanged version of the peptide. This involves a chromatographic step in which the TFA counter-ion is replaced with a more biocompatible ion — typically chloride (Cl⁻) or acetate (CH₃COO⁻). After exchange, residual TFA should be <1% by mass.
Cost: TFA-exchange adds ~20–30% to the peptide cost, but it is essential for cell-based work where bioavailability and potency matter.
For in-vitro biochemical assays (enzyme kinetics, binding affinity): Standard TFA salt is usually acceptable. The high ionic strength of your assay buffer will dilute any TFA effects.
For cell-based assays (viability, proliferation, signaling): Use TFA-exchanged peptide. Residual TFA can produce false negatives (apparent lack of activity) or off-target effects.
For in-vivo work or animal studies: TFA-exchanged peptide is strongly recommended. Systemic TFA exposure raises safety and efficacy concerns.