What is Trifluoroacetic acid?

Trifluoroacetic acid (TFA, CF3COOH, 114.02 g/mol) is acetic acid with the three methyl hydrogens replaced by fluorine. The fluorines draw electron density away from the carboxyl group, making it a far stronger acid: Roux and colleagues put its pKa at about 0, against about 4.5 for acetic acid ^[1]. It boils at about 72 °C, so excess free acid evaporates readily.

How it ends up in a peptide

TFA is used at two points in a standard synthesis. In Fmoc solid-phase peptide synthesis, a cleavage cocktail made up mostly of TFA, with scavengers such as water and triisopropylsilane, releases the chain from the resin and removes the side-chain protecting groups. Then, in reversed-phase HPLC purification, around 0.1% TFA in the mobile phase acts as an ion-pairing reagent. Free TFA leaves during freeze-drying, but trifluoroacetate held as the counter-ion of protonated amines stays, so cationic peptides are mainly obtained as trifluoroacetate salts ^[1]. A review of impurities in peptide medicines lists trifluoroacetate among the unwanted counter-ions that can persist into final products ^[2].

Why it matters

Mass. Each protonated basic site can carry one trifluoroacetate. Selank (Thr-Lys-Pro-Arg-Pro-Gly-Pro, 751.87 g/mol) has three, at the N-terminus, Lys and Arg, so its tris-trifluoroacetate salt weighs 1,093.94 g/mol and is only 68.7% peptide before water is counted. The counter-ion entry compares other sequences and salt forms, and net peptide content covers the weighing correction.

Biology. In fetal rat osteoblast cultures, TFA at 10⁻⁸ to 10⁻⁷ M reduced cell numbers and thymidine incorporation after 24 hours, with similar effects in articular chondrocytes and neonatal mouse calvariae ^[3]. When TFA and hydrochloride salts of amylin, amylin-(1-8) and calcitonin were compared, the TFA salts consistently gave less proliferation, enough to mask a proliferative effect or create an apparent antiproliferative one; the authors recommended converting peptides to a hydrochloride or biologically equivalent salt before testing ^[3]. This comes from one group working in bone and cartilage cells, but it is the main reason TFA content is treated as an experimental variable.

Spectroscopy. Trifluoroacetate interferes with infrared characterisation of peptides ^[1].

Worked example: how much TFA reaches the cells

Dilute the tris-TFA salt of Selank to 10 µM peptide in culture medium and the well also contains 30 µM trifluoroacetate, 300 to 3,000 times the 10–100 nM range at which the osteoblast study reported effects ^[3]. Even at 10 nM peptide, the accompanying 30 nM trifluoroacetate falls inside that range. A matched control (the same concentration of trifluoroacetate without peptide, or the same peptide as a different salt) separates the two.

Removing or exchanging it

The classical method, repeated freeze-drying from excess hydrochloric acid, replaces trifluoroacetate with chloride but means working below pH 1, which can degrade the peptide ^[1]. Testing alternatives on the octapeptide lanreotide, Roux and colleagues found that reversed-phase HPLC and ion-exchange resin gave partial to almost complete exchange to the counter-ion of a weaker acid such as acetic acid, and that a deprotonation–reprotonation cycle removed trifluoroacetate completely; ¹⁹F-NMR and infrared spectroscopy tracked what remained ^[1]. The exchanged products are sold as acetate salts or hydrochlorides.

The TFA versus acetate guide compares the salt forms for different experiments, and how peptides are made places TFA in the full synthesis sequence.