Short answer: Trifluoroacetic acid (TFA) is a small, very strong carboxylic acid with the formula CF3COOH (C2HF3O2) and a molecular weight of 114.02 g/mol [1]. In peptide work it does two main jobs: it cleaves finished peptides from the resin and removes side-chain protecting groups in Fmoc solid-phase synthesis, and it serves as an ion-pairing additive in reversed-phase HPLC [2][3]. Because of that, many synthetic peptides end up as TFA salts, which matters for weighing, analysis and counter-ion exchange [2][4].
This article is educational chemistry for laboratory readers. It does not describe biological effects, and it is not guidance for use in people or animals.
What is trifluoroacetic acid?
TFA is acetic acid with the three hydrogens of the methyl group replaced by fluorine. PubChem lists it under CID 6422 with these identifiers and properties [1]:
- IUPAC name: 2,2,2-trifluoroacetic acid [1]
- Formula: C2HF3O2; molecular weight: 114.02 g/mol [1]
- CAS number: 76-05-1 [1]
- Other names: perfluoroacetic acid, trifluoroethanoic acid [1]
- Boiling point: 72.4 °C; melting point: −15.4 °C [1]
- Density: 1.531 at 20 °C (relative to water = 1, about 1.5) [1]
- Acidity: pKa = 0.3 [1]
- Water solubility: miscible with water at 20 °C [1]
How strong an acid is TFA?
Ordinary acetic acid has a pKa of about 4.5 [4]. TFA's pKa is about 0 [4] (PubChem lists 0.3 [1]), about four units lower, which makes it a much stronger acid. For comparison, Roux and colleagues give hydrochloric acid a pKa of −7, stronger again [4].
TFA in solid-phase peptide synthesis
Most synthetic peptides are now produced by solid-phase procedures [4]. A 2016 review in the Journal of Peptide Science calls Fmoc solid-phase peptide synthesis (SPPS) "the method of choice for peptide synthesis" [3]. TFA's role depends on which protecting-group strategy is used.
In Boc chemistry, TFA is used again and again during the synthesis cycles. The same review notes that this repeated TFA treatment could cleave small amounts of side-chain protecting groups at each cycle and cause gradual loss of peptide from the polymer support [3]. Merrifield's original Boc approach also needed strong acids such as hydrogen fluoride (HF) for the final global deprotection [2].
In Fmoc chemistry, no TFA is needed during chain assembly [3]. Instead, TFA is used once, at the end, as the global deprotection and cleavage agent. Erckes and colleagues note that the Fmoc route allowed HF to be replaced with TFA, which they describe as less hazardous and more widely available [2].
Which protecting groups does TFA remove?
Fmoc SPPS provides an orthogonal combination of temporary and permanent protecting groups: the temporary Fmoc group is removed during each cycle, while the permanent side-chain groups come off in TFA at the end [3]. The Behrendt review lists the standard TFA-labile side-chain choices [3]:
- Arg: Pbf
- Asn, Gln, Cys, His: Trt (trityl)
- Asp, Glu: OtBu (tert-butyl ester)
- Ser, Thr, Tyr: tBu (tert-butyl ether)
- Lys, Trp: Boc
Cleavage cocktails and scavengers
Final TFA-mediated deprotection is carried out in the presence of cation scavengers [9]. Published examples include TFA/TIS (95:5) and TFA/TIS/EDT/H2O (92.5:2.5:2.5:2.5), where TIS is triisopropylsilane and EDT is ethanedithiol [3]. The choice matters for sensitive residues: in one study of (2-furyl)-L-alanine peptides, cocktails containing dithiol scavengers caused partial degradation, while cocktails of water plus triisopropylsilane or 3,6-dioxa-1,8-octanedithiol (DODT) in TFA were sufficient to minimise that side reaction [9].
For the bigger picture of how peptides are built, see our guide to peptide synthesis.
TFA in HPLC purification and analysis
TFA is widely used as an ion-pairing reagent in reversed-phase liquid chromatography, both for purifying peptides and for characterizing them analytically [2]. Erckes and colleagues explain that the trifluoroacetate anion pairs with positively charged groups, such as the N-terminus and the side chains of arginine, histidine and lysine, and this improves the separation of polar compounds on lipophilic stationary phases [2]. A 2020 proteomics study refers to 0.1% TFA as a commonly used mobile-phase condition [7].
TFA does have a downside when the column feeds a mass spectrometer. A 2020 Analytical Chemistry paper notes that TFA improves reversed-phase peptide separation but causes significant signal suppression during electrospray ionization (ESI) [6]. Labs work around this in several ways:
- adding a small amount of glycine (for example 2 mM) to TFA mobile phases, which boosted peptide MS signal-to-noise by more than an order of magnitude on average in that study [6];
- replacing TFA fully or partly with difluoroacetic acid, or, in antibody LC-MS work, using 0.03% TFA with 0.07% formic acid [8];
- using formic acid-based mobile phases with other modifiers [7][8].
Why many peptides are TFA salts
Because TFA is present during both cleavage and purification, synthetic peptides are mainly obtained in the presence of TFA and, for cationic peptides, as trifluoroacetate salts [4]. Erckes and colleagues add that the strong interaction between the trifluoroacetate anion and the peptide means that freeze-drying (lyophilizing) peptide solutions produces a peptide–TFA salt [2].
That counter-ion is not inert from an analytical point of view. Published work reports that:
- trifluoroacetate interferes with physicochemical characterization by infrared spectroscopy [4];
- it can complicate secondary-structure measurements by CD spectroscopy and FT-IR, and it can introduce a weighting error because the salt form has a different molecular weight [2];
- it can interfere with in vitro experiments, which is why the authors recommend quantifying and specifying the counter-ion in assays with synthetic peptides [2].
The weighing effect, worked through
Each trifluoroacetic acid unit adds 114.02 g/mol to the weighed salt [1]. If a peptide with a molecular weight of 1,000 g/mol carries three trifluoroacetate counter-ions, the salt weighs about 1,000 + 3 × 114.02 ≈ 1,342 g/mol, so only about 74.5% of the weighed solid is the peptide itself. This is one reason purity and net peptide content are different numbers; see net peptide content vs. purity.
Measuring and exchanging the TFA counter-ion
How residual TFA is measured
Labs have several validated options:
- 19F-NMR, 1H-NMR and ATR FT-IR were used to compare exchange methods, and ATR-IR can monitor TFA removal [4].
- FT-IR, 19F-NMR and HPLC with an evaporative light-scattering detector (ELSD) were each validated according to ICH guidelines for detecting and quantifying residual TFA [2].
- Ion chromatography (IC), capillary electrophoresis (CE) and isotachophoresis (ITP) were compared for acetate, trifluoroacetate and chloride; IC gave the best results [5].
How TFA is exchanged
The classical approach is to freeze-dry the peptide several times in the presence of an excess of a stronger acid, usually HCl [4]. Roux and colleagues note that working at pH below 1 can degrade peptides, so they tested alternatives on the octapeptide lanreotide [4]:
- reversed-phase HPLC and ion-exchange resin, which gave partial to almost complete exchange to a weaker acid's anion such as acetate [4];
- a deprotonation/reprotonation cycle of the amino groups, which uses a basic solution and gave complete removal of trifluoroacetate [4].
If a certificate of analysis states a salt form or counter-ion content, that line relates to this chemistry. See how to read a peptide COA.
Frequently Asked Questions
What is trifluoroacetic acid used for?
In peptide chemistry, mainly as the cleavage and global deprotection agent in Fmoc solid-phase synthesis and as an ion-pairing reagent in reversed-phase HPLC [2][3].
What is the formula of trifluoroacetic acid?
CF3COOH, written as C2HF3O2, with a molecular weight of 114.02 g/mol and CAS number 76-05-1 [1].
Is TFA a strong acid?
Yes, compared with ordinary carboxylic acids. Its pKa is about 0 (PubChem: 0.3), versus about 4.5 for acetic acid [1][4].
What is the boiling point of TFA?
72.4 °C, per PubChem [1].
What is a TFA salt peptide?
A peptide, usually one with positively charged groups, isolated with trifluoroacetate as its counter-ion because TFA was present during cleavage and purification [2][4].
Why exchange TFA for acetate or chloride?
Trifluoroacetate can interfere with IR and CD measurements, change the weighed mass and interfere with laboratory assays, so researchers often replace it [2][4].
Does TFA affect LC-MS?
Yes. It improves reversed-phase separation but suppresses electrospray ionization signal; glycine additives or difluoroacetic acid are among the published workarounds [6][8].
Bottom line
Trifluoroacetic acid (C2HF3O2, 114.02 g/mol, pKa about 0) is a strong, fuming, corrosive carboxylic acid [1][4]. Fmoc peptide synthesis relies on it for final cleavage and side-chain deprotection, and reversed-phase HPLC uses it as an ion-pairing reagent [2][3]. As a result many synthetic peptides are TFA salts, which affects weighing and analysis and is why counter-ion content is measured and, when needed, exchanged for acetate or chloride [2][4][5].
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