Trifluoroacetylation on N-α / Lys-Nε / Tyr-OH
TFA + ethyl trifluoroacetate (or TFE contaminated with TFA → trifluoroethyl trifluoroacetate) acylates free amines and even Tyr-OH. +96 Da.
Why it happens (mechanism)
Under TFA, residual carbodiimide or acid catalysis can convert TFA into mixed anhydrides or active esters; these acylate the most nucleophilic free amine in solution. Lys ε-amine and N-terminal α-amine are common targets. Even hot TFE can carry trace TFA → ester → acylation.
When it strikes (triggers)
Concentrated TFA exposure to peptides with multiple free amines. TFE solvent that's been stored alongside TFA. Cleavage cocktail without acid scavenger when amines are unprotected (rare in synthesis but happens in side-chain labeling).
How to spot it (MS signature)
+96.00 Da (TFA-CO group). Confirm by ¹H-NMR (CF₃ resonance) or by treating with mild base (NaHCO₃, pH 9, 1 h, RT) — the trifluoroacetyl group is base-labile and comes off cleanly.
How to prevent it
- Protect Lys with Boc (in Fmoc chemistry) or Cbz (in Boc chemistry) until needed.
- Distill TFE before use; check by ¹H-NMR for TFA contamination.
- Quench TFA cleavage thoroughly with cold ether precipitation; don't leave the peptide bathing in TFA at room temp.
- If trifluoroacetyl appears, mild aqueous base wash (pH 9, RT, 1 h) removes it (this is NOT true for stable amides like acetyl).
If it already happened (salvage)
- Reversible by mild base — the only readily-removable acylation at this temperature.
Source
Yi Yang, Side Reactions in Peptide Synthesis (Elsevier, 2016), Chapter 7, §7.2; 14.7.