Definition
A peptide carrying trifluoroacetate counter-ions left over from synthesis and HPLC purification.
Trifluoroacetic acid (TFA) is used to cleave synthetic peptides from the resin and as an ion-pairing agent in HPLC purification. It remains bound as trifluoroacetate counter-ions to basic groups: lysine, arginine, histidine and the free N-terminus. A peptide isolated in this form is a TFA salt, and sequences rich in basic residues can carry a substantial share of their mass as TFA. Trifluoroacetate has been reported to affect cell proliferation in vitro, so acetate or hydrochloride salts, produced by ion exchange, are often preferred for cell-based studies. A complete certificate of analysis states the counter-ion and, ideally, the residual TFA content.