The Salt Form Quietly Changes Your Concentration Maths

Free base is not how a synthetic peptide normally reaches anybody. What arrives is a salt, and the identity of that salt feeds into your concentration arithmetic and, in a few assay systems, into your data.

Purity and content answer different questions

Counter-ion is not a rounding error on the balance. Where a sequence carries several basic residues, trifluoroacetate can make up a considerable share of the lyophilized mass. Together with residual water it is one of the two reasons the gross weight of a vial overstates how much peptide is inside.

That has an immediate practical edge. Weigh the powder out, compute molarity from nominal peptide mass, and the concentration you think you have is higher than the concentration you do have, unless net peptide content is known. Purity asks what proportion of the peptide-related material is the intended sequence; content asks what proportion of the powder is peptide at all. A vial being 99% pure while sitting well below 99% peptide by weight involves no contradiction whatsoever. Net peptide content explained works through it in full.

How trifluoroacetate ends up in the vial

It arrives twice over. Solid-phase synthesis closes with cleavage from the resin, and trifluoroacetic acid is the usual reagent for that. Purification then proceeds on reversed-phase HPLC with TFA serving as the ion-pairing agent in the mobile phase. So by the point the material is freeze-dried, the basic sites — lysine, arginine, histidine, along with the N-terminal amine — are already paired off with trifluoroacetate.

Moving to a different salt is an added operation performed on purpose, generally ion exchange or repeated lyophilization out of dilute acetic or hydrochloric acid. Since that costs money, the sensible default is to assume TFA wherever a peptide has not been specified as the acetate or the hydrochloride.

Two occasions when it stops being bookkeeping

• Sensitive cell culture. Residual trifluoroacetate has been reported to influence viability and proliferation in certain cell types, at concentrations that are reachable once peptide loading is high. Where the endpoint is a cytotoxicity readout, a TFA vehicle control, or else a peptide supplied in the acetate form, deserves thought. The literature is indexed at PubMed.
• Spectroscopy. TFA absorbs down in the low-UV region and brings a strong carbonyl band with it in the infrared, either of which can intrude on secondary structure work by circular dichroism or FTIR.

Outside those two cases, most biochemical work treats the counter-ion as an accounting matter and nothing more.

Reading the paperwork

What you want on the page is the salt form and the net peptide content standing next to the chromatographic purity figure, so that the two distinct questions above both get answered rather than one of them being quietly left open. The third-party reports we have obtained are published on our certificates of analysis page, and how to read a certificate of analysis goes through the fields one by one.

ExoLabz supplies compounds for laboratory research use only. Nothing on this page is medical advice or a suggestion of human or veterinary use.

Related listings. The gap between vial mass and peptide mass applies to every listing, including BPC-157 5mg and TB-500 5mg.