What counter-ion should I expect on an SGN retatrutide lot?

The particulars: SGN · retatrutide.

I can parse the result. I am less sure what it licenses me to conclude.

I have deliberately not looked at anyone else’s interpretation yet.

What does this actually establish, and what does it not?

Answer first: the counter-ion is part of the mass you paid for and none of the peptide you wanted, and it can run to several per cent or more.

Quantification differs by ion: trifluoroacetate by ion chromatography or by fluorine nuclear magnetic resonance, acetate by ion chromatography or by capillary electrophoresis. Neither shows up on a UV chromatogram.

To be exact about it, net peptide content — the fraction of gross mass that is the peptide itself — is the figure that folds water and counter-ion together and is what you should ask for if you ask for anything.

Peptides from reverse-phase purification with trifluoroacetic acid mobile phases are isolated as trifluoroacetate salts, which is standard synthesis practice.

The caveat is that a purity figure is completely blind to counter-ion content, and the two are routinely conflated.

Ask for net peptide content. It folds water and counter-ion into one usable number.

The short version: peptides are supplied as salts, the salt is real mass, and neither purity nor a chromatogram will tell you how much of it there is.

Peptides purified by reverse-phase chromatography with a trifluoroacetic acid mobile phase are isolated as trifluoroacetate salts. The trifluoroacetate content depends on the number of basic residues and can run from a few per cent to well over ten by mass on a heavily basic sequence.

Concretely, acetate salts are produced by a deliberate ion exchange after purification. The exchange costs a processing step and is the reason acetate-form material is more expensive.

Counter-ion content scales with the number of basic residues, following directly from charge balance.

Net peptide content varies between lots as the process varies, so one figure does not characterise a supplier.

Count the basic residues and you can estimate the counter-ion load.

edited 12 Jul 2024 by Dr_Hanne_Solberg — added the citation requested in comments

The relevant arithmetic is that gross mass equals peptide plus counter-ion plus water, and the certificate usually reports only the first indirectly.

A supplier who reports both water and counter-ion has given you everything needed to reconcile the vial, and on this site that is a very short list.

Worked reconciliation: a 10 mg vial at four per cent water and eight per cent trifluoroacetate contains 10.0 × 0.88 = 8.8 mg of peptide, a twelve per cent shortfall against label with a perfectly clean purity figure.

Purity says nothing about counter-ion. They are unrelated measurements.

Stated carefully, a net-peptide-content figure already accounts for it, which is why net content is the number worth asking for.

Trifluoroacetate has documented biological effects at higher concentrations in cell culture, which is the technical reason acetate exchange exists rather than a marketing one.

Absence of a counter-ion figure is an absent measurement rather than a low one.

Trifluoroacetate unless a deliberate exchange was done. Ask which.

Worth being precise here: acetate is generally preferred to trifluoroacetate for biological work, which is why the exchange is done at all.

The number of basic residues drives counter-ion load, because each protonated basic site pairs with one counter-ion. A sequence with several lysines and arginines carries proportionally more.

Water plus counter-ion is why gross mass and peptide mass differ.