Why 10 mg of peptide is not 10 mg of peptide

A vial says 10 mg. You dissolve it in 2 mL and record a 5 mg/mL stock. That number is almost certainly wrong, and the size of the error is not trivial.

Where the salt comes from

Reversed-phase purification runs in an acidic mobile phase, conventionally containing trifluoroacetic acid. TFA improves peak shape and resolution, which is why it became standard.

It also does something less convenient: it pairs with every positively charged group on the peptide. When the fractions are dried, those pairs stay. The peptide leaves the process as its trifluoroacetate salt, and TFA removal is a deliberate extra step requiring its own optimisation1.

The arithmetic

Trifluoroacetate has a molecular weight of about 114. Each basic site, lysine, arginine, histidine to a degree, plus the free N-terminus, can carry one.

Take a peptide of nominal mass 1400 with three basic sites. Fully associated, the salt weighs roughly 1400 + 3×114 ≈ 1742. The peptide is therefore about 80% of the salt mass before any water is considered.

Now add moisture. Lyophilised solids are hygroscopic by construction, freeze-drying leaves a porous cake with enormous surface area, and residual moisture is a standard formulation concern2. A few percent is normal; more is easy to acquire if a cold vial is opened in humid air.

Combine the two and 10 mg of material can plausibly contain 7-8 mg of peptide. For a screening experiment that may not matter. For anything where molar concentration is the independent variable, a 20-30% error in the stock is not acceptable.

Why the choice of counter-ion also matters biologically

It would be convenient if the salt were merely dead weight. It is not always.

Residual trifluoroacetate carried into cell culture inhibited proliferation of osteoblasts and chondrocytes in a study specifically examining it as a contaminant of purified proteins3. The effect was attributable to the counter-ion rather than the protein.

This is the practical reason peptide intended for cell work is often supplied as the acetate salt: acetate is a normal metabolite and far less likely to produce an artefact. Acetate exchange is an additional processing step, which is part of why acetate-form material costs more.

If a preparation produces an unexpected inhibition in culture, the counter-ion belongs on the list of suspects before the peptide does.

The number that resolves it

Peptide content, sometimes called net peptide content, is the fraction of the vial's mass that is peptide. It is determined by amino acid analysis after hydrolysis, or by elemental nitrogen determination, and reported as a percentage.

With it, the calculation is straightforward: 10 mg of material at 78% peptide content is 7.8 mg of peptide, and your 2 mL stock is 3.9 mg/mL rather than 5.

Without it, you are estimating. A reasonable estimate is better than none, and the estimate is bounded: identify the basic residues in the sequence, assume they are salted, assume a few percent water.

What to do in practice

• Check the certificate for a salt form. "TFA salt" or "acetate salt" changes the mass calculation. Its absence is itself information.
• Do not treat net mass as molar amount in any protocol where concentration is the variable being tested.
• Weigh into the buffer, not the other way round, when working with small amounts, transferring a weighed powder loses material to static and to the vessel.
• Store closed and cold, warm before opening. Every opening at room temperature is an opportunity for the powder to gain water, which shifts the mass fraction further.

None of this is a defect in the material. It is what a purified synthetic peptide is: a salt of an organic acid, holding some water, at a purity that a chromatogram describes and a mass measurement confirms. The label states the mass in the vial, and the rest is arithmetic you have to do.