A research peptide is not harvested but built, one amino acid at a time, on a bead of resin. That method explains both why there are counter-ions in the vial and why a purity percentage is never exactly 100. This guide walks through the process.
In solid-phase synthesis the chain grows on a solid support, from C- to N-terminus, in a repeated cycle of deprotection and coupling. Every cycle leaves a small shortfall, and those shortfalls become the related impurities HPLC has to remove later.
Why a solid support?
Because it makes the separation problem disappear: as long as the growing peptide is attached to an insoluble bead, all excess reagent can simply be washed off. Without that trick, a full purification would be needed after every step, and for a chain of thirty amino acids that is not workable.
Robert Bruce Merrifield introduced the principle in 1963 and received the Nobel Prize in Chemistry for it in 1984. Virtually every synthetic research peptide in existence today comes about via this route.
The cycle
Assembly runs from C-terminus to N-terminus, the reverse of what a ribosome does, and consists of one cycle repeated as many times as there are amino acids.
- 1Anchoring
The C-terminal amino acid is fixed to the resin. That determines which end is freed later.
- 2Deprotection
The temporary protecting group on the N-terminus is removed so that side can react again. In the common Fmoc strategy that is done with a base.
- 3Coupling
The next amino acid is added in activated form and makes a new peptide bond with the freed N-terminus. Then a wash, and the cycle starts again.
- 4Cleavage
Once the chain is complete, the peptide is cleaved from the resin with strong acid and the remaining protecting groups are removed.
Where the impurities come from
Every coupling and every deprotection has a yield just under 100%, and across dozens of cycles those shortfalls add up. If one coupling fails, a chain missing exactly one amino acid appears. If a protecting group stays put, a molecule results whose mass is slightly off.
Those by-products closely resemble the target molecule — hence the requirement that the separation be fine enough to still resolve them as a separate peak. How that works is in what is HPLC. Purification in practice uses preparative HPLC: the same technique, now deployed to separate rather than to measure.
From resin bead to vial
Cleavage is typically done with trifluoroacetic acid, and that explains why TFA counter-ions come back in the final product. They weigh in inside the vial without counting toward the purity percentage — precisely the difference between purity and net peptide content.
After purification comes lyophilisation: the water leaves by sublimation and what remains is the powder you find in the vial. Identity is then confirmed by mass spectrometry, the batch receives a lot number and is tied to a certificate. Our thresholds are on quality standards.
This information is general and educational in nature. Peptide Lab supplies research peptides for laboratory and research purposes only. Not for human or animal consumption, not a medicine and not medical advice.
Frequently asked questions about peptide synthesis
What is solid-phase peptide synthesis (SPPS)?
SPPS is the standard method for making peptides chemically. The first amino acid is anchored to a solid support (a resin bead), after which the chain is built one amino acid at a time. Because the product stays attached to the support, excess reagent can simply be washed away.
In which direction is a peptide built chemically?
From the C-terminus to the N-terminus — the reverse of how a ribosome does it. That follows from the method: the C-terminal amino acid is the one anchored to the resin bead.
Why is there TFA in a synthesised peptide?
Because the peptide is cleaved from the support with trifluoroacetic acid and often purified afterwards with a TFA-containing mobile phase. What remains are TFA counter-ions: they add to the weight in the vial but not to the purity percentage from HPLC.
Why is a synthesised peptide never 100% pure?
Every cycle has a yield just under 100%, and those small shortfalls accumulate over dozens of steps. The result is chains missing one amino acid or retaining a protecting group — compounds that closely resemble the target molecule and therefore have to be purified out.
