Research peptides are built one residue at a time, on a solid support, from the C-terminus backwards. The method is solid-phase peptide synthesis, it has been the standard since the 1960s, and almost everything that shows up on a certificate — the purity figure, the identity of the impurities, the counter-ion, the limits on what lengths are practical — follows from how it works. Knowing the process makes a certificate readable rather than a set of unrelated numbers.
The cycle
The first residue is anchored to an insoluble resin bead. Then the same two steps repeat for every subsequent residue:
- Deprotection. The temporary protecting group on the growing chain's amino terminus is removed, exposing it.
- Coupling. The next amino acid — itself protected everywhere except the end that must react — is activated and joined to the exposed terminus.
Between steps the resin is washed. That is the insight the whole method rests on: because the growing chain is attached to a solid bead, excess reagents and by-products are removed by filtration rather than by purifying an intermediate. Each cycle is driven to completion with a large excess of reagent, and cleanup is a rinse.
Synthesis runs C-terminus to N-terminus, which is the reverse of how a ribosome builds a protein. Sequences on a certificate are written N to C by convention, so the first residue installed is the one written last.
Why is synthesis done backwards relative to biology?
Because the chemistry is cleaner in that direction. Activating a carboxyl group to form the new bond risks racemisation — scrambling the stereochemistry at the adjacent carbon — and the risk is lower when the activated species is the incoming amino acid rather than the growing chain. Building C to N means the activated partner is always a single protected residue, which is the arrangement that keeps stereochemical integrity manageable.
Why length is the limiting factor
Each coupling step has a yield slightly below 100%, and the losses compound. At 99% per step, a 10-residue peptide finishes around 91% intact; a 30-residue peptide around 74%; a 50-residue peptide around 61%. At 99.5% the same three come out near 96%, 86% and 78%.
| Length | At 99.0% per step | At 99.5% per step |
|---|---|---|
| 10 residues | ~91% | ~96% |
| 20 residues | ~83% | ~91% |
| 30 residues | ~74% | ~86% |
| 50 residues | ~61% | ~78% |
This is why most research peptides are short, why a 40-residue sequence costs disproportionately more than a 20-residue one, and why the practical ceiling for routine synthesis sits around 50 residues. Beyond that, fragments are made separately and joined, or the molecule is expressed recombinantly instead.
What is a deletion sequence, and why does it matter?
It is the product of a coupling step that failed. If a residue fails to attach on one chain, that chain continues growing without it, and the final product is the target sequence missing one residue. It is nearly the right molecule: almost the same mass, almost the same hydrophobicity, and therefore almost the same retention time on the column. Deletion sequences are the impurities hardest to separate, which is exactly why they dominate the missing percent on a purity figure. Distant impurities come out in preparative chromatography. The near ones are what is left.
What is capping?
A deliberate step to make failures easier to remove. After each coupling, any chain that failed to react is blocked permanently with acetic anhydride, so it cannot participate in later cycles. Instead of accumulating a deletion sequence differing from the target by one residue, the process produces a truncated fragment that stopped growing — which is much further from the target in hydrophobicity and much easier to separate. Capping does not improve yield; it improves purifiability, which is usually the better trade.
Cleavage, and where the counter-ion comes from
When the chain is complete, the peptide is cut from the resin and the side-chain protecting groups are removed — typically in one step, with trifluoroacetic acid and a mixture of scavengers that mop up the reactive fragments released. The crude product is precipitated, then purified by reversed-phase HPLC.
That purification is run with TFA as the ion-pairing additive, so the peptide pairs with trifluoroacetate when the fractions are dried. The counter-ion on the certificate is inherited from the method rather than chosen for the molecule, which is why TFA is the default and acetate is the conversion.
Why are peptides supplied as a lyophilised powder?
Because a dried solid is far more stable than a solution. Water is a reactant in most degradation routes — hydrolysis of the backbone, deamidation of asparagine and glutamine — and removing it slows all of them. Freeze-drying takes the purified fractions to a dry cake that can be stored and shipped without a cold chain over short periods. The consequence is that the powder is hygroscopic and will draw atmospheric moisture, which is why a cold vial is equilibrated to room temperature before opening rather than after.
Does the synthesis route show up on a certificate?
Indirectly, and usefully. The impurity profile is a fingerprint of the process: deletion sequences point at incomplete coupling, truncated fragments at capping, incompletely deprotected material at an insufficient cleavage step, oxidised methionine or tryptophan at exposure during workup. A certificate that names its impurities rather than reporting an anonymous remainder is describing the synthesis. Most do not, which is one reason a chromatogram is worth more than the figure derived from it.
Why do two lots of the same sequence differ?
Because each is an independent run of a process with many steps, each carrying its own small variation. Coupling efficiency depends on the sequence context — a stretch that aggregates on the resin couples less well — and on reagent quality and timing. The purification then makes its own cut decisions about where a collected fraction begins and ends. The result is that lots of the same peptide can differ in purity, in impurity profile, in water content and in counter-ion burden, all within specification. That is not a defect; it is what batch manufacture is, and it is the reason a certificate is tied to a lot number rather than to a product.
All products referenced here are supplied for laboratory and research use only. They are not drugs, foods, supplements or cosmetics, and are not for human or veterinary use.
