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How are peptides made?

A peptide is not harvested but built, one amino acid at a time, onto a grain of plastic. What happens afterwards decides whether you end up with a usable powder or a mixture of half-finished chains.

Building on a bead

The standard method is called solid phase peptide synthesis, or SPPS. The idea dates from 1963 and earned its inventor a Nobel prize. It works like this: the first amino acid is attached to a small bead of plastic. Then the next amino acid is coupled to it, then the next, until the chain is finished.

The trick is in that bead. Because the growing chain is anchored, after every step you can simply wash away everything that did not react. You do not have to purify after each coupling. That makes the process suitable for a machine, which is why a peptide of fifteen amino acids can now be built in a day or two.

Every step goes 99 percent right

A coupling rarely succeeds a hundred percent. Suppose each step succeeds 99 percent of the time: after fifteen steps you are left with about 86 percent complete chains. After thirty steps that has dropped to 74 percent. The rest is truncated chains that stalled somewhere along the way.

That small calculation explains two things. First, why long peptides cost more than short ones: more is simply lost. Second, why purification afterwards is not a luxury but a necessity. The raw product out of the machine is always a mixture.

Purifying with HPLC

After synthesis the chain is cut loose from the bead and the mixture goes through an HPLC column. HPLC stands for high performance liquid chromatography. Think of it as a long tube packed with grains through which the mixture is forced under high pressure.

Every substance in the mixture sticks to those grains slightly differently, and so comes out the other end at a different moment. By collecting only the slice where your peptide emerges, you separate it from the rest. Run the result through the column once more to measure, and the size of the peak gives you the purity figure.

Confirming with mass spectrometry

HPLC tells you what percentage of the sample is one substance, but not which substance that is. For that a second measurement is needed: mass spectrometry, usually shortened to MS. It weighs the molecule. If the measured weight matches the calculated weight of the intended sequence, you have confirmation that you hold the right substance.

A certificate with only an HPLC figure is therefore incomplete. It says something pure is present, not that it is the right thing. Only the combination of both measurements gives a real answer.

Freeze drying to a powder

What remains after purification is a solution. It is frozen and then put under vacuum, so the ice evaporates directly without becoming liquid first. That is called lyophilisation, or freeze drying.

The result is the white, airy cake you see at the bottom of a vial. Sometimes it is a solid disc, sometimes a film of powder, sometimes transport has shaken it loose into what looks like nothing at all. That last case says nothing in itself about quality: a few milligrams take up hardly any space.

Where it goes wrong

Most quality differences between suppliers arise not at synthesis but at purification. A column run costs time and money, and anyone who wants to supply more cheaply can save there most easily: cut less sharply, accept a broader peak, skip a second round.

You cannot see that in the powder. The only thing you can know about it is on the certificate of analysis for that specific batch. A certificate belonging to a different lot number than the one on your vial says nothing about what you are holding.

Updated 2026-10-03

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