How synthetic peptides are made

Most synthetic peptides are made by solid-phase synthesis. The chain is built one amino acid at a time while it is held on a solid support, a resin. When the chain is complete, it is cut free and its protecting groups are removed. The crude peptide is then purified by chromatography and is usually freeze-dried into a powder. Each step can leave by-products, which is why purity and identity are tested afterwards.

This page follows the steps using the words of the European Medicines Agency's guideline for synthetic peptides in medicines, which took effect on 1 June 2026. It is a European standard for medicines. We quote it to explain the process, not to describe how any product was made.

Where synthetic peptides sit

The guideline opens by placing peptides between two familiar groups:

"Synthetic peptides are at the interface of small molecules and proteins"

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 2, retrieved 12 October 2026.

It also separates them from proteins made by living cells:

"This guideline is not applicable to biological and biotechnological products manufactured by recombinant technologies"

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 2, retrieved 12 October 2026.

"Synthetic" here means built by chemistry, step by step, not grown in cells. For where the line between a peptide and a protein falls, see peptide vs protein.

Step 1: start on a solid support

In solid-phase synthesis, the growing chain is anchored to an insoluble material. The guideline describes the method in a few words:

"the peptide sequence is built up on a solid support"

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.2.2, retrieved 12 October 2026.

The support is a resin, often bought with the first building block already attached:

"Peptide synthesis resins preloaded with the first amino acid of the peptide sequence"

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.2.3, retrieved 12 October 2026.

Because the chain stays fixed to the resin, the chemicals used in each step can be washed away before the next one.

Step 2: couple, cap, deprotect, repeat

Each new amino acid arrives with its reactive end covered by a protecting group, so it can only join in one place. After it joins (coupling), unreacted chains may be blocked so they stop growing (capping), and the protecting group on the new end is removed (deprotection). Then the cycle repeats. The guideline names the usual check on these steps:

"The most common test for the monitoring of coupling, capping and deprotection reactions is the Kaiser test"

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.2.4, retrieved 12 October 2026.

Capping explains one kind of by-product. The guideline calls the capping step acetylation, and says:

"The acetylation procedure results in acetylated peptide fragments (truncated sequences)."

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.3.2, retrieved 12 October 2026.

In plain words: chains that were capped stop short, and those short chains are part of what purification has to remove.

Step 3: cut the chain free

When the last amino acid is in place, the chain is released from the resin and the remaining protecting groups come off. The guideline asks that this step be fully written up:

"The final cleavage/deprotection step should be described in detail, including any use of scavengers and other reagents."

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.2.2, retrieved 12 October 2026.

What comes off the resin is not the finished material:

"The crude peptide after cleavage and deprotection is typically considered to be an intermediate."

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.2.4, retrieved 12 October 2026.

Step 4: purify

The crude peptide is a mixture of the target chain and related by-products. It is cleaned up by chromatography:

"Synthetic peptides are generally purified using chromatographic techniques"

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.2.2, retrieved 12 October 2026.

The same family of methods is later used to measure purity; see peptide purity and HPLC. The finished powder also carries a salt partner, the counter-ion; see TFA and acetate salts.

Step 5: isolate as a powder

Last, the purified peptide is taken out of solution. The guideline names the usual way:

"Lyophilisation of synthetic peptides is considered common practice."

European Medicines Agency, Guideline on the Development and Manufacture of Synthetic Peptides, section 4.2.2, retrieved 12 October 2026.

Lyophilisation is freeze-drying: the solution is frozen and the water is drawn off under vacuum, leaving a dry powder. That powder still takes up water from the air, which is why storage matters; see storing lyophilized peptides.

Why the process explains the tests

Each step above can leave something behind: chains with an amino acid missing, short capped chains, altered chains, leftover salt and water. That is why a batch is tested for identity and purity at the end, and why those tests belong on a certificate of analysis. Our page on what is in a synthetic peptide besides the peptide goes through the by-products one by one.

How a peptide is made says nothing about how it may be sold. In Canada, Health Canada's position is that:

"In Canada, peptides are generally regulated as prescription drugs."

Health Canada public advisory, 9 Apr 2026, retrieved 4 October 2026.

For what that means, read are peptides legal in Canada?.

The next science pages cover impurities and the peptide-protein line. Get the free library updates by email.