How Are Peptides Made? Solid-Phase Synthesis Explained

Cover illustration: AI-generated editorial artwork. Conceptual only; not an exact molecular structure, actual laboratory result or evidence of clinical effects.

Almost every research peptide in a catalogue is built the same way: one amino acid at a time, on a solid resin bead. The method is solid-phase peptide synthesis, introduced by Bruce Merrifield, and understanding its four stages explains most of what appears on a certificate of analysis, including why purity and content are different numbers.

For laboratory research use only. The research materials supplied by HealthyWelness are not approved medicines, foods or cosmetics and are not for human or veterinary use. This article summarises published research and is not medical advice.

Key points

Four stages: chain assembly, cleavage, purification, lyophilization.
The chain is built C-terminus to N-terminus, the opposite of biological synthesis.
Each coupling is slightly less than perfect, which is where impurities come from.
HPLC with TFA explains the usual trifluoroacetate counter-ion.
Freeze-drying sets the residual moisture that affects shelf life.

Stage 1: building the chain

The C-terminal residue is anchored to an insoluble resin. Each subsequent residue is added in a repeating four-step round, and because the chain stays attached, excess reagents can simply be washed away between steps. Merrifield’s original description set out this principle (Merrifield, 1969), and the method is now routinely automated, with synthesisers making many peptides in parallel (Winkler, 2020).

Step in each round	Purpose
Deprotection	Remove the temporary group from the chain’s free end
Washing	Remove deprotection reagent and by-products
Coupling	Attach the next activated amino acid
Washing	Remove unreacted amino acid and coupling reagents

A 15-residue peptide such as BPC-157 requires 14 of these rounds; a 44-residue one such as tesamorelin requires 43. Side-chain protecting groups stay in place throughout so that reactive side chains do not interfere.

Why length matters for purity

No coupling is 100% efficient. If each step proceeds at 99%, the theoretical yield of full-length product after n couplings is 0.99ⁿ.

Peptide length	Couplings	Theoretical yield at 99% per step
7 residues (Semax)	6	~94%
15 residues (BPC-157)	14	~87%
29 residues (sermorelin)	28	~75%
44 residues (tesamorelin)	43	~65%

These are illustrative figures from a simple calculation, not manufacturing data, but they explain two real things: why longer peptides cost more, and why deletion sequences, chains missing one or more residues, are the characteristic impurity of this method. Purification has more work to do as length increases.

Stage 2: cleavage

A strong acid, usually trifluoroacetic acid with scavengers, releases the peptide from the resin and removes side-chain protecting groups at the same time. The crude peptide is then precipitated. This step is where the trifluoroacetate counter-ion enters the picture.

Stage 3: purification

Crude peptide is purified by reverse-phase HPLC, which separates the target from deletion sequences and other by-products by hydrophobicity. The mobile phase typically contains TFA as an ion-pairing agent, which is the second reason the product ends up as a trifluoroacetate salt. For cationic peptides this counter-ion binds tightly, and exchanging it for acetate or chloride needs a deliberate procedure; the classic approach of repeated freeze-drying from strong acid risks degrading the peptide (Roux et al., 2008).

This stage is also why “HPLC purity” exists as a specification: it is literally a measurement of how well this separation worked, read from the same kind of chromatogram used to perform it. See how peptide purity is tested.

Stage 4: freeze-drying

Purified peptide in aqueous solution is frozen and the ice removed by sublimation under vacuum, leaving the dry cake found in a vial. The process has three phases, freezing, primary drying and secondary drying, and the physical event that governs it is the glass transition of the freeze-concentrated solution (Nail et al., 2002). Residual moisture is a controlled parameter: too much accelerates degradation, and over-drying has its own drawbacks (Hsu et al., 1992). See what are lyophilized peptides?

What ends up in the vial

Component	Comes from
Target peptide	Successful synthesis and purification
Counter-ion	Cleavage acid and HPLC mobile phase
Residual water	Incomplete removal during secondary drying
Trace related substances	Deletion or truncated sequences surviving purification

Only the first and last of these appear in an HPLC purity figure, which is why net peptide content is reported separately. Degradation products that appear later, such as deamidation or hydrolysis species, come from storage rather than synthesis and are detected by the same analytical techniques (Reubsaet et al., 1998).

Why not make them biologically?

Recombinant expression in bacteria or yeast is how larger proteins such as growth hormone are produced. For short peptides, chemical synthesis usually wins on speed and flexibility: it allows non-natural residues such as the aminoisobutyric acid in tirzepatide and ipamorelin, D-amino acids, N-terminal acetylation and C-terminal amidation. None of those can be made directly by a ribosome.

Reconstitution of the finished product is covered in how to reconstitute peptides, and solvent choice in bacteriostatic water for peptides. Our bacteriostatic water is supplied for that purpose, and the peptide reconstitution calculator handles the maths.

Open questions

How much do deletion-sequence profiles vary between manufacturers for the same peptide?
At what length does recombinant production become preferable to synthesis?
How often is counter-ion exchange actually performed on research-grade material?

Frequently asked questions

How are research peptides made?

By solid-phase peptide synthesis: the chain is built one amino acid at a time on an insoluble resin support, then cleaved from the resin, purified by reverse-phase HPLC and freeze-dried. The method was introduced by Bruce Merrifield.

Why is synthesis done on a solid support?

Because the growing chain stays attached to the resin, excess reagents and by-products can be washed away at each step without isolating an intermediate. That is what makes the procedure repeatable and automatable.

What are deletion sequences?

Peptides missing one or more residues because a coupling step did not go to completion. They are the characteristic impurity of solid-phase synthesis and show up on HPLC and MS as species of lower mass.

Why are peptides supplied as TFA salts?

Trifluoroacetic acid is used in cleavage and in HPLC mobile phases, so the purified peptide is usually isolated as a trifluoroacetate salt unless a deliberate exchange step is performed.

Can peptides be made biologically instead?

Yes, by recombinant expression, which is how larger proteins such as growth hormone are produced. For short research peptides, chemical synthesis is usually faster and allows non-natural residues.