How a research peptide is actually made

There is a habit in this market of treating a peptide as a thing that either is or is not what the label says. The chemistry does not work that way, and the reason is worth understanding before reading any certificate of analysis.

The idea that made it possible

Before 1963, making a peptide meant purifying an intermediate after every single coupling, slow, lossy, and impractical beyond a few residues.

Merrifield's insight was to anchor the growing chain to an insoluble support1. Once the first residue is attached to a resin bead, everything else can be done by flooding the vessel with reagents and then washing them out. The product cannot be lost in the wash because it is physically tethered.

That is the whole trick, and it is why a 39-residue peptide like tirzepatide is manufacturable at all.

The cycle

Each residue is added the same way:

1. Deprotect, remove the temporary group blocking the end of the chain.
2. Wash.
3. Couple, activate the next amino acid and let it react with the freed end.
4. Wash.

Repeat. For a 15-residue peptide that is thirty chemical operations before the chain is even cut from the resin.

Two chemistries define how step 1 is done. Boc chemistry uses strong acid at every cycle and needs in-situ neutralisation to run fast and cleanly3. Fmoc chemistry uses a mild base instead, which is gentler on sensitive sequences and avoids repeated acid exposure2. Most research-grade peptide today is Fmoc-made.

Why purity is a percentage

Here is the arithmetic that governs everything downstream.

Suppose each coupling proceeds at 99% efficiency, a good number in practice. After 30 steps, the fraction of chains that received every single residue is 0.99³⁰, which is about 74%. At 98% per step it falls to roughly 55%.

The missing quarter is not dirt. It is mostly deletion sequences: chains identical to the target except that one residue never attached. They have almost the same size, almost the same charge, and often behave almost the same way in a purification column, which is exactly why removing them is the expensive part of making a peptide.

This is the honest explanation of why nobody sells "100% pure" peptide, and why a supplier who claims it is describing marketing rather than chemistry.

What happens after the chain is finished

The peptide is cleaved from the resin, usually with a strong acid mixture that simultaneously strips the side-chain protecting groups. Then it is purified, typically by reversed-phase chromatography, and dried.

Three consequences follow, and all three show up in what you receive:

• Salt form. The acid used in cleavage and purification leaves a counter-ion on the basic residues. That is why so much research peptide arrives as a trifluoroacetate or acetate salt, a topic with real consequences for how much peptide is in the vial.
• Residual solvents and scavengers. Cleavage cocktails contain scavenger reagents to mop up reactive fragments; traces can persist.
• Sequence-specific difficulty. Sequences rich in hydrophobic residues or prone to aggregation on the resin couple less efficiently, so their crude purity is lower before purification even begins.

What this means when you read a label

A stated purity of ">99% by HPLC" is a statement about the chromatographic profile of the finished, purified material, not about the synthesis. It says the purification worked, not that the synthesis was perfect.

It also says nothing about how much peptide is in the vial, because salt and residual water are not impurities in the chromatographic sense; they are simply other things present. Those are separate numbers, and we cover them in a separate piece.

The practical takeaway is narrow but useful: when a preparation behaves unexpectedly, the likely culprits are the predictable ones. A deletion sequence that survived purification. A salt form different from the one your calculation assumed. A hydrophobic sequence that never fully dissolved. Chemistry constrains the failure modes, which is what makes them worth knowing.