How research peptides are made

Almost all research peptides are made by solid-phase peptide synthesis (SPPS), the method introduced by Bruce Merrifield in 1963 and recognised with the Nobel Prize in Chemistry in 1984.

The principle is that the growing peptide chain stays anchored to an insoluble resin bead. Because the product is attached to a solid support, excess reagents and by-products can simply be washed away at every step rather than requiring isolation. That is what makes stepwise synthesis of a long chain practical at all.

The chain is assembled from the C-terminus towards the N-terminus — the opposite direction to biological ribosomal synthesis.

Each residue is added by repeating a four-step cycle:

1. Deprotection. The temporary protecting group on the N-terminus of the resin-bound chain is removed, exposing a free amine. In Fmoc chemistry — now the dominant approach — this group is removed with a base, typically piperidine.
2. Washing. Reagents and cleaved protecting group are washed away.
3. Coupling. The next amino acid, itself N-protected and side-chain protected, is activated and reacted with the free amine to form a new peptide bond.
4. Washing. Excess activated amino acid and coupling reagents are removed.

The cycle repeats once per residue. A 30-residue peptide requires 30 cycles, and each cycle is an opportunity for the reaction to be incomplete.

This is the section that explains most of what appears on a chromatogram.

No coupling step is 100% efficient. If a coupling reaction reaches 99% completion, 1% of chains on the resin did not receive that residue. Those chains continue into the next cycle and receive the following residue instead — producing a deletion sequence, a peptide identical to the target but missing one amino acid.

The arithmetic compounds. At 99% efficiency per coupling, a 30-residue peptide has a theoretical maximum yield of 0.99³⁰, or about 74% of chains being the full correct sequence. At 98% it falls to about 55%. This is why longer peptides are harder and more expensive to make well, and it is the mechanism behind why peptide prices vary.

Truncation sequences arise differently: chains that stop growing entirely, usually because the N-terminus becomes inaccessible through aggregation on the resin. These are typically capped deliberately — acetylated after each coupling — so they stop competing and become easier to separate.

Deletion sequences are the difficult impurity because they differ from the target by a single residue and therefore have very similar hydrophobicity. That similarity is exactly what makes them prone to co-elution, as covered in HPLC purity explained.

Once the chain is complete it must be released from the resin and stripped of side-chain protecting groups. In Fmoc chemistry this is done with a cleavage cocktail based on trifluoroacetic acid (TFA), usually with scavengers to mop up the reactive fragments released from the protecting groups.

The crude peptide is then precipitated, and purified by preparative reversed-phase HPLC — whose mobile phase also contains TFA.

This is where the counterion comes from. TFA associates with basic residues (lysine, arginine, histidine and the free N-terminus), and it stays with the peptide through lyophilisation. It is not a contaminant that slipped in; it is a structural consequence of the purification method. Where a different counterion is required, an explicit salt-exchange step to acetate or hydrochloride is performed afterwards.

TFA content is a significant part of the gap between nominal vial weight and net peptide content.

Preparative RP-HPLC separates the target from the deletion, truncation and oxidised species generated during assembly. Fractions are collected across the main peak, analysed, and the acceptable ones pooled.

Where the collection window is drawn is a judgement with a direct commercial consequence: a narrow window gives higher purity and lower yield, a wide one the reverse. Two suppliers running the same crude material to different specifications will legitimately produce different products at different prices.

The pooled fractions are then lyophilised — frozen and dried under vacuum by sublimation — leaving the porous solid cake supplied in the vial, along with residual water and the counterion.

Many research peptides are not plain linear sequences. Common modifications each add manufacturing steps and their own impurity classes:

• Disulfide bridges require an oxidation step after cleavage, with the risk of incorrect pairings in multi-cysteine sequences.
• C-terminal amidation requires a specific resin chosen at the outset.
• Acetylation, PEGylation or lipidation add coupling steps and produce partially-modified species.
• Cyclisation competes with intermolecular reaction, generating dimers and oligomers.

Each of these is a reason a particular compound costs more than its length alone would suggest, and a reason its chromatogram has features a simple linear peptide does not.