How synthetic peptides are made

Nearly every research peptide is made by chemical synthesis on a solid support, purified by reversed-phase HPLC and freeze-dried. Knowing the steps explains most of what a certificate of analysis reports: why peptides are salts, which impurities appear, why some sequences are harder to make than others, and why identity and purity are tested separately. This article follows a peptide from the first amino acid to the finished powder.

The principle: building on a bead

Solid-phase peptide synthesis, introduced by R. B. Merrifield in 1963, builds the chain one amino acid at a time while it stays attached to an insoluble resin bead. Because the growing peptide is anchored, each reaction can be pushed to completion with excess reagent and the excess simply washed away. The chain is built from its C-terminus toward its N-terminus, the reverse of the direction in which a sequence is written.

Each amino acid arrives with two kinds of protection. A temporary protecting group blocks its amino group so that it couples only once; a permanent protecting group blocks any reactive side chain until the very end. In the dominant chemistry today, Fmoc/tBu, the temporary group is Fmoc (9-fluorenylmethoxycarbonyl), removed by a mild base, and the permanent groups are acid-labile, such as tert-butyl on serine, threonine, tyrosine, aspartic and glutamic acid, Boc on lysine and tryptophan, trityl on cysteine, asparagine, glutamine and histidine, and Pbf on arginine.

The synthesis cycle

One cycle of Fmoc solid-phase synthesis, repeated for every residue

Step	What happens	Typical reagents
Deprotection	The Fmoc group is removed from the chain's N-terminus, exposing a free amine	20% piperidine in dimethylformamide
Wash	Piperidine and by-products are washed out	Dimethylformamide
Coupling	The next Fmoc-amino acid is activated and forms a peptide bond with the free amine	A coupling reagent such as HATU, HBTU or DIC with Oxyma, and a base
Wash	Excess amino acid and reagent are washed out	Dimethylformamide
Capping (optional)	Any chain that failed to couple is acetylated so it stops growing	Acetic anhydride

A peptide of 30 residues needs 30 cycles. Even at 99 percent completeness per step, the share of chains that are complete falls with every cycle (0.99³⁰ is about 74 percent), which is why long sequences are harder to make clean and why purification is essential.

Termini and modifications

• C-terminus. The resin decides it. A Wang or 2-chlorotrityl resin releases a free acid; a Rink amide resin releases a C-terminal amide.
• N-terminus. After the last cycle the amine is left free, or acetylated on the resin with acetic anhydride, or coupled to another acyl group.
• Non-standard residues. D-amino acids, norleucine, α-aminoisobutyric acid and other building blocks are coupled like any other Fmoc-amino acid.
• Disulfide bonds. Usually formed after cleavage, by oxidizing the free cysteines in dilute solution with air, dimethyl sulfoxide or iodine. Peptides with two or more disulfides use orthogonal cysteine protection so that each pair is joined in turn.
• Lactam bridges and other rings. Formed on the resin or in solution between side chains protected with groups that can be removed selectively.

Cleavage and isolation

When the chain is complete, a cleavage cocktail of about 95 percent trifluoroacetic acid releases the peptide from the resin and removes the permanent side-chain protection in one step. Scavengers, such as water, triisopropylsilane and ethanedithiol, capture the reactive carbocations released from the protecting groups before they can attach themselves to tryptophan, methionine, tyrosine or cysteine.

The crude peptide is precipitated in cold ether, washed and dried. At this stage it typically contains the intended peptide alongside deletion sequences, truncated chains and protecting-group adducts.

Purification and freeze-drying

The crude material is purified by preparative reversed-phase HPLC, the larger-scale form of the analytical method used to measure purity. Fractions are collected across the main peak, checked by analytical HPLC and mass spectrometry, and those that meet the purity target are pooled.

The pooled solution, mostly water and acetonitrile with a little trifluoroacetic acid, is freeze-dried. What remains is the peptide as its trifluoroacetate salt, holding some water. If another salt form is wanted, an exchange step follows; the salt forms article describes it.

Where impurities come from

The impurities in a purified peptide trace back to particular steps. They are what HPLC separates and mass spectrometry identifies, and they are the variants listed under "What to watch for analytically" on each peptide reference entry.

Synthesis-related impurities

Impurity	Cause	Mass change
Deletion sequence	An incomplete coupling, followed by a successful one in the next cycle	Minus one residue
Truncated sequence	A chain that stopped growing, often capped	Minus several residues; plus 42.011 Da if acetylated
Incomplete deprotection	A side-chain protecting group not removed in cleavage	For example +252.082 Da (Pbf), +56.063 Da (tert-butyl)
Diastereomer	Racemization of a residue during activation, notably cysteine and histidine	None
Aspartimide products	Ring closure at aspartic acid during the repeated base steps, especially before glycine, then re-opening	−18.011 Da (aspartimide); none for the β-aspartyl and D-aspartyl forms; +67.079 Da as piperidides
Scavenger and cation adducts	Reactive species from cleavage attaching to tryptophan, tyrosine, methionine or cysteine	Varies with the species
Oxidation	Methionine or tryptophan exposed to air or peroxides	+15.995 Da

Impurities with no mass change, diastereomers and some aspartimide products, are the hardest to find: mass spectrometry cannot see them, and they elute close to the main peak. Synthesis conditions designed to suppress them, such as additives in the deprotection step and lower coupling temperatures for sensitive residues, matter more than detection after the fact.

Difficult sequences

Some sequences resist synthesis. Stretches of hydrophobic or β-branched residues (valine, isoleucine, threonine) can fold and stick together on the resin, so that the growing chain becomes inaccessible and couplings fail repeatedly. Chemists respond with heat or microwave energy, stronger solvents, pseudoproline and backbone-protected building blocks that disrupt folding, and lower resin loading. Microwave heating speeds couplings but can increase racemization and aspartimide formation, so conditions are tuned for sensitive residues.

Long peptides and proteins

Stepwise synthesis becomes impractical somewhere around 50 to 70 residues, depending on the sequence. Longer chains are assembled from shorter synthetic segments, most often by native chemical ligation, which joins a peptide carrying a C-terminal thioester to one beginning with cysteine through a native peptide bond. Very long chains are usually made biologically, by recombinant expression, which brings a different set of impurities, including host-cell proteins and endotoxin.

Frequently asked questions

Why are synthetic peptides usually trifluoroacetate salts?

Trifluoroacetic acid is used to cleave the peptide from the resin and in the HPLC purification. The peptide's positive charges keep trifluoroacetate as their counter-ion through freeze-drying.

What is a deletion sequence?

A peptide missing one residue, made when a coupling step fails on some chains but the next succeeds. It is very similar to the intended peptide, so it often elutes nearby, and mass spectrometry shows it as the full mass less one residue.

Why do purity specifications get harder to meet for longer peptides?

Every cycle adds a small chance of a failed coupling or side reaction, so the crude product of a long synthesis contains more closely related impurities, which are harder to separate from the intended peptide.

Can mass spectrometry find every synthesis impurity?

No. Diastereomers and some aspartimide-derived products have exactly the same mass as the intended peptide. They are found by chromatography, ideally with a method developed to separate them.

Are synthetic peptides made by bacteria?

No. Chemical synthesis uses no living organisms. Endotoxin can still be introduced by water, buffers or equipment during purification and handling, which is why it is tested when cell-based work needs it.

Sources & Further Reading

1. Merrifield RB. Solid phase peptide synthesis. I. The synthesis of a tetrapeptide. Journal of the American Chemical Society 1963;85(14):2149–2154. doi:10.1021/ja00897a025
2. Carpino LA, Han GY. 9-Fluorenylmethoxycarbonyl amino-protecting group. The Journal of Organic Chemistry 1972;37(22):3404–3409. doi:10.1021/jo00795a005
3. Behrendt R, White P, Offer J. Advances in Fmoc solid-phase peptide synthesis. Journal of Peptide Science 2016;22(1):4–27. doi:10.1002/psc.2836
4. Isidro-Llobet A, Álvarez M, Albericio F. Amino acid-protecting groups. Chemical Reviews 2009;109(6):2455–2504. doi:10.1021/cr800323s
5. El-Faham A, Albericio F. Peptide coupling reagents, more than a letter soup. Chemical Reviews 2011;111(11):6557–6602. doi:10.1021/cr100048w
6. Mergler M, Dick F, Sax B, Weiler P, Vorherr T. The aspartimide problem in Fmoc-based SPPS. Part I. Journal of Peptide Science 2003;9(1):36–46. doi:10.1002/psc.430
7. Palasek SA, Cox ZJ, Collins JM. Limiting racemization and aspartimide formation in microwave-enhanced Fmoc solid phase peptide synthesis. Journal of Peptide Science 2007;13(3):143–148. doi:10.1002/psc.804
8. Paradís-Bas M, Tulla-Puche J, Albericio F. The road to the synthesis of "difficult peptides". Chemical Society Reviews 2016;45(3):631–654. doi:10.1039/c5cs00680e
9. Dawson PE, Muir TW, Clark-Lewis I, Kent SB. Synthesis of proteins by native chemical ligation. Science 1994;266(5186):776–779. doi:10.1126/science.7973629
10. Kent SB. Chemical synthesis of peptides and proteins. Annual Review of Biochemistry 1988;57:957–989. doi:10.1146/annurev.bi.57.070188.004521
11. D'Hondt M, Bracke N, Taevernier L, Gevaert B, Verbeke F, Wynendaele E, et al. Related impurities in peptide medicines. Journal of Pharmaceutical and Biomedical Analysis 2014;101:2–30. doi:10.1016/j.jpba.2014.06.012

Cosmic Peptides. (2026, October 25). How synthetic peptides are made. Cosmic Peptides Research Library. Retrieved October 26, 2026, from https://cosmicpeptides.com/research/methods/how-peptides-are-made

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  author       = {{Cosmic Peptides}},
  title        = {How synthetic peptides are made},
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  year         = {2026},
  month        = oct,
  url          = {https://cosmicpeptides.com/research/methods/how-peptides-are-made},
  urldate      = {2026-10-26},
  note         = {Last reviewed 2026-10-25}
}

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