"Peptide synthesis" is searched both by people curious how the powder in a vial came to exist and by people trying to read the certificate that came with it. The two questions have the same answer. Almost every research peptide is built one amino acid at a time on a resin bead, cut free with a strong acid and purified by chromatography, and each of those steps leaves something behind that the headline purity figure was never designed to measure. This page walks through the process as FDA's own reviewers describe it, with dates, and sets out what it means for reading a label. It describes; it does not rate any supplier, and how brands appear on this site is on our disclosure page. Where that factory usually is, and who has registered with FDA, is on our peptide sources explainer and our page on Chinese peptides.
The method: building a chain on a bead
FDA's evaluation of BPC-157 for its July 2026 compounding advisory committee (evaluation dated 2026-05-11) states the starting point plainly: "For most synthetic peptides, solid-phase synthesis methods are widely used by industry for peptide synthesis." The same document summarises how BPC-157 itself was first made, in the 1993 paper that introduced it:
- Anchor. The first amino acid is attached to a polymer resin — in that case a benzhydrylamine resin — so the growing chain stays fixed while everything else is washed away.
- Deprotect. Each incoming amino acid carries a temporary protecting group on its amine (Fmoc, fluorenylmethoxycarbonyl), removed with piperidine before the next one is added.
- Couple. The next Fmoc-protected amino acid is joined using a coupling reagent — diisopropylcarbodiimide in the original synthesis.
- Repeat for every residue. BPC-157 has fifteen, so fifteen cycles.
- Cleave. The finished chain is cut from the resin, and its side-chain protecting groups stripped, with a strong acid mixture — trifluoroacetic acid, trifluoromethanesulphonic acid and anisole in the original paper.
- Purify. The crude mixture is separated by high-performance liquid chromatography; the 1993 synthesis reported a purity above 95%.
- Freeze-dry. The purified solution is lyophilised into the white powder a vial holds.
Every step is a chemical reaction that goes most of the way, not all of it. Over fifteen couplings, small per-step losses compound, and that is where most of the impurity profile comes from.
What each step leaves behind
FDA's reviewers list the impurities a synthesis like this can carry, in the same evaluation:
| Where it comes from | What it leaves | Why it matters |
|---|---|---|
| Incomplete coupling, truncation, side reactions | Peptides missing a residue, cut short, or chemically altered | "Typically similar in structure to the target peptide and may be difficult to identify and quantify without sophisticated analytical methods" |
| Protected amino acid starting materials | Isomeric impurities (the mirror-image form of a residue), free amino acids | An isomer has the same mass as the correct residue, so a mass check can miss it |
| Reagents | Residual solvents, coupling reagents, activators, catalysts, scavengers | Not peptides at all, so a peptide purity figure does not see them |
| The peptide itself, in storage or formulation | Aggregates — several chains clumped together | FDA ties aggregation to immunogenicity, the body making antibodies to an injected peptide |
The first row is the hard one. A peptide missing one amino acid out of fifteen looks almost exactly like the right one: similar size, similar charge, similar behaviour on a column. Whether an HPLC method separates it depends on how that method was developed — which is why a purity figure is only as good as the method behind it, the subject of our guide to reading a certificate of analysis.
Why "99% pure" is not "99% peptide"
The acid used to cleave the chain, and usually to run the purification, is trifluoroacetic acid. The peptide therefore comes off the column as a trifluoroacetate (TFA) salt: its basic groups carry trifluoroacetate counter-ions. Many suppliers then exchange those for acetate. Both are real substances with their own identifiers — FDA lists BPC-157 free base (UNII 8ED8NXK95P, CAS 137525-51-0) and BPC-157 acetate (UNII PAR2FC72XP, CAS 216441-37-1) separately, and gives the acetate's formula only as the peptide plus "X" acetate, because the number varies.
That salt has weight, and an HPLC purity figure does not count it. The arithmetic below is ours, and it rests on one stated assumption — two counter-ions per molecule, one each for BPC-157's single lysine and its free N-terminus:
| Form | Molecular weight | Share that is BPC-157 by weight | Peptide in a vial labelled 5 mg of the salt |
|---|---|---|---|
| Free base | 1,419.5 g/mol (FDA) | 100% | 5.0 mg |
| Acetate salt, 2 acetates | ≈1,539.6 g/mol | ≈92% | ≈4.6 mg |
| TFA salt, 2 trifluoroacetates | ≈1,647.5 g/mol | ≈86% | ≈4.3 mg |
Residual water in a freeze-dried powder lowers the real figure further. This is why some certificates report net peptide content alongside purity: the first answers "how much of this powder is the peptide", the second "how much of the peptide-like material is the right peptide". A certificate with only the second leaves the first unknown, and a vial's milligram figure does not say whether it counts the salt.
FDA found exactly this confusion in the BPC-157 nominations it reviewed. Both nominators submitted a certificate of analysis for BPC-157 acetate while their identifiers, formula and molecular weight described the free base. FDA also reported that most BPC-157 certificates it could find "only contain purity testing results", with no impurity limits.
What a regulated peptide is held to
For comparison, FDA's revised draft product-specific guidance for generic semaglutide injection (recommended November 2023, revised December 2025 and July 2026) sets thresholds for peptide-related impurities in a generic made by synthesis or by recombinant methods:
- Report every one present above 0.1% of the drug substance;
- Identify every one above 0.5%;
- Keep each new one below 1.0%, and each shared one no higher than in the brand product or 1.0%, whichever is greater;
- Sum all above 0.1% and keep the total no higher than the brand's.
It also asks about particulates, microbial contaminants, residual solvents and elemental impurities, and about aggregates as a route to immune reactions. FDA withdrew its broader 2021 guidance on synthetic peptide generics on 2026-07-28 as no longer reflecting its thinking, and moved these recommendations into 17 product-specific guidances. Nothing equivalent governs a research-labelled vial: its certificate reports whatever the seller chose to test. Which tests exist, and what each can and cannot detect, is on our peptide testing page; what an independent lab report adds to a seller's certificate is on our third-party testing explainer.
What this page does not establish
It does not say how any particular supplier makes any particular peptide; synthesis routes are rarely published, and FDA's description is of the method in general. It does not claim any vial on the market is a TFA or acetate salt, or how many counter-ions it carries — the weights above illustrate the size of the effect, not a measurement. And it does not rate any product's quality. For compounds such as BPC-157, what the missing impurity data mean for safety is set out on our BPC-157 side effects page.
Sources and dates
Read 2026-09-25: FDA briefing document for BPC-157-related bulk drug substances, Pharmacy Compounding Advisory Committee meeting of July 23–24, 2026 (evaluation dated 2026-05-11) — basic-information tables, probable routes of synthesis (citing Sikirić et al. 1993), likely impurities and characterisation sections; FDA product-specific guidance for semaglutide subcutaneous solution (PSG 215256; recommended Nov 2023, revised Dec 2025 and Jul 2026), impurity section; FDA statement of 2026-07-28 on revised draft product-specific guidances for generic peptide products. Molecular weights for the salt forms are our arithmetic from FDA's free-base weight and the standard weights of acetic acid (60.05) and trifluoroacetic acid (114.02). Corrections go to the contact page.