Why CAS Numbers and Molecular Weights Differ Between Peptide Listings
Salt forms, metal complexes, two-peptide materials and superseded registry numbers: five reasons two listings of the same peptide show different numbers, with examples from our catalogue.
Published October 9, 2026 · 5 min read · Pepoderma
Laboratories and distributors regularly find two different CAS numbers, or two masses, for what looks like the same peptide. Usually both are correct — for different forms. Five common reasons, with examples from our product table.
1. Salt or free peptide
A peptide supplied as a salt has a larger formula than the peptide itself. Our product table gives SYN®-AKE as the diacetate, C23H37N5O7 (495.56 g/mol); the peptide alone is C19H29N5O3, about 375.5 g/mol (calculated). Acetate and trifluoroacetate (TFA) salts of other peptides shift the mass the same way.
2. Metal complex or free peptide
A copper complex and its peptide are different substances. GHK-Cu is C14H22CuN6O4 (401.91 g/mol); GHK, the same tripeptide without copper, is C14H24N6O4 (340.4 g/mol), each with its own CAS. For AHK-Cu our product table uses the CAS of the neutral complex, 767286-83-9, while PubChem records 682809-81-0 for a hydrochloride form.
3. One material, two peptides
The material sold as Matrixyl® 3000 contains Palmitoyl Tripeptide-1 and Palmitoyl Tetrapeptide-7, so it carries two CAS numbers (147732-56-7 + 221227-05-0) and no single molecular weight.
4. Older and superseded numbers
Registries keep old entries. For Hexapeptide-9, GSRS records 1228371-11-6 as superseded; our product table lists 885024-94-2. For Palmitoyl Tripeptide-5 another public listing gives 623172-55-4, our table 623172-56-5 — the CAS on the COA of the lot applies.
5. Average or monoisotopic mass
Catalogue molecular weights are average masses (standard atomic weights). Mass spectrometry often reports the monoisotopic mass or charged ions, which differ from the average by a small amount for larger peptides. Compare like with like.
What we show
Each product page gives the formula, the molecular weight with the form it refers to, the CAS and any known alternative number, with the source of each value. Where the table has a formula but no mass, the page shows a mass calculated from the formula and says so.
Talk to sales
Two listings, two numbers?
Send us both; we reply within 24 hours with the form we supply and the COA of the batch you are quoted.
Peptides in this guide
- Dipeptide Diaminobutyroyl Benzylamide Diacetate · on request
- AHK-Cu · stocked
- Palmitoyl Tripeptide-1 & Palmitoyl Tetrapeptide-7 · on request
- Hexapeptide-9 · on request
Trade names in this guide (for example SNAP-8®, Argireline®, Leuphasyl®, Eyeseryl® — Lipotec, Lubrizol; Matrixyl® — Sederma, Croda; SYN®-AKE — dsm-firmenich) are trademarks of their owners and are used only to identify the peptides. Pepoderma lists peptides by chemical name and is not affiliated with these companies. All products are supplied for research use only.
Frequently asked questions
- Why does the same peptide have two CAS numbers?
- Usually because the numbers belong to different forms (salt, complex, free peptide), to the components of a two-peptide material, or to an older registry entry that has been superseded.
- Which molecular weight should I compare with the MS result?
- The one for the form in the vial, without counter-ions for a salt, and keeping in mind that MS may report a monoisotopic mass or charged ions.