Peptide CAS Number: What a Registry Entry Does and Doesn’t Prove
Inventory software, safety documentation and purchasing systems all like a single identifier per chemical, and for most substances that identifier is the CAS Registry Number. For research peptides, the peptide CAS number is useful but trickier than it looks. One peptide can have several numbers depending on its salt form, end groups or metal content, some sequences have none at all, and numbers get copied between supplier pages with errors intact. This guide explains what the number actually designates, how to sanity-check one, and how a lab managing a large peptide inventory should record it.
What a CAS Registry Number is
CAS Registry Numbers are assigned by Chemical Abstracts Service, a division of the American Chemical Society, to individual chemical substances. Each number refers to one substance defined by its structure. It is not a name, not a product and not a category.
The format is up to ten digits in three hyphenated groups. The last digit exists purely as a checksum derived from the rest, which means a mistyped number usually fails the check rather than pointing to some other real substance.
Checking the check digit
The calculation is simple enough to do by hand. Take every digit except the check digit, multiply the rightmost by 1, the next by 2, and so on moving left, add the products, and take the last digit of the total. Using water, 7732-18-5, as an example:
- Digits before the check digit, right to left: 8, 1, 2, 3, 7, 7
- Products: 8×1 + 1×2 + 2×3 + 3×4 + 7×5 + 7×6 = 105
- Last digit of 105 is 5, which matches the check digit
A number that fails this test is wrong somewhere. One that passes is at least well-formed, though it can still belong to a different substance than the one intended.
Why one peptide can carry several numbers
Because assignment follows structure, any chemically meaningful difference gets its own entry. For peptides, four differences come up repeatedly:
| Difference | Example | Why it matters in the lab |
|---|---|---|
| Salt form | Free peptide vs acetate vs trifluoroacetate | Changes the weighed mass per milligram of peptide; see our article on TFA and acetate counter-ions |
| End groups | C-terminal amide vs free acid; acetylated vs free N-terminus | Different molecule and different mass; covered in terminal modifications |
| Metal complexes | GHK vs its copper complex GHK-Cu | Two substances with two masses; worked through in GHK-Cu registry numbers |
| Stereochemistry | A defined D-residue vs the all-L sequence | Diastereomers can have separate entries even with identical mass |
The practical upshot: a registry number on a certificate or supplier page should match the form you are actually buying. A number for the free peptide on a product supplied as an acetate salt is not strictly wrong about the peptide, but it does not describe the powder in the vial.
Where peptide CAS numbers go wrong
- Copied errors. Supplier listings are often built from other listings. A transcription error, or a number for a related compound, can spread across many sites.
- Form mismatch. The number for one salt form or end group gets attached to a different form.
- No number at all. Newer or less common sequences, and many custom blends, have never been registered. A number given for a blend usually belongs to one component.
- Shared trade names. A marketing name may be applied to materials of very different structure. Our article on thymosin beta-4 and the TB-500 fragment shows how far apart two products with overlapping names can be.
For these reasons, a CAS number should never be the only identity field you rely on. The sequence, the stated terminal groups and the measured mass are what confirm the molecule; the registry number is a cross-reference that should agree with them.
What the number does not tell you
A peptide CAS number identifies the intended substance and nothing more. It says nothing about:
- Purity or impurity profile
- Which batch the material came from
- Who manufactured it or how
- Regulatory status of any kind
Two lots sharing a registry number can differ widely in everything an HPLC run measures. Registration is simply an index entry; it confers no approval and implies nothing about quality.
CAS numbers on safety paperwork
The registry number is also how safety documentation finds its way to the right substance. A safety data sheet lists it in the composition section, and chemical inventory and hazard-communication systems in Canadian labs are commonly keyed to it. If the number attached to a peptide is for the wrong form, or belongs to a different compound altogether, the safety record and the physical stock drift apart. Checking the number when a new material is first logged avoids that mismatch spreading through several systems at once.
Recording CAS numbers in a lab inventory
For labs holding many peptides in quantity, a consistent record avoids confusion when safety data sheets, purchasing systems and inventory databases all need to line up:
- Record the number exactly as the supplier or certificate gives it, and note which form it refers to, if stated.
- Store the full sequence with end groups, the molecular formula and the calculated mass alongside it.
- Run the check-digit test once when a new number enters your system.
- Where the number and the sequence disagree, trust the sequence and mass, and flag the record for follow-up.
- For blends, record each component separately rather than forcing one number onto the mixture.
- Link each inventory entry to the certificate and vial group it applies to; at Bulk Peptides, vials are matched to their certificate by cap and crimp colour.
Our products are third-party tested by HPLC, and certificates are available for some products on the certificates page. Those documents, not the registry number, are what tell you about the material you received.
Bulk Peptides supplies its products for in-vitro laboratory research only; none are intended for use in humans or animals.
