Verify a peptide listing: CAS number, sequence and molecular weight

Why a product page needs checking

A product page or certificate usually gives a name and some of: a sequence, a molecular formula, a molecular weight and a CAS Registry Number. Each is a claim about what the material is. They should agree with one another and with a public reference record. When they do not, you may be comparing a different substance, a different salt form or simply a copy-paste error, and you will not know which until you ask.

This check uses only public databases. It does not prove what is in a vial; it tells you whether the supplier's description is internally consistent before you rely on any test result.

What each identifier tells you

CAS describes a CAS Registry Number as a unique and unambiguous identifier for a specific substance that does not depend on any naming system, and says a CAS RN designates only one substance. [3] CAS Common Chemistry is its open subset of nearly 500,000 substances, against a registry CAS describes as holding more than 300 million, so a peptide missing from Common Chemistry tells you nothing. [3]

PubChem compound records give computed properties such as molecular formula, molecular weight and monoisotopic mass, together with synonyms and cross-referenced registry numbers. [1] [2] Treat synonyms and cross-references as leads, not proof, for the reason shown below.

Worked reference: angiotensin II

Angiotensin II is a well-documented octapeptide, which makes it useful for practice. PubChem CID 172198 lists the synonym Asp-Arg-Val-Tyr-Ile-His-Pro-Phe (eight residues), molecular formula C50H71N13O12, molecular weight 1046.2 and monoisotopic mass 1045.53451475, with 4474-91-3 among its synonyms. [1]

PubChem CID 172197, angiotensin II acetate, lists molecular formula C52H75N13O14 and molecular weight 1106.2, with 68521-88-0 as its first synonym. [2] The difference is 60.0, the mass of one acetic acid (C2H4O2) added to the formula. A one-to-one acetate record therefore weighs about 5.7% more than the free base for the same number of peptide molecules.

Two cautions from the same records. The free-base record lists two CAS numbers in its identifiers section, 4474-91-3 and 11128-99-7, while the acetate record lists 68521-88-0 and 32044-01-2 and also carries "4474-91-3(Free Base)" among its synonyms. [1] [2] The free-base synonym list also includes names such as "Valyl(5)-Angiotensin II," although the listed sequence has isoleucine at position 5. [1] Cross-references and synonyms are aggregated from many depositors, so confirm a CAS RN against the formula and form rather than trusting a single synonym hit.

Mass spectrometry numbers on a certificate

Certificates often report an observed mass from mass spectrometry. That is usually an ion, not the average molecular weight. For angiotensin II, adding one proton (1.00728) to the monoisotopic mass of 1045.5345 gives an [M+H]+ of about 1046.54, and a doubly protonated ion [M+2H]2+ appears near m/z 523.77. These are arithmetic from the PubChem monoisotopic mass. [1] If a certificate gives only "MW 1046" with no ion type, record it as ambiguous and ask whether it is an observed ion or a calculated weight.

A second reference point: PubChem CID 9941957 (BPC 157) lists the sequence Gly-Glu-Pro-Pro-Pro-Gly-Lys-Pro-Ala-Asp-Asp-Ala-Gly-Leu-Val (15 residues), formula C62H98N16O22, molecular weight 1419.5 and CAS RN 137525-51-0. [4] Use the same four-field comparison for any listing.

A fictional mismatch and how to log it

An invented listing reads: "Angiotensin II acetate, CAS 4474-91-3, MW 1046.2, C50H71N13O12." The name says acetate, but the CAS RN, weight and formula are those PubChem gives for the free-base record. [1] [2] Either the material is the free base and the name is wrong, or it is the acetate and the numbers are wrong. The difference changes the mass basis by 60 g/mol.

Log it as a structured row rather than a note in an email.

Questions to send the supplier

Send the same short list every time: the full sequence including any terminal modifications, the salt form, the molecular formula and weight for that form, the CAS RN they assign and its source, and whether the certificate's mass result is an observed ion or a calculated value. Record the date of your lookup with each reference value, because database records are revised.