A research peptide’s identity is not a single name but a set of documented fields that agree with each other. This guide walks through the fields any careful laboratory reads first, using three catalogue compounds as worked examples and citing only the values that appear on their documentation.
The fields that define an identity
Names are the least reliable part of a peptide record. The same compound may appear under a code, a trade name, and a descriptive name, and unrelated materials sometimes share loose nicknames. The fields that actually pin down what is in the vial are the registry number, the class, the sequence descriptor, and the reported purity, cross-checked against a certificate.
CAS number
The CAS registry number is a stable, unique handle for a defined substance. It travels across catalogues, safety data sheets, and databases, so it is the single most useful field for confirming that two listings refer to the same thing.
Compound class
The class situates the peptide in a family: a secretagogue pentapeptide, a thymosin fragment, a pineal tetrapeptide, and so on. Class does not tell you the exact structure, but it frames what other identifiers you should expect to see and which analogues might be confused with it.
Sequence descriptor
A sequence descriptor ranges from an explicit residue string to a shorthand such as an amino-acid count or a named fragment. When a full sequence is published it is the most specific descriptor available; when only a fragment name or residue count is documented, that is what should be cited, and nothing more should be invented around it.
Reported purity
Purity is a measured, lot-specific figure, most often an HPLC area-percentage. It is a property of a batch, not a fixed attribute of the compound, so it belongs on a certificate tied to a lot rather than in a general description.
Three worked examples
The table below shows the documented identity fields for three catalogue compounds. Note what is present and what is deliberately absent: where a molecular formula, molecular weight, or database identifier is not part of the documented record here, it is left out rather than guessed.
| Name | Class | Sequence descriptor | CAS | Reported purity |
|---|---|---|---|---|
| TB-500 | Thymosin Beta-4 fragment | Actin-regulating fragment | 77591-33-4 | 99.4% |
| Ipamorelin | Pentapeptide secretagogue | 5 amino acids | 170851-70-4 | 99.3% |
| Epithalon | Pineal-derived tetrapeptide | Ala-Glu-Asp-Gly | 307297-39-8 | 99.5% |
Each row reads differently on purpose. TB-500 is documented as an actin-regulating fragment of thymosin beta-4, so the descriptor is a named fragment rather than a residue string. Ipamorelin is documented at the level of its class and residue count, a pentapeptide of five amino acids. Epithalon is short enough that its full four-residue sequence, Ala-Glu-Asp-Gly, is the descriptor. Reading them side by side shows that a complete identity is whatever the documentation supports, not a template filled in from memory.
What not to add
The discipline that separates a trustworthy record from a misleading one is refusing to invent fields. If a molecular formula, molecular weight, PubChem CID, InChIKey, or UNII is not on the documentation in front of you, the correct action is to leave it blank and note the gap, not to reconstruct a plausible-looking value. A fabricated identifier is worse than a missing one, because it looks authoritative while being unverifiable.
Putting it together
To read a peptide identity in practice, confirm the CAS number first, check that the stated class and sequence descriptor are consistent with each other, and treat purity as a lot-specific measurement to be verified against a certificate. When those fields agree, you have a defensible identity. For the deeper chemistry behind sequence descriptors, the notation guide in peptide profiles covers how residue codes are written and read.
Cross-checking fields against a certificate
Reading an identity is not only about knowing which fields exist; it is about checking that they agree. A practical pass over a certificate of analysis moves through the documented fields in order and stops at the first inconsistency.
- Confirm the CAS number resolves to the expected compound in a registry.
- Check that the stated class is consistent with that compound, so that a pentapeptide secretagogue does not carry a tetrapeptide sequence.
- Match the sequence descriptor to what the class implies, whether that is a full residue string, a named fragment, or a residue count.
- Read purity as a lot-specific HPLC figure, and note the method and the batch it belongs to.
Applied to the three examples, the checks read cleanly. Epithalon’s four-residue sequence Ala-Glu-Asp-Gly is consistent with its tetrapeptide class. Ipamorelin’s documentation stops at a pentapeptide class and a five-residue count, so that is exactly what should be cited, with no residue string added from memory. TB-500 is documented as an actin-regulating fragment of thymosin beta-4, so the honest descriptor is the fragment name rather than an invented full sequence.
When fields disagree
A disagreement between fields is a signal, not a rounding error. A CAS number that resolves to a different structure than the stated sequence, or a class that does not match the descriptor, means the document should be set aside and questioned rather than reconciled by guesswork. The safe response to a conflict is always to trust the registry-anchored fields and flag the rest, never to smooth over the gap with a value that was not supplied.
None of this is caution for its own sake. An inventory built on verified, self-consistent identities is one a researcher can defend under audit and reproduce months later; an inventory built on names and half-remembered values is not. The few extra minutes spent cross-checking fields at receiving are what make every downstream record trustworthy.
Common questions
What is the most reliable field for confirming a peptide's identity?
The CAS registry number. It is a stable, unique handle for a defined substance and travels across catalogues, safety data sheets, and databases, so it is the best single field for checking that two listings refer to the same compound.
What should I do if a molecular weight or CID is not documented?
Leave it blank and note the gap. If a molecular formula, molecular weight, PubChem CID, InChIKey, or UNII is not on the documentation in front of you, inventing a plausible value is worse than omitting it, because it looks authoritative but is unverifiable.
Is reported purity a fixed property of a compound?
No. Purity is a measured, lot-specific figure, usually an HPLC area-percentage. It describes a particular batch, so it belongs on a certificate of analysis tied to a lot rather than in a general description of the compound.