What it is, and what it is next to
PT-141, also called bremelanotide, is a cyclic peptide of seven residues built on the same melanocortin scaffold as Melanotan II. The two are close structural relatives: the ring is formed the same way, the chain length is the same, and they differ at one position, where PT-141 carries a free acid in place of the amide found on its relative.
| Property | Value | Where it is confirmed |
|---|---|---|
| Structure | Cyclic, closed by a lactam bridge | Synthesis record |
| Residue count | 7 | Synthesis record |
| Approximate mass | near 1,025 g/mol | Certificate of analysis |
| Relation to Melanotan II | Differs at one terminal position | Published chemistry |
| Physical form supplied | Lyophilized solid | Label and batch record |
The masses of the two compounds are close. That is precisely why the stated molecular formula on a report is worth more than a rounded mass for either of them, and why a certificate should name the compound it describes rather than leaving it to the listing.
The same 18-dalton question
Because this is a cyclic peptide, the check described in detail for its relative applies unchanged: closing the ring releases a molecule of water, so the uncyclised linear precursor weighs about 18 more than the finished cyclic product. An identity result matching the calculated cyclic mass confirms the ring closed; a result 18 higher indicates open-chain material.
Two cyclic compounds in the same catalogue with the same structural check is a useful thing to internalise: for any lactam-bridged peptide, the identity line is doing structural work, not just confirming a formula.
What else a report should show
- The linear precursor as the characteristic impurity. For a short cyclic peptide, incomplete cyclisation is a likelier explanation for a depressed purity figure than chain assembly failure.
- D-residues, and their limitation. The scaffold includes D-configured positions, which have identical mass to their L counterparts. A mass result cannot detect the wrong stereoisomer; the chromatogram usually can. See Ipamorelin for the full explanation.
- A tryptophan in the ring. Light sensitivity is a real consideration for storage, though oxidation is not the dominant route.
Storage of the lyophilized solid
Supplied as a freeze-dried solid, governed by temperature and moisture. The lactam ring is a stable covalent bond and is not the fragile part of the molecule; the tryptophan residue makes opaque storage worthwhile.
Stability is a documented property rather than an assumed one: a laboratory establishes it by holding material under defined conditions and re-analysing at intervals. Where a retest date is stated, the question worth asking is what data supports it. See storing lyophilized research materials.
Scope of this documentation
This page concerns material identification and analytical documentation for laboratory research. It does not recommend personal, human, clinical, therapeutic or veterinary applications. Published research findings do not establish the properties of Arctic stock; use the applicable lot record and its stated methods and limitations.
Related documentation
Material on this site
The documentation this article describes is published per lot for the listings below. Match the lot number on a vial label to its record before relying on it.
- Melanotan II 10mg — sealed research vial, lot number on the label; lot MT2-2502-B report
- Melanotan 1 10mg — sealed research vial, lot number on the label; lot MT1-2502-B report
- Research materials catalog — current listings with batch status
- Batch COA library — every currently published report with its verification link