What Melanotan II is
Melanotan II is a cyclic peptide of seven residues, built as an analogue of a fragment of alpha-melanocyte-stimulating hormone. Two features distinguish it analytically from most of the peptides in this category: it is cyclic rather than a straight chain, and it contains residues that do not occur in ordinary proteins.
The ring is formed by a lactam bridge — an amide bond between a side-chain amine and a side-chain carboxyl, closing part of the chain into a loop. The sequence also includes a D-configured phenylalanine and a norleucine residue, both deliberate substitutions rather than synthesis artefacts. Each of these details has a consequence on the certificate of analysis.
| Property | Value | Where it is confirmed |
|---|---|---|
| Residue count | 7 | Synthesis record |
| Structure | Cyclic, closed by a lactam bridge | Synthesis record; identity result |
| Approximate mass | near 1,024 g/mol | Certificate of analysis |
| Non-standard residues | Norleucine; D-phenylalanine | Synthesis record |
| Physical form supplied | Lyophilized solid | Label and batch record |
Treat these figures as orientation. The report accompanying a given lot is the authoritative record, and where this page and that report disagree, the report governs.
The 18-dalton question
Closing a ring by forming an amide bond releases one molecule of water. Water has a mass of about 18, and that single fact is the most useful thing to know when reading a report for any cyclic peptide.
It means the uncyclised linear precursor weighs about 18 more than the finished cyclic product. If the ring did not close, the material is not Melanotan II — it is the open chain it was made from, a different molecule with different properties — and the mass difference between them is small but entirely unambiguous.
So for this compound, a mass-based identity result is doing something quite specific: it is confirming that the cyclisation step worked. A result matching the calculated cyclic mass confirms the ring. A result 18 higher indicates open-chain material. This is a rare case where a single number on a report answers a structural question directly.
Where identity is stated instead as HPLC-RTM — retention-time matching against a reference standard run under the same method — the same question is answered a different way. Cyclisation makes a peptide more compact and generally shifts its retention time relative to the linear form, so the two separate chromatographically. The report should name the reference material the match was made against.
What to look for on the chromatogram
- A peak near the main one may be the linear precursor. For a cyclic peptide this is the characteristic impurity, and it is the one worth identifying rather than simply counting toward a percentage.
- Seven residues is a short synthesis. Deletion sequences are correspondingly less common than in a long chain, so a low purity figure on a short cyclic peptide is more likely to point at incomplete cyclisation than at chain assembly.
- No methionine or cysteine. Oxidation is not the primary degradation route here, unlike MOTS-c. The ring itself is the thing to verify.
How to read an HPLC chromatogram covers reading the plot; purity versus identity covers what each result does and does not establish.
Reading a published lot
Melanotan II 10mg lot MT2-2501-A is published in the batch documentation library with a reported purity of 99.223%, issued by Janoshik in February 2026 with a verification link to that laboratory's own portal.
Three checks apply to any report for this compound, in this order: the lot identifier on the report matches the vial label; the identity result is consistent with the cyclic mass rather than sitting about 18 higher; and the chromatogram is present so the region adjacent to the main peak can actually be looked at. The headline percentage is the least informative of the three.
Storage of the lyophilized solid
Supplied as a freeze-dried solid, the material is governed by temperature and moisture like any lyophilized peptide. Cold storage slows the chemistry; the solid is hygroscopic and will draw water from the air if a vial is opened before it has equilibrated to room temperature. The lactam ring is a stable covalent bond and is not the fragile part of this molecule — the general handling considerations in storing lyophilized research materials apply without special additions.
What the published literature covers
The research record is preclinical, consisting largely of animal-model and in vitro work on melanocortin receptor systems, receptor selectivity, and structure-activity relationships among cyclic analogues of the parent fragment. A substantial part of that literature is chemistry rather than biology — studies of how cyclisation and residue substitution change binding behaviour — which is why the structural details on this page are the ones that recur in it.
The compound has not completed the regulatory process that would establish safety or efficacy for any medical indication, and results in animal models and cell culture do not transfer automatically to other species. Arctic Lab Supply does not publish research conclusions, recommend applications, or provide guidance on experimental design.