Two compounds, one numeral apart
Melanotan I and Melanotan II are routinely discussed as though they were strengths of one product. They are not. They are structurally different molecules, and the difference is larger than the names suggest.
Melanotan I, also called afamelanotide, is a linear peptide of thirteen residues — a straight chain, analogous in length to the natural hormone fragment it derives from. Melanotan II is a cyclic peptide of seven residues, closed by a lactam bridge. One is roughly twice the size of the other and lacks a ring entirely.
| Property | Value | Where it is confirmed |
|---|---|---|
| Chain | Linear, 13 residues | Synthesis record |
| Approximate mass | near 1,646 g/mol | Certificate of analysis |
| Key substitution | Norleucine; D-phenylalanine | Synthesis record |
| Contrast: Melanotan II | Cyclic, 7 residues, near 1,024 g/mol | Its own report |
| Physical form supplied | Lyophilized solid | Label and batch record |
Treat these as orientation; the report for the lot in hand is authoritative and governs where it disagrees with this page.
What changes on the report when there is no ring
The 18-dalton question that dominates a cyclic peptide's documentation simply does not arise here. There is no cyclisation step, so there is no uncyclised precursor to look for, and the identity result is not doubling as a structural check on ring closure.
What replaces it is the ordinary concern of a longer chain:
- Thirteen residues is a substantially longer synthesis than seven. Each coupling step is another chance for a deletion sequence, so deletion products are the expected impurity class, and they elute close to the main peak because a chain missing one residue resembles the complete one.
- A wider impurity envelope. Longer chains accumulate more closely related species, so the region immediately around the main peak carries more information than the headline percentage. See how to read an HPLC chromatogram.
- No methionine or cysteine. As with its cyclic relative, oxidation is not the primary degradation route for this sequence.
The mass difference between the two compounds — roughly 620 units — is far beyond any method's tolerance, so an identity result separates them without ambiguity. If a listing does not link a lot-specific report stating a mass or a named reference standard, the product name alone does not establish which of the two you would receive.
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 every chemical process available to it, and the solid is hygroscopic, so a vial opened before it has equilibrated to room temperature will draw water out of the air.
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.
What the published literature covers
The research record is preclinical, consisting largely of animal-model and in vitro work on melanocortin receptor systems and on structure-activity relationships among analogues of the parent hormone fragment. Because the linear and cyclic compounds are studied separately and behave differently at those receptors, a study of one is not a study of the other — which is the practical reason the distinction on this page recurs throughout that literature.
The compound has not completed the regulatory process that would establish safety or efficacy for any medical indication, and findings 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.