Melanotan II: A Research Primer

Melanotan II is a synthetic cyclic heptapeptide, an analog of the endogenous hormone alpha-melanocyte-stimulating hormone (alpha-MSH). Seven residues, six of them closed into a ring, written as Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2 and abbreviated MT-2. That is the identity. Everything below is the detail behind it.

This is an identity primer, not a usage page. It covers what the molecule is, how to read the notation on the label, which corner of the research literature references it, and how it is handled and tested as a reagent.

What Melanotan II is

Alpha-MSH is a 13-residue peptide hormone in the melanocortin family. Melanotan II is a shortened, chemically modified version of it. The chain is cut down to seven residues, one residue is swapped for its mirror image, another is replaced with a non-standard amino acid, both ends are capped, and the middle of the sequence is closed into a ring.

That list of changes is why it is an analog and not a fragment. A fragment is a slice of a parent sequence copied as-is; an analog has been rebuilt. Melanotan II is fully synthetic, assembled residue by residue rather than isolated from anything.

The alias you will see most often is MT-2; the original chemistry literature usually writes it MT-II.

How to read the sequence

The line printed on the label is Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2. Every piece of it is doing a job.

• Ac- is an acetyl group capping the N-terminus.
• Nle is norleucine, a non-proteinogenic amino acid. It is the straight-chain isomer of leucine, and it sits in the position that methionine occupies in the parent sequence.
• cyclo[ ] means the residues inside the brackets are joined into a ring.
• D-Phe is phenylalanine in the D configuration, the mirror image of the L-phenylalanine natural proteins are built from.
• -NH2 is a C-terminal amide, in place of the free carboxylic acid a plain peptide chain ends with.

The ring is a lactam bridge. That is an ordinary amide bond, the same chemistry as any peptide bond, formed between the side-chain carboxyl of the aspartic acid and the side-chain amine of the lysine. Because it links two side chains rather than the two ends of the backbone, it gets described as a side-chain-to-side-chain lactam. It is not a disulfide, the other common way of closing a peptide ring, which needs two cysteines.

One practical consequence of that bond: forming it expels a molecule of water, so the cyclic peptide weighs about 18 Da less than the same residues strung out in a line. A spectrum reading 18 units light against a hand-summed total is showing the cyclization, not a bad sample.

The chemistry on the label

These are the values that pin down which molecule is in the vial.

• CAS number: 121062-08-6
• Molecular formula: C50H69N15O9
• Molecular weight: 1024.18 g/mol
• Sequence: Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2
• Class: cyclic alpha-MSH analog, melanocortin receptor agonist
• Other names: MT-2

The formula and mass above are the free base. Peptides in this class are commonly supplied as trifluoroacetate (TFA) salts, a leftover from the purification step, and the salt carries mass the free-base formula does not account for. The net peptide content line on a certificate is where that shows up: it tells you how much of the powder is peptide and how much is counter-ion and water.

When you cross-check a supplier's identity claim against a certificate, match the CAS number first and the sequence second.

Where it appears in the research literature

Melanotan II appears in melanocortin receptor research. That is the field: the melanocortin system, its five receptor subtypes MC1R through MC5R, and the ligands that bind them. Naming a field is a statement about what researchers have looked at, not about anything they concluded.

I am deliberately not summarizing findings here. There is a hard line between describing a compound's identity and describing what anyone claims it does, and a primer stays on the identity side of it. The CAS number and the sequence above are what you search on if you want the papers themselves.

How it is supplied and stored

Melanotan II ships as a lyophilized solid in a 10 mg vial. Freeze-drying is standard for peptides because a dry cake survives transit better than a solution.

Store it at -20C, sealed, and out of light, away from moisture and repeated freeze-thaw cycling. That is reagent handling, not preparation guidance, and the longer version is written out in storing lyophilized peptides.

How each lot is tested

Each tested lot gets its own certificate of analysis, published as the analysis completes. Two tests carry the weight. Mass spectrometry confirms the molecule matches the label, which for a cyclic peptide also means confirming the ring closed. HPLC measures how much of the sample is the target peptide versus related impurities, read at 220 nm, the standard wavelength for peptide bonds.

Our tested Melanotan II lot was analysed by Krause Analytical in Austin, Texas, and its full COA PDF is published on that batch's page. Each certificate names the lab that ran that specific lot. If you have not read one of these documents before, there is a walkthrough on how to read a peptide COA using real reports. The current lot and its certificate are linked from the Melanotan II product page.

*All products are supplied as laboratory reagents for research use only. Not for human or veterinary use.*