Buying Melanotan or PT-141 in the USA: Three Compounds, One Dalton

Three melanocortin compounds turn up in most catalogs, and two of them are separated by a single dalton. That fact governs how a certificate for either has to be read, and it is why this family rewards looking at the identity data rather than at the label.

Melanotan-I: linear

Melanotan-I — also written MT-1, and carrying the international non-proprietary name afamelanotide — is a thirteen-residue linear peptide: Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2. It is the natural alpha-MSH sequence with two substitutions: norleucine at position 4 in place of methionine, and D-phenylalanine at position 7 in place of the L form.

Both are stability engineering. Taking out the methionine removes the thioether sulfur, and with it the oxidation route that puts a peak 16 daltons above the parent on a methionine-containing peptide. Flipping one residue to the D configuration blocks proteases that only recognize L-amino acids. Formula C78H111N21O19, mass around 1646.8 g/mol, CAS 75921-69-6.

Melanotan-II: the same idea, closed into a ring

A different object altogether: Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2 — seven residues, six of them closed into a lactam ring between an aspartate side chain and a lysine side chain. Formula C50H69N15O9, mass around 1024.2 g/mol, CAS 121062-08-6.

So the two Melanotans are not two sizes of one thing. One is a thirteen-residue linear chain; the other a seven-residue macrocycle six hundred daltons lighter. Cyclizing a peptide constrains it into a narrower set of conformations, which is a structural strategy rather than a reduction in size. Cyclic and linear peptides covers what closing a ring changes.

One dalton apart

This is the part worth committing to memory. PT-141 is Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH. Melanotan-II is the same sequence ending in -NH2 rather than -OH.

A C-terminal amide and a C-terminal free acid differ by swapping an NH2 for an OH — about one dalton. PT-141 lands near 1025.2, Melanotan-II near 1024.2, and apart from that terminus they are the same molecule.

The practical consequence: a mass reported to whole-number precision, or a certificate quoting a mass without saying which terminus was expected, does not tell these two compounds apart. Where a laboratory reports measured mass against a calculated value and the calculated value matches the sequence as written, identity is settled. Where it reports a rounded figure on its own, it is not.

Why incomplete cyclization shows as eighteen daltons

Closing a lactam ring forms an amide bond between a side-chain carboxyl and a side-chain amine, and that bond formation releases one molecule of water. Water is 18 daltons, so any uncyclized precursor left in a batch sits 18 Da above the finished compound.

On a spectrum of Melanotan-II or PT-141, a peak 18 Da high is incomplete cyclization, and it is the first thing to look for on a certificate for either. It does not apply to Melanotan-I, which has no ring at all.

The substitution mass spectrometry cannot see

All three contain D-phenylalanine, and a D-amino acid has exactly the same atoms in the same number as its L counterpart. Identical mass. A mass spectrum cannot separate them.

An epimer — the same sequence with that residue in the wrong configuration — is a different compound of identical molecular weight, invisible to the identity test most certificates lean on. Chromatography is what separates them, because their shapes differ enough to resolve on a decent reversed-phase gradient. That is a specific reason to want the trace rather than a summary figure. Reading an HPLC chromatogram covers what to look at.

Tryptophan: easy to see, easy to oxidize

All three sequences carry a tryptophan, which is convenient and inconvenient in equal measure. Convenient because tryptophan is the strongest chromophore among the standard residues, so these compounds respond well at 280 nm as well as at 214 nm — unlike Selank, which carries no aromatic residues and is nearly invisible at the longer wavelength.

Inconvenient because tryptophan is among the most oxidation-prone residues in the set. Dry, dark, minimal air exposure is the relevant discipline; how peptides degrade covers the routes.

Side by side

What to check on a certificate

Calculated mass printed alongside the measured one, so the terminus is unambiguous. Anything 18 Da high on the two cyclic compounds. The detection wavelength. And net peptide content, which is what says how much of the vial is peptide rather than counter-ion and water — see net peptide content and what a third-party peptide test actually measures. Every report we hold is published on the certificates of analysis page.

Names you will see

Melanotan-I appears as MT-1, Melanotan 1 and afamelanotide. Melanotan-II appears as MT-2, Melanotan 2 and MT-II. PT-141 appears as PT 141, PT141 and bremelanotide. Within a row those are one compound; across rows they are not.

What we supply

We stock Melanotan-1 10mg, Melanotan-II 10mg and PT-141 10mg, each lyophilized with identity data published on its product page. All supplied for laboratory research use only. We publish no dosing information, no protocols involving human or veterinary subjects, and no statements about what any of these compounds does in a person — and we do not provide them on request.