Melanotan I is a laboratory designation, not a chemical name. What follows is an identity reference for Melanotan I: what the sequence is, how it relates to the peptide it was derived from, what mass to expect, how it behaves under reverse-phase HPLC, and how a buyer can confirm that the material in a vial is what the label says. Nothing here addresses what Melanotan I does, and it offers no preparation, handling or dosing guidance.
The sequence behind Melanotan I
Melanotan I is a linear tridecapeptide — thirteen residues — conventionally written:
Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2
Both ends are capped. The N-terminal serine carries an acetyl group and the C-terminal valine is an amide, so the molecule has neither a free N-terminal amine nor a free C-terminal carboxylate. Eleven of the thirteen residues are standard L-amino acids. The other two are the reason the compound has its own name:
- Position 4 is norleucine (Nle), a non-proteinogenic amino acid with an unbranched four-carbon side chain.
- Position 7 is D-phenylalanine, the mirror-image form of the usual residue.
The molecular formula is C78H111N21O19 and the average molecular mass is approximately 1646.9 g/mol. There is no sulfur in the formula, which becomes relevant below.
What the name Melanotan I refers to
Melanotan I is an analogue of alpha-MSH, a naturally occurring 13-residue peptide with the sequence Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2. Laid side by side, the two sequences are identical except at two positions: methionine at position 4 is replaced by norleucine, and L-phenylalanine at position 7 is replaced by D-phenylalanine. That is why the compound is also written [Nle4, D-Phe7]-alpha-MSH, or abbreviated NDP-MSH.
“Melanotan I” is a coined name, and its International Nonproprietary Name is afamelanotide. All of these labels refer to the same structure. None of them tells a buyer the sequence, which is the practical argument for buying on published sequence rather than on name. It also matters because the name invites confusion with Melanotan II, which is a different molecule: a seven-residue cyclic lactam, not a thirteen-residue linear chain, with a mass of roughly 1024 g/mol rather than 1647. The numeral in the name is not a version number, and the two compounds cannot be substituted for one another on a certificate.
Why the two substitutions matter analytically
Norleucine is roughly the same size and shape as methionine but has a methylene group where methionine has a sulfur atom. Methionine is prone to oxidation, which adds 16 Da and produces a distinct, slightly more polar species that often shows up as a small peak just ahead of the main one. With norleucine in place, Melanotan I has no methionine and no sulfur, so that particular oxidation product is not expected. A mass spectrum showing a sulfur-containing species, or a strong +16 Da companion, is worth a question.
The D-phenylalanine at position 7 is invisible to mass spectrometry. A sample in which that residue has racemised to the L form has exactly the same mass as correct material. Only a chromatographic separation capable of resolving the two diastereomers can show the difference, which is one reason a mass confirmation alone is never a complete identity check for this compound.
How Melanotan I behaves on reverse-phase HPLC
At thirteen residues with a mix of polar and hydrophobic side chains, Melanotan I retains comfortably on a C18 column and elutes in the middle of a typical acidic, ion-paired gradient. Tyrosine, D-phenylalanine, tryptophan, norleucine, proline and valine provide hydrophobic surface; arginine, lysine and histidine carry positive charge under acidic conditions, and the counter-ion in the mobile phase has a real effect on peak shape and retention.
Detection has two options. Tyrosine and tryptophan both absorb near 280 nm, so the compound is readily seen there, but purity work is normally read on the peptide bond at around 214 nm so that impurities lacking an aromatic residue — truncated or deletion sequences, for example — are counted too. A certificate should state which wavelength was used. The article on peptide synthesis impurities explains the deletion and truncation species that a thirteen-residue synthesis can produce.
What an analysis for Melanotan I should show
A useful certificate reports the column chemistry and dimensions, the mobile phase and gradient including the ion-pairing additive, the detection wavelength, the retention time, the integrated peak area with the integration window visible, and a mass confirmation consistent with approximately 1647 g/mol. For a peptide of this size, electrospray mass spectra commonly show multiply charged ions, so the reported figure should be the deconvoluted neutral mass or should state the charge state.
Net peptide content for Melanotan I belongs on the certificate as its own figure, apart from chromatographic purity, together with the named counter-ion. With three basic residues, Melanotan I is normally isolated as a salt, typically acetate or trifluoroacetate, and the counter-ion adds to the weighed mass without adding peptide. The article on purity versus net peptide content covers why those are two different numbers.
Confirming identity independently
Search on the sequence, not the name. Public chemical databases index Melanotan I under its nonproprietary name and by structure, and a search using the full sequence — with the Nle and D-Phe positions, the acetyl cap and the C-terminal amide — will show quickly whether the formula and mass a vendor quotes match the ones everyone else uses. A quoted mass near 1024 is a sign that Melanotan I and Melanotan II have been confused.
How Melanotan I is supplied here
Melanotan 1 10mg is supplied as a lyophilised powder in a sealed vial. Melanotan I is analyzed by an independent laboratory using reverse-phase HPLC, and that result is published on its product page. Vial size and HPLC purity for Melanotan I are both stated on its product page. There is no lot number on a Melanotan I vial; instead, its crimp and cap color link it to the published test.
Frequently asked questions
What is the sequence of Melanotan I?
Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2, a linear, acetylated, C-terminally amidated tridecapeptide.
Is Melanotan I the same as afamelanotide?
Yes. Afamelanotide is the International Nonproprietary Name for the same structure, which is also written [Nle4, D-Phe7]-alpha-MSH.
How is Melanotan I different from Melanotan II?
Melanotan I is a linear 13-residue peptide of about 1647 g/mol. Melanotan II is a 7-residue cyclic lactam of about 1024 g/mol. They share part of their sequence but are different molecules with different masses and different retention behaviour.
Why is there no methionine in Melanotan I?
Position 4 carries norleucine in place of the methionine found in alpha-MSH. The formula therefore contains no sulfur, and the methionine oxidation product is not an expected impurity.
What molecular weight should I expect for Melanotan I?
Approximately 1646.9 g/mol for the neutral molecule, C78H111N21O19. Because a salt adds weight, the Melanotan I counter-ion needs to be stated next to the mass.
Research use only. All products supplied by Battle Born Peptides are laboratory reference materials for in-vitro research and analytical use by qualified professionals. They are not drugs, foods, dietary supplements, cosmetics or medical devices; they are not approved by the FDA or any other regulator for use in humans or animals; and they are not intended to diagnose, treat, cure, mitigate or prevent any disease, or to affect the structure or any function of the body of humans or animals. Nothing in this article is preparation, handling or dosing guidance. See our full research-use terms.