Quick answer: Melanotan I (afamelanotide) is a synthetic 13-residue analog of alpha-melanocyte-stimulating hormone (alpha-MSH), written [Nle4, D-Phe7]-alpha-MSH. Two substitutions, norleucine at position 4 and D-phenylalanine at position 7, make it far more resistant to breakdown and more potent than the native hormone in classic melanocyte bioassays. It is studied as a melanocortin-receptor agonist, mostly at MC1R, in cell and animal models of melanogenesis and melanocortin signaling. It is a laboratory research compound only. This explainer covers the name, structure, research themes, how it differs from Melanotan II, and what to check on a certificate of analysis. It is coming soon to the V8 catalog as a 10 mg vial: see the Melanotan I product page.
Where the name Melanotan I comes from
“Melanotan” was the label given by an academic group in Arizona to a pair of synthetic alpha-MSH analogs developed in the 1980s. The first, Melanotan I, is the linear peptide; the second, Melanotan II, is a smaller cyclic one. The name points to the compound's effect on melanocytes in pigment assays. The nonproprietary name afamelanotide refers to the same molecule as Melanotan I, and in older papers it appears as NDP-alpha-MSH, short for Nle4-D-Phe7-alpha-MSH, or as [Nle4, D-Phe7]-alpha-MSH.
Because several of these labels circulate, check the sequence rather than the nickname when you compare sources.
Structure of Melanotan I
Native alpha-MSH is a 13-residue peptide, Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2, with an acetylated N-terminus and an amidated C-terminus. Melanotan I keeps that framework and changes just two positions: methionine at position 4 becomes norleucine, and L-phenylalanine at position 7 becomes D-phenylalanine. Its molecular weight is about 1,647 g/mol.
Each change has a rationale. Methionine is easily oxidized, and norleucine is an isosteric replacement that lacks the sulfur, so it removes that weak point. The D-phenylalanine at position 7 sits inside the His-Phe-Arg-Trp core that melanocortin receptors recognize, and the inverted stereochemistry both stabilizes the receptor-bound conformation and makes the chain harder for peptidases to cut. A 1980 paper in PNAS described this analog as a highly potent alpha-melanotropin with unusually prolonged activity in the frog-skin and other melanocyte assays of the day.
For notation help, see understanding amino acid sequences.
Melanocortin receptors in brief
Five G protein-coupled receptors, MC1R through MC5R, respond to peptides cut from the precursor proopiomelanocortin. MC1R is expressed on melanocytes and, when activated, raises cyclic AMP and shifts the balance of melanin production toward the dark eumelanin pigment. MC3R and MC4R are expressed in brain circuits, and MC5R in exocrine tissues. Reviews of the field describe how agonists, the endogenous antagonist agouti and agouti-related protein together control pigmentation in animals.
Melanotan I is considered an agonist with strong activity at MC1R and measurable activity at several of the others in receptor-binding and cAMP assays. How selective it looks depends on the cell system, which is one reason to read methods sections closely.
What the preclinical literature examines
Everything below is cell-based, animal-model or mechanistic. None of it is a statement about use in people, and nothing here is guidance on altering skin color.
Melanocyte and melanogenesis assays. The classic readouts are cultured melanoma and melanocyte lines, in which MC1R activation raises cAMP, induces tyrosinase activity and increases melanin content, and the isolated frog-skin assay that measures pigment dispersion. These models are used to rank analogs by potency and persistence.
Rodent coat-color genetics. Mice with different melanocortin-pathway alleles show different yellow and black pigment outcomes, and analogs such as this one have been used to ask which receptor variants remain responsive.
Receptor binding and structure-activity work. Because the His-Phe-Arg-Trp core is well defined, Melanotan I is often the reference agonist in studies that map which residues drive binding to each receptor subtype.
Immunogenicity and peptide-handling studies. Laboratory work has also evaluated whether this synthetic analog provokes antibody formation in test systems, which is part of how peptide analogs are characterized.
A caution on interpretation: potency values shift with cell line, receptor expression level and assay format. Compare numbers only when the systems match. The related article on KPV and alpha-MSH shows how a shorter fragment behaves differently from the full 13-residue hormone.
Melanotan I, Melanotan II and alpha-MSH at a glance
The table lists published structural descriptors only. It is not a ranking and says nothing about suitability for any purpose.
| Compound | Structure | Approx. mass | Typical research context |
|---|---|---|---|
| alpha-MSH (native) | Linear 13-mer, Ac-SYSMEHFRWGKPV-NH2 | 1,665 g/mol | Endogenous reference ligand; short-lived in biological media |
| Melanotan I (afamelanotide) | Linear 13-mer with Nle4 and D-Phe7 | 1,647 g/mol | MC1R-weighted agonist in melanocyte and melanogenesis assays |
| Melanotan II | Cyclic heptapeptide, Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2 | 1,024 g/mol | Broader agonist across MC1R, MC3R, MC4R and MC5R in binding assays |
| KPV | Tripeptide, Lys-Pro-Val | about 342 g/mol | Cell-signaling models of the alpha-MSH C-terminus |
The main structural contrast is linear versus cyclic. Melanotan II is held in a ring by a lactam bridge, which narrows its conformational freedom and changes its receptor profile; animal work on it has leaned toward brain melanocortin circuits and behavior. Our Melanotan II research overview, PT-141 vs. Melanotan II comparison and the Melanotan II product page cover that side.
Questions a lab might ask before ordering
Which receptor subtype is my readout tied to? If the assay is MC1R-driven cAMP accumulation in a melanocyte or transfected line, Melanotan I is a long-standing reference agonist. If the question concerns brain melanocortin circuits, check which receptor the published work actually isolated.
Is a linear or a cyclic analog the better comparator? A linear 13-mer and a cyclic heptapeptide differ in size, rigidity and selectivity, so pairing them in one experiment can separate effects of the ring from effects of the receptor.
How will I verify what I received? Plan an identity check or compare against the lot documents, because the sequence has two non-natural positions that a mislabeled or degraded sample could lose.
What vial size fits the experiment? The planned catalog format is a 10 mg vial; work out solution preparation with the calculator before opening anything.
Handling the lyophilized powder
Melanotan I is supplied as a freeze-dried powder. General practice for lyophilized peptides applies: keep sealed vials cold, dry and away from light, avoid repeated temperature cycling, and write down the date a vial is first opened. Let a cold vial warm to room temperature before breaking the seal so that moisture does not condense on the cake. The tryptophan, histidine and tyrosine residues in this sequence are reasons to limit light and air exposure over long storage.
Background on the process is in what is lyophilization. For solution-preparation arithmetic, use the peptide calculator instead of a rule of thumb.
What to check on a Melanotan I certificate of analysis
A 13-residue peptide with two engineered positions can pick up deletion sequences, oxidized species and the wrong stereoisomer during synthesis, so the documentation matters.
HPLC purity with the chromatogram, not only a percentage. Look for shoulder peaks near the main peak and ask how they are integrated. Understanding HPLC purity explains the figures.
Identity confirmation. As generic buying guidance, ask which method confirms that the molecular weight is near 1,647 g/mol, and read the report for the result. Linear Melanotan I and cyclic Melanotan II differ by several hundred mass units, so a label mix-up should be easy to catch if identity is actually checked.
Lot reference and date that match the vial, and the name of the testing laboratory.
Stated fill amount and whether it refers to total powder or net peptide.
V8 Peptides publishes certificates of analysis on the COA page for tested lots. See how to read a COA for a walkthrough and the shop for the current catalog.
Common misunderstandings
“Melanotan I and II are the same thing.” They are different molecules: one is a linear 13-mer, the other a cyclic heptapeptide with a different receptor profile and mass.
“Afamelanotide is a different compound from Melanotan I.” It is the nonproprietary name for the same sequence.
“This article explains how to change skin color.” It does not. It describes receptor and melanogenesis research in cell and animal systems.
“Regulatory status elsewhere applies here.” Any status in another country does not change that products sold by V8 Peptides are for laboratory research only.
“Animal data predicts human outcomes.” It does not, and this article makes no such claim.
Frequently asked questions
What is Melanotan I?
A synthetic 13-residue analog of alpha-MSH with norleucine at position 4 and D-phenylalanine at position 7, studied as a melanocortin-receptor agonist in cell and animal models.
Is Melanotan I the same as afamelanotide?
Yes. Afamelanotide is the nonproprietary name for the same sequence, also written NDP-alpha-MSH or [Nle4, D-Phe7]-alpha-MSH.
What is the molecular weight of Melanotan I?
About 1,647 g/mol.
How does Melanotan I differ from native alpha-MSH?
Two substitutions: Met4 becomes norleucine and L-Phe7 becomes D-Phe. These raise stability and potency in classic melanocyte assays.
How does Melanotan I differ from Melanotan II?
Melanotan I is a linear 13-mer of about 1,647 g/mol. Melanotan II is a cyclic heptapeptide of about 1,024 g/mol with a broader receptor profile in binding assays.
Which receptors does it act on?
Melanocortin receptors, with strong activity at MC1R and measurable activity at others, depending on the assay system.
What kind of studies mention Melanotan I?
Cultured melanocyte and melanoma assays, frog-skin pigment assays, rodent coat-color genetics and receptor structure-activity work. All preclinical.
Is this article advice about skin color?
No. It covers melanocortin-receptor and melanogenesis research in cell and animal models only.
What should I look for on a Melanotan I COA?
An HPLC chromatogram, an identity method confirming a mass near 1,647 g/mol, a lot reference matching your vial, a test date and a named laboratory.
Is Melanotan I available from V8 Peptides?
A 10 mg vial is planned for laboratory research use only; check the product page for current status.
References
- Sawyer TK, et al. 4-Norleucine, 7-D-phenylalanine-alpha-melanocyte-stimulating hormone: a highly potent alpha-melanotropin with ultralong biological activity. Proc Natl Acad Sci U S A. 1980;77(10):5754-5758. PMID: 6777774.
- Cone RD, et al. The melanocortin receptors: agonists, antagonists, and the hormonal control of pigmentation. Recent Prog Horm Res. 1996;51:287-317. PMID: 8701084.
- Lengweiler S, et al. Evaluation of the immunogenicity of the synthetic alpha-melanocyte-stimulating hormone (alpha-MSH) analogue afamelanotide. Skin Pharmacol Physiol. 2015;28(2):103-113. PMID: 25402764.
- Haskell-Luevano C, et al. Discovery of prototype peptidomimetic agonists at the human melanocortin receptors MC1R and MC4R. J Med Chem. 1997;40(14):2133-2139. PMID: 9216831.
- Eliason NL, et al. Melanocortin receptor agonist melanotan-II microinjected in the nucleus accumbens decreases appetitive and consumptive responding for food. Neuropeptides. 2022;96:102289. PMID: 36155088.
All V8 Peptides products are sold for laboratory research use only. They are not drugs, foods, or cosmetics and are not intended for human or animal consumption.