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Melanotan I (Afamelanotide)
Technical Compound Reference | Orion Peptides Learning Library
Melanotan I (MT-1), also known as Afamelanotide or NDP-α-MSH, is a synthetic linear peptide containing 13 amino acid residues. Its structure is derived from alpha-melanocyte-stimulating hormone (α-MSH), with two amino acid substitutions that distinguish it from the native peptide.
These modifications involve norleucine at position 4 and D-phenylalanine at position 7.
Melanotan I belongs to the melanocortin receptor agonist family. It is particularly associated with melanocortin-1 receptor (MC1R) research, including receptor recognition, intracellular cAMP signaling, and melanogenesis-related pathways.
From a chemistry perspective, Melanotan I provides a useful example of how small changes to a peptide sequence can influence conformational properties, receptor interactions, and resistance to enzymatic degradation.
This reference examines the molecular structure of Melanotan I, its relationship to α-MSH, receptor pharmacology, analytical identification, degradation pathways, and laboratory characterization.
Melanotan I Molecular Identification
| Property | Technical information |
|---|---|
| Compound name | Melanotan I |
| Scientific name | Afamelanotide |
| Alternative names | MT-1, NDP-α-MSH, [Nle⁴,D-Phe⁷]-α-MSH |
| CAS number | 75921-69-6 |
| Molecular formula | C₇₈H₁₁₁N₂₁O₁₉ |
| Molecular weight | Approximately 1,646.85 g/mol |
| Peptide length | 13 amino acid residues |
| Molecular classification | Synthetic α-MSH analog |
| Structure | Linear peptide |
| N-terminal modification | Acetyl group |
| C-terminal modification | Amide group |
| Nonstandard residue | Norleucine at position 4 |
| Stereochemical modification | D-phenylalanine at position 7 |
| Principal receptor research | MC1R |
| Receptor family | Melanocortin GPCR family |
| PubChem CID | 16197727 |
These identifiers refer to the Afamelanotide free-peptide structure. Acetate salts and other material-specific forms may have different overall compositions.
Chromatographic purity, physical appearance, water content, and material quantity are batch-specific characteristics rather than universal molecular properties.
1. The Molecular Relationship Between α-MSH and Melanotan I
Melanotan I was developed from alpha-melanocyte-stimulating hormone, commonly abbreviated α-MSH.
α-MSH is a peptide belonging to the melanocortin family.
It contains 13 amino acid residues and interacts with melanocortin receptors through specific molecular recognition mechanisms.
Researchers studying α-MSH analogs investigate how modifications to its amino acid sequence affect receptor binding and signaling.
Melanotan I retains most of the native α-MSH sequence.
Only two amino acid residues are replaced.
Despite these limited changes, the resulting molecule has distinct pharmacological and stability-related characteristics.
The Natural α-MSH Sequence
The peptide sequence of α-MSH is commonly represented as:
Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂
The structure includes an acetylated N-terminus and an amidated C-terminus.
These terminal groups form part of the peptide’s defined chemical identity.
The Melanotan I Sequence
Melanotan I has the following sequence:
Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂
Nle represents norleucine.
D-Phe represents D-phenylalanine.
Comparing the sequences shows that positions 4 and 7 are the principal amino acid substitutions.
Sequence Comparison
| Position | Natural α-MSH | Melanotan I |
|---|---|---|
| 1 | Ser | Ser |
| 2 | Tyr | Tyr |
| 3 | Ser | Ser |
| 4 | Met | Nle |
| 5 | Glu | Glu |
| 6 | His | His |
| 7 | L-Phe | D-Phe |
| 8 | Arg | Arg |
| 9 | Trp | Trp |
| 10 | Gly | Gly |
| 11 | Lys | Lys |
| 12 | Pro | Pro |
| 13 | Val | Val |
The remaining peptide backbone sequence is conserved.
This makes Melanotan I a useful example in structure-activity relationship research.
2. Why Norleucine Replaces Methionine
One of the defining modifications in Melanotan I occurs at position 4.
In native α-MSH, the fourth residue is methionine.
In Melanotan I, it is replaced by norleucine.
What Is Norleucine?
Norleucine is a non-proteinogenic amino acid with a straight-chain hydrocarbon side group.
Its structure differs from methionine because it does not contain the sulfur atom present in methionine’s side chain.
This difference is important in peptide chemistry.
Methionine and Oxidation
Methionine contains a thioether group that can undergo oxidation.
Under suitable chemical conditions, methionine oxidation can produce methionine sulfoxide and related oxidation compounds.
These changes may alter the chemical and physical characteristics of a peptide.
Replacing methionine with norleucine removes that specific sulfur-containing oxidation site.
However, this substitution does not make the complete peptide immune to oxidation.
Other amino acid residues and functional groups may still participate in degradation reactions.
Does Norleucine Change the Peptide’s Shape?
Amino acid substitutions can influence local molecular conformation and interactions.
Norleucine and methionine have different chemical structures even though both contain relatively hydrophobic side chains.
The effect of the substitution depends on the surrounding sequence and molecular environment.
For Melanotan I, the change is part of the engineered molecular design.
3. The Role of D-Phenylalanine at Position 7
The second major substitution occurs at position 7.
Natural α-MSH contains L-phenylalanine at this position.
Melanotan I contains D-phenylalanine.
What Is the Difference Between D- and L-Amino Acids?
Many amino acids exist in two stereochemical forms.
These forms are known as enantiomers.
They have the same molecular formula but different spatial arrangements around a chiral center.
Most amino acid residues incorporated into naturally synthesized proteins have the L-configuration.
D-phenylalanine has the opposite configuration from L-phenylalanine.
Why Does Stereochemistry Matter?
Receptors recognize molecules through three-dimensional interactions.
Changing the stereochemistry of one amino acid can influence how a peptide fits within a receptor-binding region.
It can also affect local conformational preferences and interactions with peptide-degrading enzymes.
In Melanotan I, the D-Phe7 substitution contributes to its characteristic receptor pharmacology and stability profile.
Does D-Phenylalanine Change the Molecular Formula?
No.
D-phenylalanine and L-phenylalanine have the same elemental composition.
Replacing one with the other changes the stereochemistry, not the molecular formula.
This is an important point for analytical characterization.
A molecular mass measurement alone cannot distinguish these stereoisomers.
Methods capable of evaluating stereochemical identity may be required where this distinction is critical.
4. Why the Core Melanocortin Sequence Matters
The central region of α-MSH contains a sequence important for melanocortin receptor recognition.
A commonly identified pharmacophore includes the residues:
His-Phe-Arg-Trp
This is often abbreviated HFRW.
In Melanotan I, the phenylalanine within this region has the D-configuration.
What Is a Pharmacophore?
A pharmacophore describes structural features important for molecular recognition and biological activity.
For peptide ligands, these features may involve particular amino acid side chains and their spatial arrangement.
The HFRW region is an important part of melanocortin peptide research.
Does the Core Sequence Work Independently?
The activity of a peptide cannot necessarily be predicted from four residues alone.
Other parts of the molecule influence conformation, stability, and receptor interactions.
Therefore, the receptor activity of a complete 13-residue peptide should not automatically be assigned to a short isolated sequence.
5. MC1R and Melanocortin Receptor Pharmacology
Melanotan I is primarily associated with the melanocortin-1 receptor, or MC1R.
MC1R belongs to the G protein-coupled receptor family.
Melanocortin receptors are divided into five subtypes, designated MC1R through MC5R.
These proteins share related structural characteristics but have different biological distributions and functions.
Receptor Family Overview
| Receptor | General research association |
|---|---|
| MC1R | Melanocyte signaling and pigment-related pathways |
| MC2R | ACTH-dependent receptor signaling |
| MC3R | Melanocortin signaling in several cellular systems |
| MC4R | Central melanocortin receptor pharmacology |
| MC5R | Peripheral melanocortin signaling |
Melanotan I is often described as an MC1R agonist because this receptor is central to its well-characterized activity.
However, this classification should not be interpreted as proof that the peptide has absolutely no activity at other melanocortin receptor subtypes.
Receptor selectivity is a quantitative characteristic that depends on the assay and comparison being made.
How Does MC1R Activation Begin?
When a compatible ligand interacts with MC1R, it can stabilize an active receptor conformation.
The receptor can then communicate with intracellular G proteins.
MC1R primarily signals through Gs-associated pathways.
This leads to activation of adenylyl cyclase and increased production of cyclic adenosine monophosphate (cAMP).
Why Is cAMP Important?
cAMP acts as a second messenger inside cells.
It transmits information from activated membrane receptors to downstream signaling proteins.
In melanocyte research, MC1R-related cAMP signaling is studied alongside pathways involving protein kinase A, CREB, and MITF.
These proteins participate in the regulation of melanogenesis-related gene expression.
Molecular Signaling Summary
| Stage | Cellular event |
|---|---|
| 1 | Melanotan I interacts with MC1R |
| 2 | Receptor activation stimulates Gs signaling |
| 3 | Adenylyl cyclase activity increases |
| 4 | Intracellular cAMP rises |
| 5 | Downstream signaling proteins respond |
| 6 | Melanogenesis-related gene expression may change |
This is a simplified description of receptor signaling under appropriate experimental conditions.
Different cell systems may produce different measured responses.
6. How Melanotan I Is Characterized in Receptor Assays
Receptor pharmacology experiments investigate how a compound interacts with a defined molecular target.
For Melanotan I, researchers can examine binding and functional activity using experimental systems expressing melanocortin receptors.
Binding Affinity
Binding affinity describes the association between a ligand and its receptor.
It can be measured using suitable binding assays.
An affinity measurement does not automatically establish how strongly a ligand activates intracellular signaling.
Functional Potency
Functional potency describes the concentration associated with a specified response.
It is commonly expressed through an EC50 value.
EC50 represents the concentration producing half of the maximal response measured in a particular experimental system.
Emax
Emax describes the maximum measured effect under the assay conditions.
Two compounds may have similar EC50 values but different maximal responses.
Comparing Experimental Results
When interpreting Melanotan I receptor assays, researchers should consider receptor expression, reference ligands, signal detection, assay duration, and cellular background.
Results obtained from different experimental systems may not be directly interchangeable.
7. Melanotan I and Melanotan II: A Structural Comparison
Melanotan I and Melanotan II belong to the broader group of synthetic melanocortin peptides.
However, they are different molecules.
Melanotan I is a linear 13-residue analog of α-MSH.
Melanotan II is a shorter, cyclic heptapeptide.
Why Does Cyclization Matter?
A cyclic peptide contains a covalent linkage that constrains part of its structure.
Melanotan II contains a lactam bridge.
Melanotan I does not contain this cyclic linkage.
Differences in peptide length and geometry can affect receptor recognition, conformation, and enzymatic susceptibility.
Comparison Table
| Molecular characteristic | Melanotan I | Melanotan II |
|---|---|---|
| Alternative name | Afamelanotide | MT-II |
| Amino acid residues | 13 | 7 |
| Peptide structure | Linear | Cyclic |
| Structural template | α-MSH analog | Shortened α-MSH analog |
| Main modification | Nle4 and D-Phe7 | Cyclic lactam structure and modified residues |
| Molecular formula | C₇₈H₁₁₁N₂₁O₁₉ | C₅₀H₆₉N₁₅O₉ |
| Molecular weight | Approximately 1,646.85 g/mol | Approximately 1,024.18 g/mol |
| Receptor profile | Primarily associated with MC1R | Broader melanocortin receptor agonism |
The differences mean that analytical information and receptor assay results from one compound should not automatically be applied to the other.
8. Mass Spectrometry and Molecular Identity
Mass spectrometry is an important tool for identifying synthetic peptides.
It measures the mass-to-charge ratios of ions generated from a sample.
For Melanotan I, the expected molecular mass is calculated from its defined elemental composition.
Expected Molecular Mass
The molecular formula C₇₈H₁₁₁N₂₁O₁₉ corresponds to an average molecular weight of approximately 1,646.85 g/mol.
Analytical mass spectrometry may report monoisotopic mass, average mass, or the mass-to-charge ratio of a particular ion.
These values should not be confused.
Why Does Ion Charge Matter?
In electrospray ionization mass spectrometry, a peptide can produce ions carrying different numbers of charges.
For example, a singly charged ion and a doubly charged ion have different measured mass-to-charge ratios.
The signals must be interpreted according to the charge state.
Can Mass Spectrometry Confirm D-Phe7?
A matching intact molecular mass cannot establish whether phenylalanine has the D- or L-configuration.
The two forms are stereoisomers with the same elemental composition.
Additional stereochemically informative methods may be required.
Why Is This Important for Melanotan I?
The D-Phe7 modification is central to the molecule’s identity.
Therefore, a complete characterization strategy should distinguish basic molecular mass agreement from confirmation of the expected stereochemical structure.
9. Chromatographic Characterization
Chromatography is used to separate components in a chemical sample.
For synthetic peptides, reversed-phase high-performance liquid chromatography is a common analytical method.
What Does RP-HPLC Measure?
RP-HPLC separates compounds according to their interactions with the chromatographic stationary and mobile phases.
The resulting chromatogram contains peaks corresponding to components detected under the method.
For Melanotan I, chromatography can help evaluate the main peptide peak and selected impurities.
What Can Influence Retention?
Chromatographic retention depends on several factors, including molecular hydrophobicity, mobile-phase composition, stationary phase, and analytical conditions.
Melanotan I contains aromatic amino acid residues and other groups that affect its chromatographic behavior.
What Does 99% HPLC Purity Mean?
A reported 99% HPLC purity typically refers to a chromatographic calculation under specified conditions.
It does not necessarily mean 99% of the total sample mass consists of the intended molecule.
The reported value may not account for substances not detected by the method.
HPLC vs LC-MS
| Technique | Main purpose |
|---|---|
| RP-HPLC | Separation and chromatographic purity assessment |
| LC-MS | Molecular mass information linked to chromatographic components |
| LC-MS/MS | Additional fragmentation and structural evidence |
| Chiral or stereochemical analysis | Investigation of amino acid configuration |
| Content assay | Quantification under a specified analytical method |
Complementary techniques are often necessary for a more complete characterization.
10. Melanotan I Stability: Findings From Recent Research
The stability of Melanotan I is particularly relevant because its molecular structure includes chemically modified amino acids and terminal groups.
A 2026 analytical study examined the degradation profile of Afamelanotide using liquid chromatography and high-resolution mass spectrometry.
The investigators subjected the peptide to selected chemical and physical stress conditions.
Their research identified 14 degradation compounds.
Reported Degradation Pathways
The study reported molecular changes involving:
- Peptide chain truncation
- Oxidation
- Deacetylation
- Methylation-related changes
These findings show that amino acid substitutions associated with improved resistance to certain degradation processes do not make the molecule chemically inert.
Why Is N-Terminal Acetylation Important?
Melanotan I contains an N-terminal acetyl group.
This modification changes the chemical structure of the peptide’s terminal amino group.
Loss of the acetyl group would produce a different molecular species.
Such changes can be investigated using appropriate chromatographic and mass spectrometric methods.
Does the Peptide’s Linear Structure Affect Stability?
A linear peptide has different conformational and chemical characteristics from a cyclic peptide.
However, stability cannot be predicted from this distinction alone.
Individual amino acids, terminal modifications, formulation components, and environmental conditions also matter.
Lyophilized and Solution-State Stability
The physical state of a material may influence the chemical processes that occur during storage.
Lyophilized peptides contain relatively little water but can still experience degradation.
Solution-state materials may undergo additional reactions depending on the chemical environment.
Specific stability claims require experimental evidence covering the actual formulation and conditions being discussed.
Important Research Reference
Chawathe A. and Sharma N. (2026), Investigation of the Stability Profile of Therapeutic α-MSH Analogue: Insights from Liquid Chromatography-High Resolution Mass Spectrometry Analysis of Afamelanotide.
Journal: Journal of Pharmaceutical and Biomedical Analysis.
https://doi.org/10.1016/j.jpba.2026.117362
11. Reading a Melanotan I Certificate of Analysis
A Certificate of Analysis is a record of tests performed on a particular sample or batch.
For Melanotan I, analytical documentation should account for its molecular mass, sequence-related properties, and relevant chemical modifications.
Useful COA Fields
| Field | Scientific significance |
|---|---|
| Compound identity | Name of the material analyzed |
| CAS number | Reference chemical identifier |
| Batch number | Traceability of the analyzed material |
| Molecular formula | Theoretical elemental composition |
| Expected molecular mass | Reference for mass-related analysis |
| HPLC chromatogram | Chromatographic separation profile |
| HPLC purity | Reported purity under the specified method |
| Mass spectrometry | Molecular mass-related evidence |
| Peptide content | Quantitative content, if tested |
| Water content | Residual moisture, if analyzed |
| Testing laboratory | Identification of the organization performing analysis |
| Analytical date | Date of the measurements |
Does a COA Confirm the Complete Molecular Structure?
Only to the extent supported by its tests.
Mass spectrometry may establish consistency with an expected molecular mass.
HPLC may provide chromatographic purity information.
Neither alone necessarily establishes stereochemical identity.
If confirming D-Phe7 is an analytical requirement, the testing strategy should address that property specifically.
Why Is Batch-Specific Testing Important?
Different production batches may have different impurity profiles.
Results from one batch cannot automatically be assigned to another.
A batch-matched analytical report provides stronger traceability than a generic compound reference.
12. Technical FAQs About Melanotan I
What is Melanotan I?
Melanotan I is a synthetic 13-amino-acid analog of α-MSH containing norleucine at position 4 and D-phenylalanine at position 7.
What is another name for Melanotan I?
Its established scientific name is Afamelanotide. It is also called NDP-α-MSH or MT-1.
What is the molecular formula of Melanotan I?
C₇₈H₁₁₁N₂₁O₁₉.
What is its molecular weight?
Approximately 1,646.85 g/mol.
What is the CAS number?
75921-69-6.
How many amino acids does it contain?
13 amino acid residues.
Is Melanotan I a cyclic peptide?
No. It is a linear peptide with terminal chemical modifications.
What is the Melanotan I sequence?
Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH₂.
What does NDP-α-MSH mean?
It refers to the norleucine and D-phenylalanine substitutions in the α-MSH analog.
What does Nle stand for?
Norleucine, a non-proteinogenic amino acid.
Why does Melanotan I contain D-phenylalanine?
The D-configuration influences peptide conformation and receptor interactions.
What is the primary receptor associated with Melanotan I?
MC1R, the melanocortin-1 receptor.
Is Melanotan I the same as Melanotan II?
No. They differ in peptide length, structure, and receptor activity.
Is Melanotan I the same as melatonin?
No. Melatonin and Melanotan I are chemically unrelated compounds.
How is Melanotan I identified in laboratory analysis?
Methods can include mass spectrometry, chromatography, and additional structural characterization.
Can HPLC measure peptide purity?
HPLC can provide chromatographic purity information under specified conditions.
Does HPLC verify D-phenylalanine?
Conventional reversed-phase HPLC alone may not conclusively establish the stereochemical identity of D-phenylalanine.
Can Melanotan I undergo oxidation?
Yes. The complete molecule remains susceptible to certain chemical degradation processes.
What did the 2026 stability study find?
The researchers identified 14 degradation compounds using high-resolution mass spectrometric techniques.
Does Melanotan I have a universally established research-material shelf life?
No. Shelf life must be established using stability data for the specific material and formulation.
Is Afamelanotide an FDA-approved active ingredient?
Yes. Afamelanotide is the active ingredient in the FDA-approved prescription compound Scenesse. That approval does not extend to independently marketed research peptide compounds.
13. Scientific References
Chemical Identity and Structure
PubChem: Afamelanotide
Reference information: Molecular formula, molecular weight, CAS number, sequence, chemical identifiers, and structural representation.
Peptide Structure and Engineering
Exploring FDA-Approved Frontiers: Insights into Natural and Engineered Peptide Analogues (2024)
Research relevance: Amino acid modifications, melanocortin receptor pharmacology, and engineered α-MSH analogs.
Afamelanotide Stability Research
Chawathe A. and Sharma N. (2026)
Investigation of the Stability Profile of Therapeutic α-MSH Analogue: Insights from Liquid Chromatography-High Resolution Mass Spectrometry Analysis of Afamelanotide.
Journal: Journal of Pharmaceutical and Biomedical Analysis.
Research relevance: Peptide degradation compounds, chromatographic methods, and structural identification by HRMS/MS.
https://doi.org/10.1016/j.jpba.2026.117362
Melanocortin Receptor Reference
IUPHAR/BPS Guide to Pharmacology
Research relevance: Melanocortin receptor classification, ligand recognition, and receptor activity.
Regulatory and Chemical Quality Reference
FDA: Scenesse (Afamelanotide), NDA 210797
Research relevance: Official chemical identity, approved compound documentation, and pharmaceutical quality evaluation.
https://www.accessdata.fda.gov/drugsatfda_docs/nda/2019/210797Orig1s000ChemR.pdf
Conclusion
Melanotan I is a 13-residue synthetic analog of α-melanocyte-stimulating hormone characterized by two major amino acid substitutions: norleucine at position 4 and D-phenylalanine at position 7.
These modifications distinguish it from native α-MSH and contribute to its molecular properties and melanocortin receptor pharmacology.
The compound is primarily associated with MC1R research, including ligand recognition and cAMP-mediated signaling.
From an analytical perspective, Melanotan I presents important questions involving stereochemical identity, peptide sequence verification, chromatographic purity, mass spectrometric characterization, and degradation pathways.
Recent stability research has further demonstrated that the compound can undergo multiple chemical transformations under selected experimental conditions.
Accurate technical characterization requires distinguishing the reference molecule’s established chemical identity from the analytical results obtained for a specific material or batch.
Research reference notice: This page provides information about peptide chemistry, molecular structure, receptor pharmacology, and analytical research. It does not include dosing, reconstitution, administration, or personal-use instructions. FDA approval of a specific Afamelanotide medicine does not establish approval or clinical suitability of separately supplied research materials.
