commonly confused

Melanotan I (Afamelanotide) vs Melanotan II

Melanotan I is a linear thirteen-residue analog, while Melanotan II is a cyclic lactam heptapeptide. The shared name does not identify the same molecule.

Published: 2026-10-03Library record comparison: 2026-10-03

Identity side by side

Recorded molecular identities of Melanotan I (Afamelanotide) and Melanotan II
Recorded field	Melanotan I (Afamelanotide)	Melanotan II
Canonical name	Afamelanotide	Melanotan II
Aliases	Melanotan I; Melanotan-1; MT-I; MT-1; NDP-MSH; NDP-alpha-MSH	Melanotan-II; MT-II; MT-2
Development codes	CUV1647; MBJ05	not recorded
CAS	75921-69-6	121062-08-6
UNII	QW68W3J66U	UPF5CJ93X7
PubChem CID	16197727	92432
Molecular formula	C78H111N21O19	C50H69N15O9
Molecular weight (Da)	1646.85	1024.18
Sequence length (residues)	13	7
Terminal modifications	N-terminal acetylation; C-terminal amide	N-terminal acetylation; C-terminal amide
Structural modifications	Met4 to L-norleucine substitution; L-Phe7 to D-Phe substitution	Side-chain to side-chain lactam bridge, Asp to Lys; Norleucine substitution; D-phenylalanine substitution

Melanotan I (Afamelanotide): nomenclature notes

Melanotan I and afamelanotide are the same defined peptide. One name is commercial, the other the standardised substance name; they are not two different sequences. The opposite error is equally wrong: shared peptide identity does not make a research vial equivalent to a finished pharmaceutical product, and a product brand name is not an additional peptide. Most importantly, Melanotan I is NOT Melanotan II — this is a linear thirteen-residue analog, that is a cyclic lactam heptapeptide, and they differ in formula and mass. The unqualified word 'melanotan' is not a usable identity field.

Melanotan II: nomenclature notes

The unqualified word melanotan is not a usable identity field. Melanotan II is this cyclic lactam analog; Melanotan I is afamelanotide, a linear thirteen-residue peptide with a different formula and mass. They are distinct molecules and are never interchangeable as identities — but neither are they unrelated: both are analogs of alpha-melanocyte-stimulating hormone and both belong to the melanocortin family. What differs is the structure.

Recorded modification details

An aligned one-letter sequence is not recorded for this pair. The stored modification and sequence notes are shown below.

Melanotan I (Afamelanotide)

Terminal modifications: N-terminal acetylation; C-terminal amide
Structural modifications: Met4 to L-norleucine substitution; L-Phe7 to D-Phe substitution
Stored three-letter notation: Ac · Ser · Tyr · Ser · Nle · Glu · His · D-Phe · Arg · Trp · Gly · Lys · Pro · Val · NH2
Sequence notes: Thirteen residues, LINEAR — no disulfide and no lactam bridge. Nle is L-norleucine, substituted for the methionine of alpha-MSH; position 7 is D-phenylalanine. N-terminally acetylated and C-terminally amidated.

Melanotan II

Terminal modifications: N-terminal acetylation; C-terminal amide
Structural modifications: Side-chain to side-chain lactam bridge, Asp to Lys; Norleucine substitution; D-phenylalanine substitution
Stored three-letter notation: Ac · Nle · Asp · His · D-Phe · Arg · Trp · Lys · NH2
Sequence notes: A linear one-letter string cannot describe this molecule and none is published here. The peptide is seven residues, closed into a ring by a lactam bridge between the aspartate side-chain carboxyl and the lysine side-chain amine; the N-terminal norleucine sits OUTSIDE that ring, so the cyclized portion spans six residues. It also carries a D-phenylalanine. Written as a plain seven-residue sequence it would denote a different, linear compound.