Melanotan-II (MT-II): Cyclic Lactam Structure, Sequence, CAS 121062-08-6 and Salt Form

The CAS number of Melanotan-II (MT-II) is 121062-08-6, the registry number for the free peptide Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2, formula C50H69N15O9, molecular weight 1024.18 Da. Melanotan-II is a seven-residue cyclic peptide amide whose ring is a lactam, an amide bond between two side chains, not a disulfide. This page explains how that ring is formed and written, how it differs from PT-141 on paper and on a mass spectrum, and what the acetate salt adds to the weighed mass.

How is the Melanotan-II lactam ring formed, and how is it written?

The Melanotan-II ring is a side-chain lactam: the beta-carboxyl group of aspartate at position 2 and the epsilon-amine of lysine at position 7 are joined by an amide bond, which removes one water (18.01 Da) from the open chain. The open-chain form of the same seven residues computes to C50H71N15O10; closing the lactam gives C50H69N15O9 and 1024.18 Da, the values on the Pepta Labs listing and in PubChem CID 92432. The molecule contains no cysteine, so there is no disulfide bond anywhere in it.

Counted atom by atom, the ring has 23 members: 16 from the peptide backbone between the Asp2 and Lys7 alpha-carbons (both included), 2 from the aspartate side chain and 5 from the lysine side chain, the last being the lactam nitrogen (counted from the structure). Norleucine at position 1 carries an N-terminal acetyl group and sits outside the ring; the C-terminal lysine carboxyl is a primary amide (-NH2).

How is the ring written in sequence notation?

In three-letter notation the ring is written with "cyclo" and brackets around the residues it contains: Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2. The bracket opens at Asp2 and closes at Lys7, Ac- marks the acetylated N-terminus, and -NH2 marks the C-terminal amide. Because a bracket alone does not say which atoms are bonded, full records add a note naming the lactam between the Asp2 side-chain carboxyl and the Lys7 side-chain amine. The peptide sequence notation guide explains each element and the mass change it carries.

What ring-closure by-products show up by mass?

Two by-products of ring closure are visible by mass. An open-chain peptide that never cyclized is 18.01 Da heavier (C50H71N15O10, [M+H]+ 1042.558, calculated). Two chains joined by two lactams between them, each Asp2 bonded to the other chain's Lys7, form a cyclic dimer with exactly twice the Melanotan-II formula, so the dimer's [M+2H]2+ ion falls at m/z 1024.548, the same value as the monomer's [M+H]+; the isotope peaks of that ion are 0.5 m/z apart instead of 1.0 (calculated).

How does the Melanotan-II structure differ from PT-141?

PT-141 has the same seven residues, the same N-acetyl norleucine, the same D-Phe4 and the same Asp2-Lys7 lactam as Melanotan-II; the only difference is the C-terminus, a free carboxylic acid in PT-141 and a primary amide in Melanotan-II. Replacing -NH2 with -OH swaps one nitrogen and one hydrogen for one oxygen, so the formulas differ by N, H and O and the masses by 0.984 Da (calculated). Melanotan-II has no INN; the INN bremelanotide belongs to PT-141. The PT-141 structure and CAS guide covers that record.

Read the lowest-mass peak of each isotope cluster, not any peak near 1025.5. Melanotan-II has 50 carbon atoms, so its first isotope peak (mostly one carbon-13) is about 60% as tall as the monoisotopic peak and sits at m/z 1025.551, only 0.019 away from the PT-141 [M+H]+ at 1025.532 (calculated). A Melanotan-II spectrum therefore always shows a peak near 1025.5; what identifies it is the monoisotopic peak at 1024.548 below it. The expected-mass table gives the same calculated ions for every listed compound.

What does CAS 121062-08-6 identify, and what does the acetate change?

CAS 121062-08-6 identifies the free Melanotan-II peptide, not a salt, a vial size or a purity grade. A synthetic peptide with basic groups is isolated from its final purification as a salt, and Melanotan-II has two basic side chains (His3 and Arg5) but no free N-terminal amine and no free carboxyl, because the N-terminus is acetylated, the Asp and Lys side chains form the lactam and the C-terminus is an amide. The listing records the salt form as typically acetate or trifluoroacetate; the counter-ion adds weight to the solid without changing the peptide's CAS number, its formula or the m/z of its protonated ions.

The rows add 60.05 per acetic acid and 114.02 per trifluoroacetic acid to 1024.18 and ignore water; they show the arithmetic for each assumed counter-ion count, not the composition of any vial. A salt form can carry a registry number of its own, so a listing that pairs "Melanotan-II acetate" with 121062-08-6 is quoting the free-peptide number. A listing that quotes 189691-06-3 for Melanotan-II is quoting the PT-141 number. The TFA versus acetate guide explains counter-ions across the catalog.

What is Melanotan-II's FDA status, and where are its storage and report details?

Pepta Labs represents one kind of testing: independent third-party HPLC purity analysis and mass-spectrometry identity confirmation, commissioned through the purchasing group on a sampled production run from the source. Pepta Labs does not test in-house, and vial labels do not carry lot or batch numbers. The report is sent by email on request, before you order if you want to see it first (request a report); the CoA verification guide and HPLC versus mass spectrometry explain how to read one. The listing gives storage as 2-8 C on arrival or -20 C for long-term storage, protected from light and moisture; the peptide storage guide covers sealed powders in general. On the FDA compounding page whose content is current as of April 22, 2026, Melanotan II appears in the table of substances nominated but withdrawn; the FDA category page gives its row. For the wider category, start with what research peptides are. Pepta Labs also lists Melanotan-I; its identity data is on the Melanotan-I product page.

Frequently asked questions

Is Melanotan-II a cyclic peptide?

Yes. Melanotan-II is cyclic: a lactam bond between the Asp2 side-chain carboxyl and the Lys7 side-chain amine closes a 23-membered ring spanning six of its seven residues, and the acetylated norleucine sits outside the ring. The molecule has no cysteine, so the ring is not a disulfide.

How do I tell Melanotan-II from PT-141 on a report?

By the C-terminus and the monoisotopic mass. Melanotan-II (amide, C50H69N15O9) gives [M+H]+ at m/z 1024.548 and PT-141 (acid, C50H68N14O10) at 1025.532 (calculated); because the Melanotan-II carbon-13 isotope peak also sits near 1025.55, read the lowest-mass peak of the cluster, and check that the report's CAS is 121062-08-6 rather than 189691-06-3.

What is the CAS number of Melanotan-II?

The CAS number of Melanotan-II is 121062-08-6, assigned to the free peptide Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2 (C50H69N15O9, 1024.18 Da); the same record is PubChem CID 92432 and UNII UPF5CJ93X7, and acetate or trifluoroacetate salts do not change the free-peptide number the listing quotes.

How many residues does Melanotan-II have?

Melanotan-II has 7 residues: norleucine, aspartate, histidine, D-phenylalanine, arginine, tryptophan and lysine, with an N-terminal acetyl group, a C-terminal amide and one D-residue (D-Phe4).

Does Pepta Labs list Melanotan-I?

Yes. Pepta Labs lists Melanotan-I as a separate 10 mg listing, and its identity data is on the Melanotan-I product page.

Sources

• PubChem compound records: CID 92432 (Melanotan II) and CID 9941379 (bremelanotide).
• FDA Global Substance Registration System: UNII UPF5CJ93X7 and UNII 6Y24O4F92S.
• FDA human compounding page listing bulk substances under category 2 of the interim policies, content current as of April 22, 2026: the table of bulk substances nominated but withdrawn.
• Calculated values: monoisotopic masses and ion m/z from isotope masses (carbon-12, hydrogen-1, nitrogen-14, oxygen-16) with a proton mass of 1.007276; isotope heights from natural isotope abundances; salt formula weights by adding average counter-ion masses to the listing's 1024.18 Da.

All products are supplied for laboratory research use only and are not for human or animal use.