Melanotan II (MT-2)

1 Identity

Melanocortin receptor ligands. Synthetic cyclic heptapeptide closed by a side-chain lactam, carrying one non-proteinogenic residue and one D-residue. Not a linear peptide and not a disulfide-cyclized peptide. which is commonly published but not confirmed against a primary registry. and CAS 121062-08-6 for the non-salt form. Not a synonym, and the single most important distinction on this record: bremelanotide (PT-141) is the same seven residues, the same lactam ring and the same N-acetyl, differing only in that the C-terminal amide is hydrolyzed to the free acid — it is +0.98 Da away.

Sequence

Ac-Nle1-cyclo[Asp2-His3-D-Phe4-Arg5-Trp6-Lys7]-NH2; seven residues. N-terminal acetyl on Nle1; C-terminal carboxamide on Lys7; cyclized through A side-chain lactam between the Asp2 beta-carboxyl and the Lys7 epsilon-amino group. The bridge is an amide bond, not A disulfide: there is no cysteine anywhere in the molecule, and a document describing the ring as a disulfide is describing something else. NORLEUCINE at position 1, a non-proteinogenic residue; D-phenylalanine at position 4, which is what makes the chiral method mandatory. Because the Lys7 epsilon-amine is consumed in the lactam and the N-terminus is capped, only His3 and Arg5 remain basic, so the counterion burden is bounded near two equivalents — bounded, not zero, and still to be measured. Written as a bare seven-letter linear string this sequence describes a different compound.

Molecular formula

C50H69N15O9 (free peptide, salt-free). Acetate salt written by suppliers as C50H69N15O9 . XC2H4O2 with the stoichiometry undeclared. Uncyclized linear precursor C50H71N15O10. Bremelanotide (des-amido free acid) C50H68N14O10. Trp6-oxidized form C50H69N15O10.

Average mass

1,024.20 Da (C50H69N15O9), from the formula on IUPAC 2021 abridged conventional atomic weights: 1,024.198. One seller prints 1024.2 and the computed value matches it exactly. Comparators: uncyclized linear precursor 1,042.21 (+18.02); bremelanotide 1,025.18 (+0.98); Trp6-oxidized 1,040.20 (+16.00); Melanotan I 1,646.87. Two equivalents of acetate is +120.10 Da, a 10.5 percent w/w burden on the vial.

Monoisotopic mass

1,023.54027 Da neutral (C50H69N15O9). [M+H]+ 1,024.54755; [M+2H]2+ 512.77741. Comparators: uncyclized linear precursor 1,041.55083; bremelanotide 1,024.52428; Trp6-oxidized 1,039.53518; cyclodimer 2,047.08054. Two arithmetic traps. First, this molecule's A+1 carbon-13 isotope peak lies at 1,024.5436 and bremelanotide's monoisotopic peak at 1,024.5243 — 0.0193 Da apart, which demands a resolving power of roughly 1 part in 53,000 at that m/z; a single-quadrupole LC-MS cannot do it, and a certificate that does not state its resolving power has not measured the des-amido limit whatever number it prints. Second, a bare "1024" is simultaneously this molecule's average mass (1,024.20) and bremelanotide's monoisotopic mass (1,024.52).

Salt / variant note

(1) acetate salt versus free peptide, sold under the same name: one seller writes C50H69N15O9 . XC2H4O2 with the stoichiometry undeclared; with the Lys7 epsilon-amine consumed in the lactam and the N-terminus capped, only His3 and Arg5 are basic, so the burden is bounded near two equivalents, up to +120.10 Da and 10.5 percent w/w. Bounded is not zero, and it is still to be measured per lot. (2) bremelanotide (PT-141), C50H68N14O10, 1,025.18 average / 1,024.52428 monoisotopic — same seven residues, same lactam, same acetyl, C-terminal amide hydrolyzed to the free acid, +0.98 Da, and the two arithmetic traps set out in the mass fields above follow from it. (3) uncyclized linear precursor, C50H71N15O10, 1,042.21 / 1,041.55083, +18.02 Da — detectable by intact accurate mass, and the reason the lactam is an analytical claim rather than a drawing. (4) CYCLODIMER, exact double mass, 2,047.08054 monoisotopic — invisible to reversed-phase chromatography alone and requiring size-exclusion HPLC with mass confirmation. (5) Trp6-oxidized species, C50H69N15O10, 1,040.20 / 1,039.53518, +16.00 Da. (6) Melanotan I, C78H111N21O19, 1,646.87 — a 13-residue linear peptide traded under the adjacent name, and confused with this one in both directions. (7) the isobaric class: norleucine at position 1 is exactly isobaric with leucine and isoleucine, and D-Phe4 is exactly isobaric with L-Phe4, so both the all-Leu impostor and the all-L impostor weigh the same to every decimal place and no mass measurement of any resolution reaches either. Those two isobaric classes together are why the chiral method and a documented amino acid analysis resolution are mandatory here rather than confirmatory.

2 Class & testing panel

Form

Single article

Testing panel

P3 — panel definition

3 Primary sources & evidence

The index reports the design and provenance of the literature, not a conclusion about effect.

Published literature exists and is thin where it matters. Human data: small early-phase studies from the University of Arizona and Palatin programs in the 1990s and early 2000s, typically single-dose, open-label or small crossover designs with subject counts in the low double digits. The development line was subsequently carried by bremelanotide, not by melanotan II, and melanotan II itself was never taken through a controlled Phase 3. The larger recent literature is not efficacy literature at all: it consists of dermatology and toxicology case reports, and of analytical surveys of gray-market material, several of which report vial contents at wide variance from the stated label — those surveys are the most directly relevant published work for a catalog record and are indexed as such. No modern controlled human trial of melanotan II was identified.

4 Storage & specification

Storage

Not offered, so no condition is qualified and no lot is held. The condition that would apply: lyophilized solid, -20 degrees C plus or minus 5 degrees C, desiccated, protected from light because of Trp6, inert headspace, vials equilibrated to ambient temperature before opening. Moisture control would be critical rather than routine, and the reason is specific to this molecule rather than general: the C-terminal amide hydrolyzes to bremelanotide, so water in the vial is not a purity issue but a change of identity toward the active moiety of an approved drug.

Shelf life

Not established — no lot will be purchased. The deamidation route to bremelanotide would in any case have forced a short, data-set retest interval rather than a nominal one, with the des-amido species reported individually at every pull and the retest interval driven by that single attribute rather than by gross purity.