What mass should a melanotan II certificate report?

PubChem lists melanotan II, C50H69N15O9, at an average molecular weight of 1024.2 g/mol and a monoisotopic mass of 1023.54 Da. In electrospray the peptide usually carries two charges, so the raw spectrum often shows the [M+2H]2+ ion near m/z 513, the precursor used in published identification work. The report should convert this back to the neutral mass.

Are melanotan II vials usually filled to the label?

The best published evidence says not reliably. In one study of vials from three online shops, all labelled 10 mg, the measured content ranged from 4.32 to 8.84 mg. Identity was correct, but every vial was short, the lowest by more than half. That is why an amount test is worth adding to an identity result for this compound.

Is melanotan I the same as melanotan II?

No. Melanotan I is another name for afamelanotide, a linear 13-amino-acid peptide with an average mass of 1646.8 g/mol, authorised in the EU as Scenesse. Melanotan II is a cyclic 7-residue peptide at 1024.2 g/mol. Their masses are far apart, so identity testing tells them apart immediately when the expected compound is stated.

One of the 30 compound testing profiles

Melanotan II

Melanotan II is a 7-amino-acid cyclic peptide (C50H69N15O9, average mass 1024.2 g/mol); a test should confirm that mass and its C-terminal amide, and measure the amount in the vial, because published analysis of products sold online found every vial short of its label.

Melanotan II is a synthetic cyclic heptapeptide that PubChem describes as an analogue of alpha-melanotropin (alpha-MSH) (PubChem).

Test your Melanotan II

Identity card

Formula: C50H69N15O9
Average mass: 1,024.2 g/mol
Monoisotopic mass: 1,023.54 Da
Length: 7 residues
Modifications: N-terminal acetyl; Norleucine at position 1; D-phenylalanine at position 4; Side-chain lactam ring between Asp2 and Lys7; C-terminal amide
Also sold as: MT-II, MT2, Melanotan 2, MTII
Sequence: Ac-Nle-cyclo(Asp-His-D-Phe-Arg-Trp-Lys)-NH2
Note: PubChem lists the line notation Ac-Nle-Asp(1)-His-D-Phe-Arg-Trp-Lys(1)-NH2, where (1) marks the two residues joined through their side chains. Bremelanotide (PT-141) has the same ring with a free C-terminal acid instead of the amide.

Source: melanotan-II compound record (CID 92432).

Compare the mass printed on a certificate with the values above. The mass checker explains charge states and which mass a lab usually reports.

What usually goes wrong with Melanotan II. Documented failure modes, with sources.

Vials holding less than the label

A research team bought melanotan II from three online shops and measured identity, content and purity with LC-UV and MS/MS methods validated against guidelines for authorised medicines. Every shop claimed 10 mg per vial; the measured content ranged from 4.32 to 8.84 mg (Breindahl et al., 2015). Identity was correct in every vial. The failure was quantity, which an identity result alone never shows, so an amount test against a named reference standard is worth adding for this compound.

Unidentified impurities of several percent

In the same study, vials from two shops contained unknown impurities of 4.1 to 5.9%, and the third shop's impurities were below the quantification limit (Breindahl et al., 2015). A UV purity figure says how much of the signal is something else, not what it is. Mass data on each impurity peak is what shows whether it is a related peptide or an unrelated contaminant.

Melanotan I and melanotan II are different molecules

Afamelanotide, also known as melanotan-1, is a 13-amino-acid peptide (Chawathe and Sharma, 2026) recorded in PubChem as the linear sequence Ac-Ser-Tyr-Ser-Nle-Glu-His-D-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2, C78H111N21O19, 1646.8 g/mol (PubChem, afamelanotide), and is authorised in the EU as Scenesse (EMA). Melanotan II is a 7-residue ring at 1024.2 g/mol. The masses differ by more than 600 g/mol, so identity testing separates them at once, provided the report states which one it expected.

Bremelanotide, the free-acid twin

Bremelanotide (PT-141) has the same ring as melanotan II but ends in a free acid, at a monoisotopic mass of 1024.52 Da against 1023.54 Da (PubChem, bremelanotide). Hydrolysis of a C-terminal amide is a known degradation route: in stressed oxytocin, deamidation of the C-terminal Gly9-NH2 was identified by ESI-MS/MS (Hawe et al., 2009). The same change at melanotan II's Lys7 amide gives exactly the bremelanotide formula. A companion peak at that mass can therefore mean degradation or a mixed product, and only accurate mass resolves it from the main peak.

Which tests matter most

LC-MSMS/MS on the doubly charged ion, with qualifying fragment ions and retention time, is the published identification approach for melanotan II.
HPLC-UVQuantifies both the peptide and its impurities; the published method used UV detection at 218 nm.
HRMSSeparates the amide (1023.54 Da) from the bremelanotide-type free acid (1024.52 Da), which low-resolution mass blurs together.
Counter-ion contentPubChem lists an acetate salt form among melanotan II's synonyms, and the salt adds weight that is not peptide.

Reading Melanotan II results

In the published method, melanotan II was identified by MS/MS of its doubly charged precursor ion, [M+2H]2+, at m/z 513, using retention time and the relative abundance of five qualifying fragment ions (Breindahl et al., 2015). A certificate may therefore show a peak near m/z 513 rather than 1024; it should say that it converted the ion back to the neutral mass, or list the fragments it matched.

Afamelanotide, a relative from the same melanocortin family, formed fourteen degradation products under acid, base, neutral, oxidative, UV light and 60 °C stress, through truncation, methylation, deacetylation and oxidation (Chawathe and Sharma, 2026). Melanotan II also carries an N-terminal acetyl and a tryptophan. The numbers do not transfer, but the kinds of related peaks to expect in a badly stored sample do.

Public Tested.ph results

No public Melanotan II records yet

Each public Melanotan II record will appear here with its identity, HPLC purity and measured amount against the label, once the first vial is tested.

FIG 13Amount against the label

Questions about testing Melanotan II

What mass should a melanotan II certificate report?: PubChem lists melanotan II, C50H69N15O9, at an average molecular weight of 1024.2 g/mol and a monoisotopic mass of 1023.54 Da. In electrospray the peptide usually carries two charges, so the raw spectrum often shows the [M+2H]2+ ion near m/z 513, the precursor used in published identification work. The report should convert this back to the neutral mass.
Are melanotan II vials usually filled to the label?: The best published evidence says not reliably. In one study of vials from three online shops, all labelled 10 mg, the measured content ranged from 4.32 to 8.84 mg. Identity was correct, but every vial was short, the lowest by more than half. That is why an amount test is worth adding to an identity result for this compound.
Is melanotan I the same as melanotan II?: No. Melanotan I is another name for afamelanotide, a linear 13-amino-acid peptide with an average mass of 1646.8 g/mol, authorised in the EU as Scenesse. Melanotan II is a cyclic 7-residue peptide at 1024.2 g/mol. Their masses are far apart, so identity testing tells them apart immediately when the expected compound is stated.

Sources

melanotan-II compound record (CID 92432), PubChem, US National Library of Medicine
Breindahl T et al. Identification and characterization by LC-UV-MS/MS of melanotan II skin-tanning products sold illegally on the Internet. Drug Test Anal. 2015;7(2):164-72, Drug Testing and Analysis (PubMed)
Afamelanotide compound record (CID 16197727), PubChem, US National Library of Medicine
Chawathe A, Sharma N. Investigation of the stability profile of therapeutic alpha-MSH analogue afamelanotide by LC-HRMS. J Pharm Biomed Anal. 2026;272:117362, Journal of Pharmaceutical and Biomedical Analysis (PubMed)
Scenesse (afamelanotide) European public assessment report, European Medicines Agency
Bremelanotide compound record (CID 9941379), PubChem, US National Library of Medicine
Hawe A et al. Towards heat-stable oxytocin formulations: analysis of degradation kinetics and identification of degradation products. Pharm Res. 2009;26(7):1679-88, Pharmaceutical Research (PMC)

Last reviewed 6 Oct 2026.