What is MT-2's CAS number?

121062-08-6, with PubChem CID 92432 at 1024.2 Da.

Why does searching 'melanotan' give a different mass?

It returns afamelanotide at 1,646.8 Da — Melanotan I, a different and approved compound, 622 Da heavier.

Can mass spectrometry confirm the D-amino acid?

No. L- and D-phenylalanine have identical mass. The stereochemistry must be stated on the certificate and confirmed by other means.

MT-2 (Melanotan II)

MT-2 Chemical Identity and the Database Trap

JMWritten & reviewed by Jack Muncaster · Founder, UK PeptidesLast reviewed 2026-08-233 cited sources

MT-2 is PubChem CID 92432, CAS 121062-08-6, molecular weight 1024.2 Da, formula C50H69N15O9. Searching the bare name melanotan instead returns afamelanotide at 1,646.8 Da — a different and approved compound.

Key facts

PubChem CID: 92432
CAS: 121062-08-6
Molecular weight: 1024.2 Da
Formula: C50H69N15O9
'melanotan' returns: CID 16197727, afamelanotide
That mass: 1,646.8 Da — 622 Da heavier
Correct search: Melanotan II, with the numeral

The correct identifiers

CID 92432 at 1024.2 Da for C50H69N15O9, CAS 121062-08-6. PubChem also carries the full IUPAC name, which specifies the cyclic (2-7) architecture, the D-phenylalanine and both capped termini — an unusually complete record.

What the bare name returns

Afamelanotide, CID 16197727, at 1,646.8 Da for C78H111N21O19. That is Melanotan I and it is an approved medicine. The discrepancy against MT-2 is 622 Da — obvious once both are in view, and invisible if only one record is consulted.

Research material referenced

MT-2 10mg — third-party HPLC tested

View — £19.99

Why this collision is the worst of the set

This site has found several. A tirzepatide record cited as retatrutide. Thymosin beta-4's CAS quoted as TB-500's. An organic acid returned for KPV. This one is more consequential because it resolves from an unlicensed compound to a licensed one, so the wrong record does not merely mislead about mass — it invites the inference that the compound has regulatory standing it does not have.

Verifying a batch

Mass spectrometry against 1024.2 Da. A cyclic peptide typically shows characteristic fragmentation behaviour differing from a linear one, since the ring must open before the chain can fragment. No cysteine means no disulfide species; no methionine means no +16 Da oxidation satellite.

What a certificate should state

The full sequence including the D-phenylalanine, the norleucine and the cyclisation. A sequence written without stereochemistry is not a complete specification for this molecule — a peptide with L-phenylalanine at position 4 would be a different compound with the same mass, and mass spectrometry alone would not distinguish them.

Extended research context

The MT-2 (Melanotan II) deep dive

Deep dive: the identifier collision that points the wrong way

This site has now found several plausible-looking identifiers that resolve to the wrong molecule. A PubChem record for tirzepatide cited as retatrutide. Thymosin beta-4's CAS quoted as TB-500's. An unrelated organic acid returned for KPV. This one is the most consequential of the set, and the reason is direction. Searching PubChem for melanotan returns CID 16197727 - afamelanotide, at 1,646.8 Da for C78H111N21O19. That is Melanotan I, a linear thirteen-residue peptide, and it is an APPROVED MEDICINE, marketed as Scenesse for erythropoietic protoporphyria. Melanotan II is CID 92432: cyclic, seven residues, 1024.2 Da, 622 Da lighter, and assessed by no regulator anywhere. So the wrong record does not merely mislead about mass. It resolves from an unlicensed compound toward a licensed one, and the sequential naming - I and II - makes them look like versions of a single product rather than the structurally distinct compounds they are. Anyone verifying a certificate by searching the bare name lands on an approved drug's record and may never notice which compound it names.

Deep dive: four stability features in seven residues

PubChem's IUPAC name for CID 92432 is unusually complete and reads as a full specification: N-acetyl-L-norleucyl-L-alpha-aspartyl-L-histidyl-D-phenylalanyl-L-arginyl-L-tryptophyl-L-lysinamide, cyclic (2-7)-peptide. Unpack it and there are four independent protease-resistance features in a molecule of seven residues. A lactam bridge joins the aspartate side chain at position 2 to the lysine side chain at position 7, closing the ring - which both locks conformation for receptor engagement and eliminates the free termini exopeptidases require. Position 4 is D-phenylalanine, the mirror image of the natural form, and proteases are stereospecific enough that they largely cannot process it. The N-terminus is acetylated and the C-terminus amidated, capping both ends. And position 1 is norleucine, which is not among the twenty proteinogenic amino acids at all - so even setting aside the cyclisation and the D residue, this peptide could never have been a gene product. It is a fully synthetic construction rather than a modified natural sequence, and it is among the most chemically robust compounds in this catalogue as a direct result.

Deep dive: why the safety literature is included rather than omitted

There is a published dermatological literature here, and the editorial choice was to state it. Reid and colleagues documented atypical melanocytic naevi following melanotan injection in the Irish Medical Journal in 2013. Eijmael and colleagues published a risk review in the Nederlands Tijdschrift voor Geneeskunde in 2022 - a national medical journal considering the topic worth addressing directly. Case reports have a real limitation: they establish an observation without a denominator, so they cannot establish causation or incidence, which is the same constraint that applies to pharmacovigilance disproportionality signals. But that limitation cuts both ways. A case report is not proof of harm and it is not dismissible either. What makes the absence of better data significant here is that no regulator has assessed this compound - so there is no label, no contraindications, no adverse event reporting requirement and no monitoring framework. The information that would normally exist for an injectable product simply does not, and what is available instead is what individual clinicians chose to publish after seeing something. Presenting the compound without that context would give an incomplete picture of what is actually known.

Research applications

▸Melanocortin receptor pharmacology and selectivity research
▸Cyclic peptide and lactam bridge design studies
▸D-amino acid substitution and protease resistance research
▸Structure-activity work on constrained peptide analogues
▸Comparative work on alpha-MSH derivatives
▸Analytical method development for cyclic peptides

Handling checklist

✓Verify against CID 92432, 1024.2 Da, C50H69N15O9, CAS 121062-08-6
✓Never verify by searching 'melanotan' alone - that returns afamelanotide at 1,646.8 Da
✓Require stereochemistry on the certificate - D-Phe4 is not detectable by mass
✓Protect from light; tryptophan at position 6 is photochemically reactive
✓Store lyophilised, cold, dry and dark
✓Expect no disulfide or oxidation satellites - no cysteine, no methionine
✓Quantification at 280 nm is available thanks to the tryptophan

Common research-handling mistakes

Learnt from thousands of researcher orders across our UK labs.

✗ Searching PubChem for 'melanotan' to verify identity

Fix: That returns CID 16197727, afamelanotide, at 1,646.8 Da - an approved medicine and a different molecule. Use CID 92432 or search Melanotan II with the numeral.

✗ Treating Melanotan I and II as versions of one compound

Fix: They differ by 622 Da, by six residues, and by whether the peptide is cyclic or linear. Only Melanotan I has been assessed by regulators.

✗ Reading afamelanotide's approval as covering MT-2

Fix: Different molecule, different structure, different evidence, different assessment. Nothing transfers.

✗ Accepting a sequence written without stereochemistry

Fix: L- and D-phenylalanine have identical mass. A peptide with the L form at position 4 is a different compound that mass spectrometry cannot distinguish.

✗ Assuming a non-selective agonist affects only its intended receptor

Fix: Five melanocortin receptors share binding surfaces across different tissues. Engagement elsewhere is the same pharmacology in another place, not a side reaction.

Continue researching

Peer-reviewed guides, comparators and matched reference materials.

Frequently asked questions

What is MT-2's CAS number?: 121062-08-6, with PubChem CID 92432 at 1024.2 Da.
Why does searching 'melanotan' give a different mass?: It returns afamelanotide at 1,646.8 Da — Melanotan I, a different and approved compound, 622 Da heavier.
Can mass spectrometry confirm the D-amino acid?: No. L- and D-phenylalanine have identical mass. The stereochemistry must be stated on the certificate and confirmed by other means.

Primary sources & clinical trials

Peer-reviewed research and registered trials from PubMed, ClinicalTrials.gov, PubChem, FDA and NIH. All links open in a new tab and point to the primary source, so every claim can be verified at origin.

Written and reviewed by

Jack Muncaster · Founder, UK Peptides

Jack founded UK Peptides in Manchester after repeatedly receiving research compounds with missing or recycled paperwork. He is responsible for supplier selection, batch release decisions and the content published in this research library. Every article here is sourced to primary literature and every product page to a signed third-party certificate.

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