Compound Notes · Jul 28, 2026
PT-141 vs Melanotan II: One Gram per Mole Apart
This is the most closely related pair in the catalogue, and the one where the analytical detail actually matters.
Almost the same mass
| PT-141 | Melanotan II | |
|---|---|---|
| Molecular formula | C50H68N14O10 | C50H69N15O9 |
| Molar mass | 1,025.18 g/mol | 1,024.18 g/mol |
| CAS | 189691-06-3 | 121062-08-6 |
One gram per mole apart. Both have exactly fifty carbons. The formulas differ by a single nitrogen swapped for an oxygen, and one hydrogen.
That is not a coincidence. PT-141 is a metabolite-derived analogue of Melanotan II, differing at the C-terminus. Both belong to the melanocortin family and derive from the same parent structure.
Why that makes identity checking harder
Mass spectrometry confirms identity by measuring mass. When two candidate compounds differ by 1 g/mol out of about 1,025, a low-resolution instrument reporting a rounded mass cannot reliably tell them apart.
This is the one place in this catalogue where identity confirmation deserves more scrutiny than purity. Points worth checking on a certificate for either compound:
- Is the observed mass reported to enough decimal places to distinguish 1,024
- from 1,025, or is it rounded?
- Is retention time reported? Under a stated HPLC method the two separate
- cleanly even though their masses nearly coincide, so retention time carries
- identity information the mass alone does not.
- Is the theoretical mass stated alongside the observed one?
A certificate reporting "MS confirmed" with no numbers is not confirming much for this pair specifically. How to spot a forged certificate covers the general checks; this is the case where the identity section matters more than the purity headline.
Handling and testing status
Both are mid-sized peptides, straightforward to reconstitute, and neither is unusually fragile.
Melanotan II lot KP-MELA-2608 carries an independent certificate from Krause Analytical reporting 99.82% HPLC purity. PT-141 has no analysed lot at present.
Related
- PT-141 research primer
- Melanotan II research primer
- How to read a certificate of analysis
- Peptide glossary
Both are supplied as laboratory reagents for research use only. Neither is approved by Health Canada for human or veterinary use, and nothing here describes such use.
Compounds covered here
- Melanotan IIIn stock
- PT-141Out of stock
Supplied as a laboratory reagent for research use only. Not for human or veterinary use.
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