MT-II vs PT-141
Structural and analytical comparison · research use only
PT-141 is a metabolite-derived analog of MT-II, which makes these two the most closely related pair in this library — and the easiest to mix up. Their molecular weights differ by about one unit.
| Property | MT-II | PT-141 |
|---|---|---|
| Classification | Synthetic cyclic melanocortin-receptor analog | Synthetic cyclic melanocortin-receptor analog (bremelanotide) |
| Structural relationship | Parent structure | Deaminated analog of MT-II |
| CAS number | 121062-08-6 | 189691-06-3 |
| Molecular formula | C50H69N15O9 | C50H68N14O10 |
| Molecular weight | 1024.18 g/mol | 1025.16 g/mol |
| Ring structure | Cyclic lactam | Cyclic lactam |
| Supplied format | Lyophilized powder, sealed vial | Lyophilized powder, sealed vial |
| Purity standard | ≥99.2% (HPLC) | ≥99.2% (HPLC) |
Why the ~1 Da difference matters
PT-141 differs from MT-II by the replacement of a terminal amide with a carboxylic acid — a change of roughly one mass unit and one nitrogen for one oxygen. On a low-resolution instrument the two can look nearly identical. This is precisely why identity confirmation uses high-resolution mass spectrometry rather than mass alone, and why a lot-specific COA matters more for this pair than for almost any other in the catalog.
Practical implication for buyers
If you are specifying one of these two as a reference material, confirm the CAS number on the Certificate of Analysis rather than relying on the vial label alone. The two CAS numbers — 121062-08-6 and 189691-06-3 — are unambiguous where the masses are not.
Supplied strictly as a research reference material for in-vitro laboratory use. Not for human or veterinary use.
Frequently asked questions
Is PT-141 the same as MT-II?
No. PT-141 (bremelanotide) is a distinct compound derived from MT-II, differing by replacement of a terminal amide with a carboxylic acid. Their molecular weights differ by roughly one unit (1024.18 vs 1025.16 g/mol).
How can I be sure which one I have?
Check the CAS number on the lot Certificate of Analysis: 121062-08-6 for MT-II, 189691-06-3 for PT-141. Because the masses are within about one unit, identity is confirmed by high-resolution mass spectrometry.