Alpha-MSH

1 Identity

Linear tridecapeptide, all-L, N-terminally acetylated and C-terminally amidated. No cysteine and no disulfide. The endogenous post-translational cleavage product of proopiomelanocortin, sequence-identical to ACTH residues 1-13. It is also sold under seller-specific listing names, by one seller as a 10 mg vial and by another as an aliquot.

Sequence

Ac-Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly-Lys-Pro-Val-NH2 (Ac-SYSMEHFRWGKPV-NH2). N-terminal acetyl on Ser1; C-terminal primary amide on Val13. Both modifications are load-bearing and neither is optional.

Molecular formula

C77H109N21O19S

Average mass

1664.907 Da (C77H109N21O19S).

Monoisotopic mass

1663.79293 Da for the neutral molecule; [M+H]+ 1664.80021.

Salt / variant note

The melanocortin family is dense and this article sits at the center of it. (1) Melanotan I, afamelanotide — [Nle4, D-Phe7]-alpha-MSH, a superpotent analog of this exact molecule, already in this catalog. The two differ by a single Met-to-Nle substitution and a single L-to-D inversion at Phe7; the D-inversion is invisible to mass spectrometry and to achiral chromatography, so a lot of Melanotan I mislabeled as alpha-MSH is not excluded by any routine test. This is the sharpest identity risk on the record. (2) Melanotan II, a cyclic lactam heptapeptide, also in this catalog — different mass, easily excluded. (3) KPV, alpha-MSH (11-13), also in this catalog — the C-terminal tripeptide of this molecule. (4) N-Acetyl KPV, Ac-Lys-Pro-Val-NH2, likewise in this catalog — the acetylated and amidated form of that same tripeptide, and note that it carries this molecule's N-terminal acetylation pattern applied to its C-terminal fragment, which is a naming trap in both directions. (5) ACTH(1-13) free acid, unacetylated and unamidated — differs by 42 Da at the N-terminus and 1 Da at the C-terminus. This article is the parent of the four catalog entries named above.

2 Class & testing panel

Form

Single article

Testing panel

P1 — panel definition

3 Primary sources & evidence

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

Strong for identity and characterization: a classical endogenous peptide with a settled structure, four concordant institutional listings and a PubChem entry whose computed mass reproduces the value the formula gives. No physiological claim is made or adopted anywhere in this record.

4 Storage & specification

Storage

Lyophilized powder, stored at -20 degrees C plus or minus 5 degrees C, desiccated and protected from light. Met4 is an oxidation liability, and the peptide is not stored in solution.

Shelf life

Provisional 24 months at -20 degrees C, assigned by protocol. Met4 oxidation is the expected degradation route and is the stability-indicating attribute.