Peptide reference · Cyclic heptapeptide (lactam bridge)

Melanotan II COA check: molecular weight, formula and what to look for

By the PeptideCOACheck team · Last updated November 2, 2026 · How we check

Use these reference values to check the identity result on a Melanotan II Certificate of Analysis. The theoretical average molecular weight of Melanotan II is 1,024.18 Da and the monoisotopic mass is 1,023.540 Da.

Check a Melanotan II COA now

FP · 1,023.5403 Fingerprint calculated from the formula: positions of the protonated ions, the natural isotope pattern of the neutral molecule (relative heights are calculated; ion heights are schematic) and the atom mix. It is a reference picture, not measured data.

Melanotan II reference data

Name	Melanotan II
Also known as	MT-2, MT-II
Type	Cyclic heptapeptide (lactam bridge)
Molecular formula	C₅₀H₆₉N₁₅O₉
Average molecular weight	1,024.18 g/mol (Da)
Monoisotopic mass	1,023.5403 Da
Residues	7
Sequence	Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH₂
Reference source	PubChem CID 92432 (formula and mass cross-checked)

Expected mass spectrometry ions

A Melanotan II COA may list one of these m/z values instead of the neutral mass. That is still consistent with Melanotan II. How charge states work.

Ion	m/z (monoisotopic)	m/z (average)
[M+H]⁺	1,024.55	1,025.19
[M+2H]²⁺	512.78	513.10
[M+3H]³⁺	342.19	342.40

What else can show up on a Melanotan II COA

Masses calculated from Melanotan II's formula and sequence for the adducts, degradation products and synthesis impurities a lab report can show. A match tells you what the extra peak probably is.

Signal	Value	What it means
Na adduct, deconvoluted (M + 21.98)	1,045.52 Da	Sodium adduct: an MS artifact, not a separate impurity. The [M+Na]+ ion itself is at M + 22.99
K adduct, deconvoluted (M + 37.96)	1,061.50 Da	Potassium adduct: an MS artifact. The [M+K]+ ion itself is at M + 38.96
TFA adduct (M + 113.99)	1,137.53 Da	Trifluoroacetic acid from purification seen as an adduct; residual TFA salt is real vial content
M + 16 (oxidation)	1,039.54 Da	Oxidation of Trp
M + 0.98 (free acid)	1,024.52 Da	C-terminal amide hydrolysed or free-acid synthesis error
M + 18.01 (uncyclized precursor)	1,041.55 Da	Linear precursor that never closed its lactam ring
M − 42 (missing acetyl)	981.53 Da	N-terminal acetyl group missing

Adduct rows are deconvoluted masses (the shift seen against [M+H]⁺); the singly charged [M+Na]⁺ ion itself sits at M + 22.99. Deletion masses alone can't say which copy of a repeated residue is missing.

Short peptides like Melanotan II (7 residues) are easy to make cleanly, so 98% HPLC purity should be the floor, not the goal. The full checklist is in how to read a peptide COA.

Melanotan II-specific things to watch

Melanotan II is a cyclic peptide (lactam bridge between Asp and Lys) with a C-terminal amide, about 1024.2 Da.

It differs from PT-141 (bremelanotide) by only about 1 Da: Melanotan II ends in an amide, PT-141 ends in a free acid. A low-resolution MS result can't reliably distinguish them, so the COA should report the mass to at least one decimal place and, ideally, the lab method.

Expect a [M+H]⁺ ion near 1024.5 m/z and a [M+2H]²⁺ ion near 512.8 m/z.

If the lactam ring never closed, the linear precursor weighs about 18 Da more (near 1,042.2 Da average). A missing N-terminal acetyl gives a peak about 42 Da light (near 982.2 Da). Aspartimide formed during synthesis has the same mass as the finished product, so MS alone can't reveal it; HPLC or MS/MS can.

Melanotan II has one tryptophan, so a +16 Da peak (near 1,040.2 Da average) means oxidation. Don't confuse it with Melanotan I (afamelanotide), a linear 13-residue peptide of about 1,646.9 Da.

Compounds with similar mass

If a COA's mass is off, compare it with these neighbors. A match to one of them can point to a mislabeled product.

PT-141: 1,025.16 Da (+0.98 Da)
TB-500: 889.01 Da (-135.17 Da)
Hexarelin: 887.04 Da (-137.14 Da)

Sources

PubChem CID 92432: Melanotan II formula and reference mass.
CIAAW: Standard atomic weights: Used to calculate average molecular weights.
NIST CODATA: proton mass: Used to convert m/z charge states to neutral mass.
PeptideCOACheck methodology: How the checker's thresholds work.

Last updated November 2, 2026. Written and maintained under our editorial policy. Found an error? See corrections.

Frequently asked questions

What does a Melanotan II peak near 1,042 Da mean?

The uncyclized linear precursor, about 18 Da heavier than the finished ring.

How do I tell Melanotan II from Melanotan I?

By mass: about 1,024.2 Da for Melanotan II versus about 1,646.9 Da for Melanotan I (afamelanotide).

What does 512.8 m/z mean on a Melanotan II COA?

The [M+2H]2+ ion of the 1,023.5 Da (monoisotopic) molecule.

For laboratory research use only. This page describes analytical chemistry data. It is not medical advice and makes no claim about effects in humans or animals.