Peptide reference · Cyclic heptapeptide (free acid C-terminus)

PT-141 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 PT-141 Certificate of Analysis. The theoretical average molecular weight of PT-141 is 1,025.16 Da and the monoisotopic mass is 1,024.524 Da.

Check a PT-141 COA now

FP · 1,024.5243 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.

PT-141 reference data

Name	PT-141
Also known as	Bremelanotide
Type	Cyclic heptapeptide (free acid C-terminus)
Molecular formula	C₅₀H₆₈N₁₄O₁₀
Average molecular weight	1,025.16 g/mol (Da)
Monoisotopic mass	1,024.5243 Da
Residues	7
Sequence	Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH
Reference source	PubChem CID 9941379 (formula and mass cross-checked)

Expected mass spectrometry ions

A PT-141 COA may list one of these m/z values instead of the neutral mass. That is still consistent with PT-141. How charge states work.

Ion	m/z (monoisotopic)	m/z (average)
[M+H]⁺	1,025.53	1,026.17
[M+2H]²⁺	513.27	513.59
[M+3H]³⁺	342.52	342.73

What else can show up on a PT-141 COA

Masses calculated from PT-141'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)	1,046.51 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)	1,062.48 Da	Potassium adduct: an MS artifact. The [M+K]+ ion itself is at M + 38.96
TFA adduct (M + 113.99)	1,138.52 Da	Trifluoroacetic acid from purification seen as an adduct; residual TFA salt is real vial content
M + 16 (oxidation)	1,040.52 Da	Oxidation of Trp
M + 18.01 (uncyclized precursor)	1,042.53 Da	Linear precursor that never closed its lactam ring
M − 42 (missing acetyl)	982.51 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 PT-141 (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.

PT-141-specific things to watch

PT-141 (bremelanotide) shares Melanotan II's cyclic core but has a free carboxylic acid at the C-terminus instead of an amide, making it about 1 Da heavier (about 1025.2 Da).

Because the two are so close, a COA that only says "MS: 1024–1025" is not enough to confirm which one you have. A precise monoisotopic value (1024.52 for PT-141 versus 1023.54 for Melanotan II) is far more convincing.

HPLC retention time also differs slightly between the two; a lab that ran a reference standard alongside the sample gives you the strongest identity evidence.

PT-141's uncyclized precursor weighs about 18 Da more (near 1,043.2 Da average), and a missing acetyl gives a peak about 42 Da light (near 983.1 Da). Like Melanotan II, aspartimide formed during synthesis has the same mass as the product so MS alone can't reveal it; HPLC or MS/MS can.

Its single tryptophan gives an oxidation peak about 16 Da high (near 1,041.2 Da average). Bremelanotide is the generic name for the same molecule.

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.

Melanotan II: 1,024.18 Da (-0.98 Da)
TB-500: 889.01 Da (-136.15 Da)
Hexarelin: 887.04 Da (-138.12 Da)

Sources

PubChem CID 9941379: PT-141 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

Is bremelanotide the same as PT-141?

Yes. Bremelanotide is the generic name for PT-141. The COA mass should be about 1,025.2 Da average or 1,024.52 Da monoisotopic.

What does a PT-141 peak near 1,043 Da mean?

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

What does 513.3 m/z mean on a PT-141 COA?

The [M+2H]2+ ion of the 1,024.5 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.