PT-141 is a laboratory designation, not a chemical name. This page sets out the identity of PT-141: what the sequence is, how the ring is closed, what mass to expect, how it behaves under reverse-phase HPLC, and how a buyer can confirm that the material in a vial is what the label says. It leaves aside any question of what PT-141 does, and it offers no preparation, handling or dosing guidance.
The sequence and structure behind the name
PT-141 is a cyclic heptapeptide, conventionally written Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH. Seven residues, with three features that set it apart from an ordinary linear peptide:
- An acetylated N-terminus. The first residue, norleucine (Nle), carries an acetyl group, so there is no free N-terminal amine.
- A lactam bridge. The side-chain carboxyl of the aspartic acid is joined to the side-chain amine of the lysine by an amide bond, closing the six residues from Asp to Lys into a ring. The norleucine sits outside the ring as an acetylated tail.
- Two residues that are not ordinary protein residues. Norleucine is a non-proteinogenic amino acid with an unbranched four-carbon side chain, and the phenylalanine is the D-enantiomer.
The C-terminus is a free carboxylic acid, not an amide. That detail matters more for this compound than for almost any other in the catalogue, for reasons covered below. The molecular formula is C50H68N14O10 and the average molecular mass is approximately 1025.2 g/mol.
What the name PT-141 refers to
“PT-141” is a development code. The compound’s International Nonproprietary Name is bremelanotide, and the two names refer to the same structure. Neither name carries any structural information: nothing in “PT-141” tells you the molecule is cyclic, acetylated or a free acid.
That is a practical problem because PT-141 has a very close structural neighbour. Melanotan II has the identical ring, the identical acetylated norleucine tail and the identical residues — the only difference is that Melanotan II ends in a C-terminal amide where PT-141 ends in a free acid. The two formulas differ by swapping one oxygen for one NH group, a mass difference of about 1 Da (approximately 1025.2 versus 1024.2 g/mol). Both are also related to the linear 13-residue Melanotan I, but that compound is far easier to tell apart.
How PT-141 behaves on reverse-phase HPLC
PT-141 is a mixed molecule. Norleucine, D-phenylalanine and tryptophan give it substantial hydrophobic surface; arginine and histidine give it positive charge under the acidic conditions used in most peptide methods. On a C18 column with an acidic, ion-paired mobile phase it retains well and elutes in the middle of a typical gradient, far from the solvent front where very short polar peptides appear.
The cyclic structure also helps the chromatography. A lactam-bridged ring is conformationally restricted, and constrained peptides tend to give sharper, more symmetrical peaks than flexible linear chains of similar size. A broad or split main peak is therefore worth a question rather than an assumption.
Detection is straightforward. Tryptophan absorbs strongly near 280 nm, so PT-141 can be read there as well as on the peptide bond at around 214 nm. A certificate should say which wavelength was used, because impurities that lack an aromatic residue are nearly invisible at 280 nm and a purity figure read only at that wavelength can look better than the same sample read at 214 nm. The guide to reading an HPLC chromatogram covers why the integration wavelength changes the number.
What an analysis for PT-141 should show
A useful certificate reports the column chemistry and dimensions, the mobile phase and gradient including the ion-pairing additive, the detection wavelength, the retention time, the integrated peak area with the integration window visible, and a mass confirmation consistent with approximately 1025 g/mol.
The free-acid versus amide question is where the mass confirmation earns its keep. A 1 Da difference is clearly resolved on a high-resolution instrument, but on a lower-resolution one the two species can be hard to separate, and the retention times of the acid and the amide can sit close together under some conditions. A report that states the observed mass to one decimal place, and names the instrument type, is far more informative than one that simply says “mass confirmed”. The article on mass spectrometry and peptide identity explains what an observed mass can and cannot prove.
Two further numbers belong on the certificate. Net peptide content is not the same as chromatographic purity: purity describes what fraction of the peptide present is the target sequence, content describes how much of the vial’s mass is peptide at all. And the counter-ion should be named. With arginine and histidine both able to carry a positive charge, PT-141 is normally isolated as a salt, typically acetate or trifluoroacetate, and the counter-ion adds mass that a purity figure does not show.
Finally, the D-phenylalanine is a reminder that stereochemistry is invisible to a mass measurement. A sample in which that residue has racemised has exactly the same mass as correct material, and only a chromatographic separation can reveal it.
Confirming identity independently
Search on the structure, not the code. Public chemical databases index PT-141 under its nonproprietary name and by structure, and a search on the sequence — including the cyclo notation and the free-acid C-terminus — will show quickly whether the formula and mass a vendor quotes match the formula and mass everyone else uses. If a supplier quotes a mass near 1024 for material sold as PT-141, the likely explanation is that the amide and the acid have been confused somewhere, and that is worth resolving before the vial is opened.
How PT-141 is supplied here
PT-141 10mg is supplied as a lyophilised powder in a sealed vial. An independent reverse-phase HPLC analysis of PT-141 is published with the product. You will find the PT-141 vial size and its published HPLC purity on the product page. PT-141 vials carry no batch identifiers; instead, its crimp and cap color link it to the published test.
Frequently asked questions
What is the sequence of PT-141?
Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH: an acetylated, cyclic heptapeptide with a lactam bridge between aspartic acid and lysine and a free-acid C-terminus.
Is PT-141 the same as bremelanotide?
Yes. PT-141 is a development code and bremelanotide is the International Nonproprietary Name for the same structure.
How is PT-141 different from Melanotan II?
Only at the C-terminus. PT-141 is a free acid and Melanotan II is an amide. The ring and every residue are otherwise identical, and the masses differ by about 1 Da.
What molecular weight should I expect for PT-141?
Approximately 1025.2 g/mol for the neutral molecule, C50H68N14O10. A salt form will read higher in bulk, which is why the counter-ion has to be declared.
Can PT-141 be detected at 280 nm?
Yes. The tryptophan residue absorbs near 280 nm, although most purity work is still read at around 214 nm so that non-aromatic impurities are counted too.
Research use only. All products supplied by Battle Born Peptides are laboratory reference materials for in-vitro research and analytical use by qualified professionals. They are not drugs, foods, dietary supplements, cosmetics or medical devices; they are not approved by the FDA or any other regulator for use in humans or animals; and they are not intended to diagnose, treat, cure, mitigate or prevent any disease, or to affect the structure or any function of the body of humans or animals. Nothing in this article is preparation, handling or dosing guidance. See our full research-use terms.