What Is PT-141? Structure and Chemistry

PT-141, also known by its non-proprietary name bremelanotide, is a synthetic cyclic heptapeptide with the sequence Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH and, by receptor class, a ligand of the melanocortin receptor family. A lactam bond between the side chains of aspartic acid and lysine closes a ring of six residues, leaving N-acetyl-norleucine outside the ring and the C-terminal carboxylic acid free. Its molecular formula is C50H68N14O10, its average molecular weight is 1025.18 g/mol and its CAS number is 189691-06-3.

This page covers PT-141 as a molecule: the sequence and its notation, the ring and how it is closed, the two non-standard residues, the formula, mass and charge, and how identity and purity are confirmed by HPLC and mass spectrometry.

Reading the PT-141 sequence

The notation packs a lot of structure into one line. "Ac-" is an acetyl group, CH3CO–, on the N-terminal amine. "Nle" is norleucine. "cyclo[…]" marks the residues that form the ring, here aspartic acid through lysine, and the bond that closes it runs between their side chains rather than between the chain ends. "D-Phe" is the D-enantiomer of phenylalanine, and the closing "-OH" is the free carboxylic acid of the C-terminal lysine. The conventions are covered in peptide sequence notation.

The core His-D-Phe-Arg-Trp is a four-residue motif shared across the melanocortin-ligand peptide class. In PT-141 it sits inside the ring, held in a constrained arrangement by the lactam bridge at either side.

The lactam ring

A lactam is a cyclic amide. In PT-141 it forms between the β-carboxyl group of aspartic acid at position 2 and the ε-amino group of lysine at position 7. Counting the atoms that make up the ring, from the aspartic acid α-carbon through the backbone of the four residues in between to the lysine α-carbon and back through the two side chains, gives a 23-membered macrocycle.

Ring closure changes the formula by exactly one water. The open-chain peptide, Ac-Nle-Asp-His-D-Phe-Arg-Trp-Lys-OH, is C50H70N14O11, monoisotopic mass 1042.5348 Da. Joining the two side chains releases H2O, 18.0106 Da, and leaves C50H68N14O10 at 1024.5243 Da. A mass 18 Da above the parent is therefore the signature of material that has not cyclised.

Because the ring closes through side chains, both chain ends remain available. The N-terminus is capped with an acetyl group, and the C-terminus of lysine stays a free carboxylic acid. The free acid adds one ionisable carboxyl group to the molecule and is a defining feature of its formula.

Two non-standard residues

Norleucine at position 1

Norleucine, 2-aminohexanoic acid, has a straight four-carbon side chain, –CH2CH2CH2CH3. It is not one of the twenty proteinogenic amino acids, so it enters the chain as a protected synthetic building block. It has the same formula as leucine and isoleucine, so the three residues share a residue mass of 113.0841 Da and cannot be distinguished by mass spectrometry alone. Norleucine is also a common sulfur-free counterpart to methionine in peptide design, with a side chain of similar length and shape but no thioether to oxidise.

D-Phenylalanine at position 4

D-Phe is the mirror-image form of phenylalanine at its α-carbon. A peptide with L-Phe at the same position is a diastereomer: it has exactly the same formula and mass, so it can only be separated chromatographically. In three-letter notation a D-residue always carries its prefix, while a residue written without one is L by convention.

Formula, molecular weight and charge

The average molecular weight, 1025.18 g/mol, is the catalogue value; the exact figure depends slightly on the atomic-weight table used.

The ring closure and the acetyl cap remove three of the usual ionisable groups: the N-terminal amine, the aspartic acid carboxyl and the lysine amine. What remains near pH 7 is the guanidinium group of arginine, positively charged, the C-terminal carboxylate, negatively charged, and the histidine imidazole, mostly neutral. The net charge is close to zero, slightly positive. At the acidic pH of reversed-phase HPLC the carboxyl group is neutral and both arginine and histidine are protonated, giving a charge of about +2, which is why the doubly charged ion is prominent in electrospray.

Counter-ions and the solid form

PT-141 is isolated from purification as a salt, with counter-ions such as acetate or trifluoroacetate paired with its protonated arginine and histidine. Those counter-ions, together with residual water, add weight to the lyophilised powder without adding peptide, so the net peptide content of a vial sits below its gross weight. The free C-terminal acid is the one group that can itself form a salt with a basic counter-ion, which is why the certificate should state the salt form explicitly rather than leave it to be inferred.

For a 10 mg vial, the figure that matters for calculating the amount of peptide present is the net content, not the weight of solid. Peptide purity vs net peptide content explains how the two are related and how each is measured.

The tryptophan chromophore

Tryptophan at position 6 gives PT-141 a useful ultraviolet absorbance at 280 nm, with a calculated extinction coefficient of about 5,500 M⁻¹ cm⁻¹ from its single indole ring. That makes 280 nm a practical second wavelength alongside the peptide-bond region at 210 to 220 nm. The indole is also the residue most open to oxidation in this sequence, giving related substances 16 and 32 Da above the parent, and it is the reason the solid is kept away from light.

How PT-141 is analysed

The mass difference between the free acid and the corresponding amide is small, 0.9840 Da, and close to the spacing of the natural carbon-13 isotope peaks. A low-resolution spectrum can therefore blur the two, and the check relies on adequate mass resolution together with HPLC retention, since the acid and amide forms elute at different times.

A cyclic peptide also fragments differently in MS/MS. Opening the ring takes one backbone cleavage, which leaves the mass unchanged, and a second cleavage is needed before a fragment separates. The ring portion therefore yields fewer, more complex sequence ions than a linear chain of the same length, while the exocyclic Ac-Nle unit gives straightforward fragments. Mass spectrometry and peptide identity explains the fragment naming.

Certificates, specification and storage

Our PT-141 is supplied at ≥99% purity (HPLC) as a lyophilised powder in a sealed 10 mg vial. When reading a certificate for PT-141, check that the sequence is given as the cyclic, N-acetylated free acid, that the observed mass matches 1024.5 Da or the ions above, that the HPLC method states its detection wavelength, that the salt form is stated, and that the report carries a reference and an analysis date. How to read a peptide certificate of analysis walks through each line of a report.

Store the sealed vial at 2–8 °C for short-term storage, or at −20 °C and below for long-term storage. Protect it from light, which matters for a tryptophan-containing peptide, and avoid repeated freeze-thaw cycles. Storing lyophilised peptides explains why cold, dark and dry storage matters for a freeze-dried solid.

PT-141 belongs to the Melanocortin & Copper Peptides research area, alongside MT2 and the copper complex GHK-Cu, a very different molecule built on a tripeptide. Terms used on this page, such as lactam, diastereomer and monoisotopic mass, are defined in the glossary.