What Is PT-141 (Bremelanotide)? Melanocortin Peptide Structure

Key takeaways

• PT-141 (bremelanotide) is a synthetic cyclic heptapeptide in the melanocortin-receptor-ligand class, with the sequence Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH.
• Its ring is formed by an intramolecular lactam (amide) bridge between an aspartic acid side chain and a lysine side chain, which constrains the backbone conformation.
• The molecular formula is C50H68N14O10 and the monoisotopic/average molecular weight is approximately 1025.2 g/mol; sequence length is seven residues (a heptapeptide).
• Structurally, PT-141 is the de-amidated relative of Melanotan II (MT-2): the two share the same cyclic core, but PT-141 carries a C-terminal carboxyl (hydroxyl-terminated) group where MT-2 has a C-terminal amide.
• Ascend Bio Labs supplies PT-141 as a lyophilized research compound with a public per-batch Certificate of Analysis tying each vial's batch ID to independent third-party HPLC (purity) and LC-MS (identity) results, fully US-domestic. Research use only.

PT-141, known by its non-proprietary chemical name bremelanotide, is a synthetic cyclic peptide that belongs to the melanocortin-receptor-ligand family. For laboratories sourcing it as a reference material, the useful description is purely structural: what residues it is built from, how the ring is closed, what it weighs, and what class of receptor ligands it sits in. This page is written entirely in those terms. It makes no statements about physiological effects, uses, or dosing — PT-141 is described here strictly as a research chemical.

Because PT-141 is closely related to other melanocortin peptides, the clearest way to understand it is comparatively: its core ring is shared with Melanotan II, and it belongs to the same broad class as the alpha-MSH-derived compounds. The sections below cover the sequence and ring chemistry, molecular weight, the precise structural relationship to MT-2, where it falls in the melanocortin class, and how a lyophilized peptide of this type is handled and verified.

Sequence and cyclic structure

PT-141 is a heptapeptide — a chain of seven amino acid residues — that is cyclized rather than linear. Its standard sequence notation is Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH. Reading that left to right: an N-terminal acetyl cap (Ac) sits on a norleucine residue (Nle), followed by the ring, which is written inside the cyclo[...] brackets and runs aspartic acid (Asp), histidine (His), D-phenylalanine (D-Phe), arginine (Arg), tryptophan (Trp), and lysine (Lys), terminating in a free C-terminal carboxyl (-OH).

Two notation details matter for identity verification. First, the residue is norleucine (Nle), a straight-chain isomer of leucine, not leucine itself — substituting Nle is a deliberate, common feature in melanocortin analogs. Second, the phenylalanine is the D-enantiomer (D-Phe), not the more common L-form. The presence of a D-residue and a non-canonical residue (Nle) are both things a sequence-aware identity check, such as LC-MS fragmentation, can help corroborate alongside intact mass.

• Class: synthetic melanocortin-receptor-ligand peptide.
• Length: 7 residues (heptapeptide), cyclized.
• Sequence: Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH.
• Non-standard residues: norleucine (Nle) and D-phenylalanine (D-Phe).
• Termini: N-terminal acetyl cap; free C-terminal carboxyl (-OH).

The lactam bridge that closes the ring

What makes PT-141 cyclic is a side-chain-to-side-chain lactam bridge. A lactam is simply an amide bond formed within a molecule; here it links the carboxyl-bearing side chain of the aspartic acid residue to the amine-bearing side chain of the lysine residue. That covalent connection closes the five interior residues (Asp through Lys) into a ring, leaving the acetyl-norleucine portion as a short exocyclic 'tail' at the N-terminus.

The functional point of the bridge, from a chemistry standpoint, is conformational constraint: a cyclic backbone has far fewer freely rotating bonds than a linear chain, so the molecule populates a much narrower set of three-dimensional shapes. Cyclization of this kind is also generally associated with greater resistance to exopeptidase trimming than an equivalent linear sequence, which is one reason cyclic scaffolds are common across the melanocortin analog family. None of this implies any biological activity — it is a statement about the molecule's covalent topology and rigidity.

Molecular formula and weight

PT-141 has the molecular formula C50H68N14O10. Its average molecular weight is approximately 1025.2 g/mol (sources commonly cite figures in the 1025.16-1025.2 range depending on rounding and isotope convention). That mass is a primary identity anchor: an LC-MS run reports the observed intact mass, and a batch's measured value should land on the expected ~1025 figure within instrument tolerance for the result to corroborate identity.

It is worth distinguishing the two analytical questions a Certificate of Analysis answers, because they are different. Purity — how much of the material is the intended compound versus synthesis-related impurities — is measured chromatographically by HPLC and reported as a percentage. Identity — whether the molecule is in fact PT-141 and not a different sequence of similar mass — is confirmed by mass spectrometry (LC-MS). A meaningful COA reports both, because a high purity number on the wrong molecule is not a useful result.

• Molecular formula: C50H68N14O10.
• Average molecular weight: ~1025.2 g/mol.
• Sequence length: 7 residues (heptapeptide).
• Primary identity check: LC-MS intact mass near ~1025 g/mol.
• Primary purity check: HPLC percent-area purity.

Structural relationship to Melanotan II (MT-2)

PT-141 and Melanotan II share the same cyclic heptapeptide core; they are close structural relatives rather than unrelated compounds. The single defining difference is at the C-terminus. Melanotan II terminates in a C-terminal amide, while PT-141 terminates in a free carboxyl group (-OH). In other words, PT-141 is the de-amidated form of MT-2 — the amide has been replaced by the corresponding acid.

That one substitution is visible in the molecular formulas: PT-141 is C50H68N14O10 (~1025.2 g/mol) whereas Melanotan II is C50H69N15O9 (~1024.2 g/mol). The pair differs by replacing an -NH2 (amide) with an -OH (carboxyl), which trades a nitrogen for an oxygen and changes the hydrogen count accordingly. Because the two are so close in mass (about one mass unit apart) and identical across the ring, they are a textbook example of why mass spectrometry plus a purity-grade HPLC method — not mass alone — is needed to tell related melanocortin analogs apart on a COA. For a side-by-side on the broader Melanotan pair, see Melanotan I vs Melanotan II: Structural Differences Explained.

• Shared: the same cyclic core ring of residues.
• Differs: C-terminus — MT-2 is amide-terminated, PT-141 is carboxyl (-OH) terminated.
• PT-141 (C50H68N14O10, ~1025.2 g/mol) vs MT-2 (C50H69N15O9, ~1024.2 g/mol).
• Practical implication: closely related mass means identity must be confirmed by LC-MS plus HPLC, not by mass alone.

Where PT-141 sits in the melanocortin peptide class

PT-141 is categorized as a melanocortin-receptor ligand. The melanocortin family of receptors comprises five subtypes (MC1R through MC5R), and the natural melanocortin peptides are derived from the proopiomelanocortin (POMC) precursor — alpha-MSH being the most familiar member. Synthetic analogs in this class, including PT-141 and the Melanotan compounds, are built around a conserved core motif (His-Phe-Arg-Trp) that recurs across melanocortin ligands; you can see that His-D-Phe-Arg-Trp stretch inside PT-141's ring.

This is a classification statement, not a claim about what PT-141 does. Describing a compound as a 'melanocortin-receptor ligand' identifies the receptor family its structure is designed to engage in binding studies — it does not assert any in-vivo outcome. For a map of the wider class and its members, see Melanocortin Research Peptides: Class Overview and Members. PT-141 also sits alongside other commonly catalogued research peptides such as the tetrapeptide covered in What Is Epithalon? Tetrapeptide Sequence and Molecular Weight.

Reconstitution, storage, and handling (research)

PT-141 is supplied as a lyophilized (freeze-dried) powder. In its dry, lyophilized state it is comparatively stable and is the appropriate form for shipping and for longer-term storage. As a general handling reference for water-soluble research peptides of this type, the lyophilized powder is typically held cold and protected from light and moisture until use, with the unopened vial often kept frozen for extended storage.

Reconstitution is the laboratory step of dissolving the lyophilized powder into a suitable sterile diluent (commonly bacteriostatic or sterile water for research preparations) to produce a working solution at a defined concentration. Once reconstituted, a peptide solution is generally less stable than the dry powder and is kept refrigerated and used within a shorter window. These are generic laboratory handling references for a compound of this class — not instructions, protocols, or any indication of human use. For a fuller treatment of cold-chain practice, see Peptide Storage and Cold-Chain Handling: A Research Reference.

• Supplied form: lyophilized (freeze-dried) powder.
• Dry storage: cold, dark, dry; frozen for extended storage of unopened vials.
• Reconstituted solution: refrigerate; shorter use window than dry powder.
• Always confirm the specific batch's COA before relying on identity or purity figures.

How Ascend Bio Labs documents its PT-141 batches

For a research compound, the documentation matters as much as the powder. Every Ascend Bio Labs batch ships with a public Certificate of Analysis, and each vial carries a unique batch ID that links directly to that batch's COA — so the purity (HPLC) and identity (LC-MS) data you are relying on belong to the exact lot in your hand, not a generic specimen result. The identity step is where the ~1025 g/mol mass and the cyclic heptapeptide structure described above are corroborated by mass spectrometry.

Testing is performed by independent third-party laboratories: HPLC for percent-area purity and LC-MS for molecular identity. Synthesis, testing, storage, and shipping are fully US-domestic, with no overseas transshipment, and orders move via insulated, tracked shipping. PT-141 is sold strictly as a research chemical — labeled for research use only, not for human or veterinary use. The verifiable signals to look for in any vendor, including this one, are the same three: a published per-batch COA, named analytical methods (HPLC and LC-MS), and a batch ID that ties the certificate to the physical vial.

Related research notes

• Melanotan I vs Melanotan II: Structural Differences Explained
• Melanocortin Research Peptides: Class Overview and Members
• What Is Epithalon? Tetrapeptide Sequence and Molecular Weight
• Peptide Storage and Cold-Chain Handling: A Research Reference