--- title: "Melanotan II vs PT-141 (Bremelanotide): Structural Differences and Receptor Selectivity" description: "Melanotan II and PT-141 share one cyclic lactam scaffold and differ by one C-terminal group: melanocortin receptor subtypes, the 0.98 Da gap, and verification." url: https://www.certuspeptides.com/research/melanotan-ii-vs-pt-141 markdown: https://www.certuspeptides.com/research/melanotan-ii-vs-pt-141.md site: Certus Peptides --- # Melanotan II vs PT-141 (Bremelanotide): Structural Differences and Receptor Selectivity Melanotan II and PT-141 are the closest pair of compounds in the Certus catalog. They share the same seven-residue cyclic scaffold, the same lactam bridge and the same pharmacophore, and they differ by one atom's worth of mass at the C-terminus. That single change shifts receptor preference across the melanocortin family and makes the two the easiest compounds on the market to confuse, deliberately or otherwise. This guide covers the structure, the receptor pharmacology and the analytical work that keeps them apart. ## One scaffold, one group's difference Melanotan II is Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2, formula C50H69N15O9, molecular weight 1024.18 Da. PT-141, also called bremelanotide, is Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH, formula C50H68N14O10, molecular weight 1025.16 Da. The only difference is at the C-terminal lysine: an amide in Melanotan II, a free carboxylic acid in PT-141. Both are analogs of alpha-melanocyte stimulating hormone. Norleucine replaces the oxidation-prone methionine at position 4 of the parent, D-phenylalanine replaces L-phenylalanine at position 7, and the His-D-Phe-Arg-Trp core is the minimal pharmacophore recognized by every melanocortin receptor. The N-terminal acetyl and the cyclization complete the design. PT-141 is the C-terminal deamidated form of Melanotan II. That relationship is not only a matter of nomenclature: it is a degradation pathway, and a Melanotan II lot that has aged in solution accumulates PT-141 as an impurity. ## The lactam bridge Both compounds are cyclized through a lactam bond between the side-chain carboxyl of aspartate at position 5 and the side-chain amine of lysine at position 10. The bridge constrains the ring into the conformation that the receptors recognize and confers resistance to proteolysis that the linear parent lacks. It is the last step in the synthesis and the least reliable one. Uncyclized material differs from the cyclic product by one molecule of water, 18.01 Da, on a 1,024 Da peptide. That is under 2 % of the mass and within the tolerance a careless mass spectrometry method might accept. The cyclic and linear forms also elute close together on a routine reversed-phase gradient. Certus resolves them on an orthogonal gradient and the certificate reports linear content as its own line with a specification of 0.5 % maximum. Uncyclized material is the single most common defect in these compounds across the market. ## Melanocortin receptor subtypes There are five melanocortin receptors, MC1R through MC5R, all class A G protein-coupled receptors signaling through Gs and cAMP. MC1R is the receptor expressed on melanocytes and is the subject of pigmentation biology. MC2R is the ACTH receptor and does not respond to alpha-MSH analogs. MC3R and MC4R are expressed centrally and are the subject of the energy homeostasis and central signaling literature. MC5R is expressed in exocrine tissue. Melanotan II is a non-selective agonist at MC1R, MC3R, MC4R and MC5R. That lack of selectivity is precisely what makes it useful: it is the tool for establishing that a response is melanocortin-mediated at all, before selective antagonists are used to assign it to a subtype. PT-141 is MC4R-preferring. The free C-terminal carboxylate measurably shifts binding preference toward MC4R and away from MC1R relative to the parent, which is why the PT-141 literature centers on MC4R-mediated central signaling and why it is studied in models where MC1R engagement would be a confound. ## What the C-terminal change does at the receptor At physiological pH the C-terminal carboxylate of PT-141 carries a negative charge that the amide of Melanotan II does not. That charge sits adjacent to the lysine that closes the lactam ring and alters the electrostatic surface of the molecule near the bridge. Receptor subtypes differ in the residues lining the binding pocket, and the shift in preference toward MC4R is the observable consequence. It is a modest structural change with a measurable selectivity effect, which is why the pair is a standard example in melanocortin structure-activity work. For experimental design the implication is direct: the two compounds are not interchangeable, and a study intended to isolate MC4R that receives Melanotan II, or one mapping the full receptor family that receives PT-141, is compromised by the substitution. Stock identity has to be proven, not assumed from the label. ## Telling them apart analytically The mass difference is 0.98 Da, the loss of NH and gain of O. At the 1.0 Da tolerance used for identity confirmation, that difference is at the edge of what low-resolution mass spectrometry can call, and a certificate that reports 'conforms' for a 1,024 Da observed mass has not distinguished the two. High-resolution mass spectrometry resolves them cleanly by exact mass; more practically, the two separate on reversed-phase HPLC because the carboxylate is more polar than the amide, and retention time against a qualified reference standard of each is the decisive line. Certus runs both compounds against their own standards on the same gradient so that the retention-time comparison is meaningful for anyone holding both. The same chromatography quantifies PT-141 as a deamidation impurity in an aged Melanotan II lot, which is worth checking on any lot that has been stored in solution. Beyond the pair itself, both compounds carry the same standard panel: HPLC purity against 99.0 %, observed mass, net peptide content, water, residual trifluoroacetate, endotoxin, sterility, and the orthogonal-gradient linear content line. Arginine's Pbf protecting group at +252 Da and tryptophan oxidation at +16 Da are the other impurities worth looking for on the chromatogram. ## Stability and storage Both are stored lyophilized at -20 °C protected from light, because the single tryptophan photo-oxidizes. Both dissolve readily in water. Reconstituted stock is held at 2-8 °C and aliquoted before freezing. The specific stability concern for Melanotan II is slow deamidation of the C-terminal amide in solution, which converts it to PT-141; the concern for both is that the lactam bridge, once formed, is stable but that any uncyclized material present will not cyclize on the bench. ## Reading a certificate for either compound The lines that decide identity are the retention time against a reference standard and, where the laboratory has it, an exact mass from high-resolution mass spectrometry. The line that decides quality is linear content under 0.5 %. A certificate that reports a purity of 99.5 % with no linear content line and an observed mass to one decimal place has established that the vial contains a peptide of about the right size, and not much more. For a pair separated by 0.98 Da and a design flaw separated by 18 Da, that is not enough. ## Questions ### What is the structural difference between Melanotan II and PT-141? One C-terminal group. Both are Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys] with a lactam bridge between Asp and Lys side chains. Melanotan II ends in an amide (1024.18 Da); PT-141 ends in a free carboxylic acid (1025.16 Da). PT-141 is the C-terminal deamidated form of Melanotan II. ### Which melanocortin receptors do Melanotan II and PT-141 act on? Melanotan II is a non-selective agonist at MC1R, MC3R, MC4R and MC5R. PT-141 is MC4R-preferring: the free C-terminal carboxylate shifts binding preference toward MC4R and away from MC1R. Neither acts at MC2R, the ACTH receptor. All four responsive subtypes signal through Gs and cAMP. ### How can a laboratory tell Melanotan II from PT-141? The mass difference is 0.98 Da, at the edge of a 1.0 Da low-resolution tolerance, so high-resolution mass spectrometry or reversed-phase retention time against a qualified reference standard of each compound is required. The carboxylate makes PT-141 more polar, and the two separate on the same gradient. Certus runs both against standards. ### What is the most common defect in Melanotan II lots? Uncyclized linear material. The lactam bridge is the last and least reliable synthesis step, and the linear precursor is only 18 Da heavier on a 1,024 Da peptide, easy to miss on routine mass spectrometry and close to co-eluting on a standard gradient. Certus reports linear content on an orthogonal gradient with a 0.5 % maximum. ## Compounds discussed - https://www.certuspeptides.com/products/melanotan-ii - https://www.certuspeptides.com/products/pt-141 ## Keep reading - [HPLC vs Mass Spectrometry: Why Peptide Testing Needs Both](https://www.certuspeptides.com/research/hplc-vs-mass-spec) - [98 % vs 99 % Peptide Purity: What the Extra Percent Means and When It Matters](https://www.certuspeptides.com/research/peptide-purity-98-vs-99) - [Lyophilized Peptide Shelf Life: Temperature, Light, Moisture and Reconstituted Stability](https://www.certuspeptides.com/research/lyophilized-peptide-shelf-life) - [How to Read a Peptide Certificate of Analysis](https://www.certuspeptides.com/research/how-to-read-a-certificate-of-analysis) Updated 2026-09-26. 7 minute read. Published by Certus Peptides. --- For laboratory and research use only. Not for human or veterinary consumption. Not a drug, food or cosmetic. Sold exclusively to qualified researchers and licensed laboratories. Support: support@certuspeptides.com. Every certificate of analysis: https://www.certuspeptides.com/lab-reports.