Melanocortin Peptides

Melanocortin receptor ligands, supplied as lyophilised powder for laboratory research use only. This is a small category in which selectivity between receptor subtypes is the whole research question, and where two of the three products are cyclic peptides.

Five receptors, and why subtype matters

The melanocortin system has five receptor subtypes, MC1R through MC5R, distributed across different tissues with different downstream effects. A compound that engages MC1R and one that engages MC4R are studying different biology, and a ligand that engages both is a less specific tool than either.

Selectivity claims in this literature need reading carefully: a compound can bind two subtypes with similar affinity and activate them with very different potency, because the subtypes couple differently. See melanocortin receptor subtypes and selectivity, and Ki, IC50 and EC50 for why a binding ratio and a functional ratio are not the same claim.

Cyclic and linear, which changes the analysis

PT-141 (bremelanotide) is cyclic: Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH, C₅₀H₆₈N₁₄O₁₀, about 1,025.2 g/mol, CAS 189691-06-3. Melanotan II is the closely related cyclic amide, C₅₀H₆₉N₁₅O₉, about 1,024.2 g/mol, CAS 121062-08-6 — roughly one mass unit apart from PT-141, which makes them the pair most easily confused by mass alone on a low-resolution instrument. Melanotan I (afamelanotide) is linear and much longer: a 13-residue sequence, C₇₈H₁₁₁N₂₁O₁₉, about 1,646.8 g/mol, CAS 75921-69-6.

Cyclisation constrains conformation, which is why these molecules bind as they do; it also means a linear impurity of the same composition has the same mass as the cyclic target. A ring-opened species is therefore invisible to a mass measurement and has to be resolved chromatographically. See cyclic versus linear peptides.

An oxidation route worth noting

All three sequences contain tryptophan, and two contain histidine. Tryptophan has an oxidation route available to it, so a certificate that was clean at manufacture is not a guarantee about material that has been stored badly. See where peptide impurities come from and storage and stability.

Analysis and handling

Products are third-party tested for HPLC purity; certificates are published on product pages and on the certificates of analysis page, and vials are matched to their certificate by cap and crimp colour. Vials ship as lyophilised powder from within the United States.

Supplied for laboratory research use only. Not approved by the FDA; not manufactured to pharmaceutical GMP standards; not for human or veterinary use.