Melanotan II: structure, nomenclature and handling

Alpha-melanocyte-stimulating hormone is a linear 13-amino-acid peptide. Melanotan II reproduces only its central active region, shortened to seven residues, and then closes that chain into a ring through a lactam bridge — an amide bond formed between a side-chain amine and a side-chain carboxyl rather than between the backbone termini.

Cyclisation has two structural consequences. It constrains the peptide conformationally, holding the residues that engage the receptor in a more defined arrangement than a flexible linear chain would sample. And it removes the free termini that exopeptidases attack, so a cyclic peptide is generally more resistant to enzymatic degradation than a linear one of the same composition.

Melanotan II is characterized against the melanocortin receptor family. That family has several members, and analogues in this group differ in how selectively they engage one member over another — a structural property arising from which residues sit at which positions in the ring.

The molecule contains a D-phenylalanine residue in place of the L form found in the native sequence. Substituting a D-amino acid is a common strategy in peptide engineering: it changes local backbone geometry and further resists proteolysis, since most peptidases are specific to L-residues.

Melanotan II is catalogued as MT-II and MT-2, and the HEEZ product title uses the latter. It also appears in the literature as MTII without punctuation, and database searches treat these as distinct strings.

The compound should not be confused with Melanotan I, catalogued as afamelanotide, which is a linear 13-residue analogue rather than a cyclic heptapeptide. The two share a parent hormone and a naming convention but are structurally distinct molecules.

PT-141, catalogued as bremelanotide, is a metabolite of Melanotan II and shares its cyclic architecture, differing at the terminus. It is stocked separately in the HEEZ catalogue, and the structural relationship is worth knowing when reading either listing.

Mass spectrometry confirms identity against the theoretical mass of the cyclic heptapeptide. Cyclisation is detectable in the mass itself: forming the lactam bond eliminates a water molecule, so a cyclic peptide is 18 mass units lighter than its linear precursor. A mass matching the linear form indicates incomplete cyclisation.

High-performance liquid chromatography reports purity as the main peak against total peak area. In a cyclic peptide the informative impurities include uncyclised linear precursor and dimeric material formed when the lactam bond bridges two molecules rather than closing one.

Receptor characterization, where reported, is performed in in vitro systems expressing melanocortin receptor subtypes, and measures binding affinity as a biochemical property of the isolated molecule.

HEEZ Research supplies Melanotan II in one format: a 10 mg lyophilized vial, listed as MT-2. There is no second strength in the current catalogue.

The peptide is freeze-dried to a solid. Lyophilized material is markedly more stable than the same peptide in solution, and tolerates ambient temperature in transit without insulated packaging.

At seven residues Melanotan II is a light molecule, so 10 mg represents comparatively many molar units against the longer peptides in the catalogue. Protocols specified in molar terms should use the molecular weight from the certificate rather than assuming parity with a similar mass of a larger peptide.

Concentration equals mass over volume. A 10 mg Melanotan II vial with 1 ml of diluent gives 10 mg per ml; with 2 ml, 5 mg per ml; with 5 ml, 2 mg per ml.

Bacteriostatic water is the standard diluent where a vial is entered more than once, its 0.9% benzyl alcohol acting as preservative. Preservative-free sterile water is used for single-entry work.

peptcalc.com converts the 10 mg vial mass and a target concentration into the volume to add, and handles molar concentrations where the molecular weight is provided.

Diluent is added slowly down the vial wall rather than onto the cake, and the vial swirled rather than shaken. Cyclic peptides are less prone to some degradation routes than linear ones but are not exempt from aggregation under mechanical stress.

Sealed lyophilized Melanotan II is ambient-stable for the duration of transit, so shipping requires no cold chain or ice packs. On arrival the vial is refrigerated and kept out of light.

Reconstituted material is refrigerated and used within the window the receiving facility's protocols specify. Freeze-thaw cycling is avoided.

Light protection deserves particular attention here. Peptides containing aromatic residues — and this one contains both phenylalanine and tryptophan — are more susceptible to photo-oxidation than those without, so storage in the dark is not merely conventional caution.

The certificate records what an independent facility measured for one specific batch, so the lot number on the document must match the vial in hand before anything else on it is relevant.

Identity is observed versus theoretical mass for the cyclic form. Given the 18-unit difference between cyclic and linear, this section is where incomplete cyclisation would show up, and it is worth reading rather than skimming.

Purity is the HPLC main-peak percentage with the chromatogram attached. Peptide content, where reported separately, accounts for counter-ions and residual water and determines how much peptide a nominal 10 mg represents.