Melanotan II: structure and analysis

What is Melanotan II?

Melanotan II is a synthetic cyclic heptapeptide analog of alpha-melanocyte-stimulating hormone. Seven residues, closed into a ring rather than left as a linear chain.

The cyclisation is the defining structural feature. A covalent bridge joins two points in the sequence, constraining the molecule into a fixed conformation instead of the flexible chain a linear peptide of the same length would be.

How is Melanotan II produced?

Assembled by solid-phase peptide synthesis: the chain is built one residue at a time on an insoluble resin support, then cleaved from the resin, deprotected and purified — most commonly by reversed-phase high-performance liquid chromatography. The purified material is lyophilized to a dry solid.

The impurity profile that appears on a certificate is a record of where that process was imperfect: deletion sequences from an incomplete coupling, truncated chains, and species retaining a protecting group.

What are the molecular figures for Melanotan II?

Calculated from the formula C50H69N15O9, not measured. The average molecular weight counts the natural abundance of each element's isotopes; the monoisotopic mass is the mass of the single most abundant isotopic composition, which is the figure a mass spectrometer is usually compared against.

The expected ions follow from the monoisotopic mass by adding one proton per charge. They are a starting point for recognising a spectrum, not a substitute for one: adducts, salts, modifications and multiply charged species all move the observed m/z, and the certificate reports what was actually measured for a batch.

Adducts, salts, modifications and multiply charged species all move an observed m/z away from any figure calculated here, and a certificate reports what was measured for a batch rather than what was expected of it.

What is the sequence of Melanotan II?

No one-letter amino-acid sequence is published for Melanotan II on this page. The chain contains norleucine and a D-amino acid, neither of which has a standard one-letter code.

What establishes the material in a specific vial is the identity method and result stated on that batch's certificate, not a notation on a reference page.

How does Melanotan II compare with related compounds?

Related compounds are not interchangeable. Published figures differ between them, and in this catalogue three compounds share a GHRH backbone while several share the same structural class — which is exactly why identity is confirmed analytically rather than inferred from the label.

How is the identity of Melanotan II confirmed?

Ring closure is what mass spectrometry confirms here beyond the sequence itself. Forming the bridge eliminates a small molecule — water, in the case of a lactam — so the cyclic product has a lower mass than the linear precursor by a predictable amount.

That makes the linear, uncyclised species a detectable and distinct impurity rather than an invisible one. It is the specific thing an identity result on this compound should settle.

Which impurities can appear in Melanotan II?

Cyclic and linear forms of the same sequence usually separate well chromatographically, because the constrained conformation changes how the molecule interacts with the stationary phase. The uncyclised precursor is therefore both mass-distinguishable and retention-distinguishable, which is a favourable analytical position.

Conformational constraint also confers real stability: a cyclic peptide is a poorer substrate for proteases than its linear counterpart, since the enzyme requires a degree of backbone flexibility to bind.

PT-141 is structurally a metabolite of this compound and the two are closely related. Distinguishing them requires attention to a small mass difference, which makes the resolution of the identity method a genuine consideration.

What does a certificate of analysis for Melanotan II report?

A certificate for a batch of this material names the batch, names the issuing laboratory, and states for each parameter both the method used and the specification the result was measured against. Identity and purity are the two core results; net peptide content states how much of the fill weight is actually peptide.

A certificate belongs to a batch, not to a product line. When a new batch is released it is tested again, and the certificate published against it is that batch's certificate — a previous batch's result is never carried forward.

Is Melanotan II for human use?

No. This page describes structure and analysis. It makes no claim about what this compound does, and none should be inferred from it. Material is supplied for laboratory research use only: it is not a drug, food, dietary supplement, cosmetic or medical device, it is not for human or veterinary consumption, and no dosing, administration or reconstitution guidance is provided for any such use.

References

• Melanotan-II — PubChem, National Library of Medicine
• Melanotan Ii — ChEMBL, European Bioinformatics Institute
• Effect of MELANOTAN, [Nle(4), D-Phe(7)]-alpha-MSH, on melanin synthesis in humans with MC1R variant alleles — Peptides, 2006