Melanotan I: structure, nomenclature and handling

The parent sequence is alpha-MSH, a thirteen-residue melanocortin peptide that is acetylated at its amino terminus and amidated at its carboxyl terminus. Its central region carries the four-residue motif histidine–phenylalanine–arginine–tryptophan, the stretch that the melanocortin receptor family engages, and synthetic analogues in this family are generally built by stabilising that motif rather than replacing it.

Melanotan I keeps all thirteen positions and both terminal modifications, and changes two residues. At position 4, norleucine replaces methionine, removing the sulphur-containing side chain that is the most oxidation-prone residue in the natural sequence — the same substitution logic used in other analogues of this family. At position 7, the phenylalanine of the core motif is present in the D configuration rather than the natural L form, a change that both shifts the preferred backbone conformation and resists cleavage by proteases, which act on L-amino acid sequences.

The result is a linear peptide — no ring, no bridge — with a molecular weight of approximately 1,647 daltons. It is characterized against the melanocortin receptor family, five G-protein-coupled receptors designated MC1R through MC5R.

The material is catalogued as Melanotan I, Melanotan-1, MT-1 and MT-I, and in the literature notation [Nle4, D-Phe7]-alpha-MSH, which states the two substitutions explicitly. Its international non-proprietary name is afamelanotide, and it carries CAS 75921-69-6. As with most synthetic peptides of this class, it is typically isolated as an acetate salt, which affects the mass weighed out without altering identity.

The numbering invites the assumption that Melanotan II is a later revision of the same molecule. It is not. Melanotan II is a cyclic heptapeptide of roughly 1,024 daltons — seven residues closed by a lactam bridge — while Melanotan I is a linear tridecapeptide some six hundred daltons heavier. The two share the core motif and little else structurally, and because their masses sit far apart, mass spectrometry separates them unambiguously. That makes this an easier identity question than the one-dalton separations found elsewhere in the melanocortin family.

Mass spectrometry confirms the intact molecular mass, and for this compound the theoretical figure encodes three things at once: the substituted sequence, the N-terminal acetyl group and the C-terminal amide. A missing acetyl group lightens the molecule by a defined 42-dalton increment, and a free acid in place of the amide shifts it by roughly one dalton — both are defined deviations that adequate resolving power detects.

High-performance liquid chromatography resolves the main peak from related substances and yields the purity figure. In a thirteen-residue synthesis the familiar related substances are deletion sequences, where a residue failed to couple. The substitution-specific one is the diastereomer: the D-phenylalanine at position 7 can invert to the L form during synthesis, producing a compound of identical mass that mass spectrometry cannot detect at all. It is separable chromatographically, which is why the two methods are run together rather than either being treated as sufficient.

HEEZ Research supplies Melanotan I in a single format: a 10 mg lyophilized vial. There is no second strength in the current catalogue, so the format decision that arises with two-size products does not apply here.

The material is supplied as a lyophilized powder. Freeze-drying removes water and leaves the peptide as a solid, markedly more stable than the same peptide in solution, and stable enough at ambient temperature to ship without refrigeration in transit.

Reconstitution converts the dried cake into a working solution, and it is a standard laboratory step rather than a product specification. Which solvent is used, and at what final concentration, is determined by the protocol in force at the receiving facility.

What HEEZ specifies is the contents of the vial: lyophilized Melanotan I in a single 10 mg format, stated on the label and on the batch certificate of analysis. Where a protocol is expressed in molar rather than mass terms, the additional figure it needs is the molecular weight, which the certificate records for the batch supplied.

A thirteen-residue linear peptide sits in the middle of the size range: long enough to have some conformational flexibility once dissolved, short enough that the aggregation pressure affecting forty-residue chains is largely absent. It dissolves readily, and vigorous agitation is unnecessary and avoided as a matter of routine.

Sealed lyophilized Melanotan I is ambient-stable for the duration of transit, so no cold chain or insulated packaging is required for shipping. On receipt the vial goes to refrigeration and is kept out of direct light.

Reconstituted material is held refrigerated and used within the window the receiving facility's protocols specify, and repeated freeze-thaw cycling is avoided. Solutions are treated as short-lived relative to the lyophilized form.

The norleucine substitution removes methionine, and with it the most common oxidation route in the natural sequence — one of the two engineering decisions in the molecule working directly in storage's favour. Tryptophan remains at position 9, and it is photosensitive, which is reason enough to keep solutions out of direct light; amber vials or foil wrapping during extended handling is standard practice for sequences containing it.

The certificate records what an independent facility measured for one specific batch. Its value lies in being batch-specific, so the first check is that the lot number on the document matches the vial in hand.

The identity section compares observed molecular mass against a stated theoretical mass, and the theoretical figure should correspond to the acetylated, amidated, substituted sequence — approximately 1,647 daltons — rather than to native alpha-MSH, which differs by a defined margin.

The purity figure is the HPLC main-peak area as a percentage of total peak area, and the attached chromatogram shows how that figure was arrived at. Peaks eluting close to the main peak are the ones to note, since a position-7 diastereomer of identical mass separates by a small retention difference rather than a large one.

Peptide content, or net peptide content, is sometimes reported separately. It accounts for counter-ions and residual water, and it is the figure determining how much actual peptide a nominal 10 mg represents.