Quick answer
Melanotan-II is verified by reversed-phase HPLC for purity and mass spectrometry for identity and cyclization state.
- Structure: Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2, lactam ring, C50H69N15O9
- Identity: average mass about 1024 Da; linear precursor about 1042 Da (+18)
- Common impurities: linear precursor, dimers, Trp oxidation (+16 Da)
- Product: MT-2 (Melanotan II), 10 mg lyophilized
- Concentration: mg ÷ mL; see the calculator
For laboratory research use only; not for human consumption.
What melanotan-II is, analytically
Melanotan-II is a cyclic peptide, and confirming both its sequence and its cyclization state is part of verifying a lot before research use. The sequence is Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-NH2. The ring is a lactam: an amide bond formed between the side chains of aspartic acid and lysine. Norleucine replaces the oxidation-prone methionine of the native hormone, the N-terminus is acetylated, and the C-terminus is amidated. The molecular formula is C50H69N15O9, which corresponds to the average mass of about 1024 Da used throughout this article. Pharmacology and receptor background are in the melanotan-II research overview.
Each structural feature creates a testable question. Is the ring closed? Is the D-phenylalanine intact, or has it racemized to the L form during synthesis? Is the tryptophan oxidized? Are the terminal caps present? A single purity percentage cannot answer all of these, which is why a lot is normally characterized by both HPLC and mass spectrometry.
| Structural feature | Analytical question | Where it shows up |
|---|---|---|
| Lactam ring (Asp to Lys side chain) | Is cyclization complete? | Mass: linear precursor is 18 Da higher; HPLC retention differs |
| D-Phe at position 4 | Has it racemized to L-Phe? | Same mass; separates only on a suitable gradient or chiral analysis |
| Trp | Has it oxidized? | +16 Da; earlier-eluting peaks |
| N-terminal acetyl and C-terminal amide | Are the caps present? | Mass shifts of roughly 42 Da (acetyl) and 1 Da (amide vs. acid) |
| Norleucine | Correct residue in place of Met? | Isobaric with Leu and Ile, so not distinguishable by intact mass alone |
Reversed-phase HPLC
HPLC separates correctly cyclized melanotan-II from linear (uncyclized) intermediate and other synthesis byproducts, with purity read per understanding HPLC purity. Cyclic peptides are more compact than their linear precursors and generally elute differently, so the two forms usually resolve under a reasonable gradient.
Beyond the linear precursor, cyclization chemistry can produce dimers, in which two chains are linked to each other instead of each closing on itself, and these elute later than the monomer. The tryptophan residue is the main site of oxidative degradation during storage, giving more polar species that elute earlier. A chromatogram of a good lot is dominated by one symmetric peak, with small, identifiable early and late peaks.
Typical impurity map
| Species | Mass relative to parent (about 1024 Da) | Elution relative to main peak |
|---|---|---|
| Linear (uncyclized) precursor | +18 Da (about 1042 Da) | Usually different; commonly earlier |
| Cyclic dimer | About twice the monomer mass | Later |
| Trp-oxidized product | +16 Da | Earlier |
| Deletion sequence | Minus one residue | Close to main peak |
| Diastereomer (D-Phe to L-Phe) | Same as parent | Very close; needs shallow gradient |
Note the final row: stereochemical changes are invisible to intact-mass measurement, and in a single fast gradient they may co-elute with the main peak. That is a reason to look at the stated method before trusting a high percentage.
Mass spectrometry for identity
Mass spectrometry confirms molecular identity and can help flag whether cyclization is complete, since a linear precursor carries slightly different mass than the finished cyclic peptide described in the chemistry article. See HPLC vs. mass spectrometry for how the two techniques complement each other.
The theoretical average mass is about 1024 Da. Forming the lactam releases one molecule of water, so uncyclized precursor appears 18 Da higher, at about 1042 Da, and a cyclic dimer appears at twice the monomer mass. Oxidized tryptophan adds 16 Da. Each of these has a distinct signature in the spectrum. At this size, electrospray typically gives mainly singly and doubly charged ions, so the spectrum is easy to read by eye: the [M+H]+ ion sits near m/z 1025 and the [M+2H]2+ ion near m/z 513.
Tandem mass spectrometry (MS/MS) fragmentation can go further and check sequence, but the ring makes fragmentation patterns of cyclic peptides more complex than those of linear ones. Routine certificates therefore usually rely on intact mass; labs needing sequence-level proof can request MS/MS data.
Why this matters for a broad-receptor agonist
Because melanotan-II's research profile depends on engaging multiple melanocortin receptors as detailed in its mechanism of action, an incompletely cyclized or impure lot could produce a receptor-engagement pattern that differs from the more MC4R-focused PT-141 for reasons unrelated to the intended comparison. Constraining a peptide into a ring is what holds the His-D-Phe-Arg-Trp core in the conformation the receptors recognize, so open-chain contaminants are not neutral bystanders.
The comparison is a fine one analytically as well as pharmacologically: the two molecules differ by roughly 1 Da, as described in PT-141 purity testing. Background on how the compound is used in model systems is in the melanotan-II preclinical research guide. The same tracing logic applies to the related tripeptide work in KPV vs. alpha-MSH, where melanocortin pathways are compared on a native-hormone basis.
Reading the Melanotan-II chromatogram
A purity percentage is a ratio of peak areas: the area under the main Melanotan-II peak divided by the total area of every integrated peak at the detection wavelength, usually 214–220 nm, where the peptide bond absorbs. Because this peptide contains tryptophan and phenylalanine, some labs also record 280 nm as a cross-check; peaks that respond strongly at 280 nm relative to 214 nm are aromatic-rich, which can help assign an oxidized or modified species.
The figure depends on method conditions. A shallow gradient on a C18 column resolves closely related impurities that a steep, fast gradient would fold into the main peak, so two reports quoting the same percentage are only comparable when column, gradient, and wavelength are stated. On the trace itself, look for a flat baseline, a symmetrical main peak without shoulders, and an integration table that lists minor peaks instead of omitting them.
What an HPLC percentage leaves out
HPLC purity describes the peptide fraction only. A lyophilized vial also contains counter-ions left from purification, commonly trifluoroacetate or acetate, plus residual moisture, and neither appears in a UV chromatogram. Net peptide content can therefore be lower than gross powder mass even in a high-purity lot. Labs preparing quantitative stock solutions should account for this.
The peptide calculator works out concentrations after reconstitution from the labeled mass and diluent volume. As an illustration of the arithmetic, a 10 mg vial brought up in 2 mL of diluent gives a nominal 5 mg/mL stock, and the molar concentration follows by dividing by the molecular weight (about 1024 g/mol), giving roughly 4.9 mM. If a lot's net content is, say, 80% of gross mass, true concentrations scale by the same factor. These are laboratory concentrations used to set up assays, nothing more.
| Vial (labeled) | Diluent volume | Nominal concentration | Approx. molarity (MW about 1024) |
|---|---|---|---|
| 10 mg | 1 mL | 10 mg/mL | About 9.8 mM |
| 10 mg | 2 mL | 5 mg/mL | About 4.9 mM |
| 10 mg | 4 mL | 2.5 mg/mL | About 2.4 mM |
Documentation to expect
A lot-specific certificate of analysis with HPLC purity and mass-spec identity is the baseline; see how to read a CoA. In practice, a complete record for a Melanotan-II lot includes:
- A lot or batch number matching the vial label
- The HPLC chromatogram with method conditions and integrated purity
- A mass spectrum showing observed versus theoretical mass
- The test date and the laboratory that performed the analysis
Current batch documents are published on the certificate of analysis page. The MT-2 (Melanotan II) product page lists the vial as 10 mg of lyophilized powder with a specified HPLC purity of at least 98%, the sequence above, and the formula C50H69N15O9, and notes where a lot's certificate is still in progress; read the page for the current status of any given batch.
Keeping a verified lot verified
A certificate describes a lot at the moment it was tested. Lyophilized Melanotan-II is generally kept cold, dry, and protected from light to hold that profile, and reconstituted solutions degrade faster than dry powder. The product page lists refrigeration at 2–8 °C with −20 °C for long-term storage, protected from heat, moisture and light. Repeated freeze-thaw cycles, moisture ingress, and prolonged time at room temperature can all generate new impurity peaks, so labs running long studies often aliquot material and re-test retained samples when results drift. General guidance is in storing research peptides.
Tryptophan oxidation is the change to watch for first: it is promoted by light and air, so amber vials, minimal headspace and limited time open on the bench all help. A new +16 Da signal in a retained sample compared against its original certificate is a clean way to show that storage, not synthesis, is responsible for a change.
Melanotan-II FAQ
What mass indicates correctly cyclized melanotan-II?
About 1024 Da. A signal near 1042 Da points to linear, uncyclized peptide, since ring closure removes one water molecule from the precursor.
Can HPLC distinguish cyclic from linear material?
Yes. The two forms differ in shape and polarity and separate under a suitable gradient, which is why the chromatogram and the mass spectrum are read together.
What degradation should be expected in older material?
Oxidation at tryptophan is the most common change, encouraged by light and air. It shows up as early-eluting peaks and as signals 16 Da above the parent mass.
Can mass spectrometry detect racemization of D-Phe?
Not by intact mass, because stereoisomers have identical mass. It requires a high-resolution gradient that separates diastereomers or a dedicated chiral analysis.
How does melanotan-II differ analytically from PT-141?
The two differ by roughly 1 Da, so they need a high-resolution instrument to tell apart by mass; see PT-141 purity testing.
What does the 214 nm detection wavelength measure?
Absorbance of the peptide bond, which all peptides share. It lets area percent represent all peptide-related material rather than only aromatic residues.
Why might two certificates show different purities for the same compound?
Different columns, gradients, wavelengths and integration rules resolve different numbers of impurities. Compare methods before comparing percentages.
Where can I see the specifications of the V8 vial?
On the MT-2 (Melanotan II) product page, with batch documents on the CoA page.
Research Use Only. For laboratory research use only; not for human consumption. This article is research education, not usage guidance.