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Molecular Weight
Purity
Storage
Appearance
Solubility
01
PT-141, international non-proprietary name bremelanotide, is a synthetic cyclic peptide with the sequence Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH. It is identical to Melanotan II except at the C-terminus, where it has a free carboxylic acid instead of an amide.
That small change adds about 0.98 Da, introduces an extra negative charge and is associated with a different melanocortin receptor profile. Because the two compounds are so similar, distinguishing them is a classic analytical challenge. Bremelanotide is the active ingredient of a medicine approved in some countries; the material listed here is supplied solely as a lyophilized reference compound for in vitro and analytical research.
02
PT-141 is defined by its relationship to Melanotan II.
Shared ring
The same 23-membered lactam ring between Asp2 and Lys7, the same His-D-Phe-Arg-Trp core and the same acetylated norleucine outside the ring.
C-terminal free acid
The only difference: –OH replaces –NH₂ at the C-terminus. The formula changes from C₅₀H₆₉N₁₅O₉ (MT-II) to C₅₀H₆₈N₁₄O₁₀, a gain of about 0.98 Da.
Charge
The free acid is negatively charged above about pH 3–4, so PT-141 carries roughly one less positive charge than MT-II at neutral pH, with a net charge of about 0 to +1.
Conformation
Structural studies indicate that the ring conformation is essentially the same as in MT-II.
Sensitive residue
Trp6 can photo-oxidise, as in MT-II.
03
PT-141 is studied as a melanocortin receptor agonist and as a comparator to MT-II.
Receptor profile
Binding and cAMP assays show agonist activity at several melanocortin receptors, notably MC4R and MC1R. Published comparisons report differences in subtype profile relative to MT-II, although both remain relatively non-selective.
Structure–activity
Pairing PT-141 with MT-II isolates the effect of the C-terminal group on receptor preference, since everything else is identical.
Relation to MT-II degradation
Hydrolysis of MT-II’s C-terminal amide gives PT-141, so it is also relevant as a degradation product in MT-II stability studies.
These findings are cited as research context only; no effects are claimed for this material.
04
High-resolution MS validation
A demanding test pair, with MT-II, for checking mass resolving power and isotope deconvolution.
Chromatographic differentiation
A defined method-development target: baseline separation of PT-141 from MT-II.
Receptor selectivity comparisons
Paired with MT-II in melanocortin receptor panels to measure the effect of the C-terminal acid.
Degradation standard
A reference standard for quantifying amide hydrolysis in MT-II stability studies.
05
Storage
Store the lyophilized powder at −20 °C, sealed, desiccated and protected from light.
Light sensitivity
Trp6 can photo-oxidise to kynurenine-type products. Handle under subdued light.
Esterification
The free C-terminal acid can form methyl or ethyl esters in acidified alcohol. Avoid prolonged exposure to acidic methanol or ethanol during sample preparation.
Lactam stability
The ring bond is robust under normal conditions and unaffected by reducing agents.
Reconstitution
Soluble in water and DMSO. Aliquot into single-use volumes in low-binding tubes and freeze.
06
The central analytical task is distinguishing PT-141 from MT-II.
Mass spectrometry
PT-141 gives [M+H]⁺ at m/z 1025.5, compared with 1024.5 for MT-II (monoisotopic). A pure sample is easy to identify by mass.
In mixtures
MT-II’s natural ¹³C isotope peak falls at about m/z 1025.55, only about 0.02 Da from PT-141’s main peak. Separating these needs a resolving power of roughly 50,000 to see two peaks and around 100,000 for clean separation; at lower resolution, chromatographic separation is required.
HPLC
The C-terminal acid usually shortens reversed-phase retention relative to MT-II. The difference is modest in acidic TFA mobile phases, where the acid is partly protonated, and larger in less acidic buffers where it is ionised.
MS/MS
Fragments that contain the C-terminus are offset by about 0.98 Da between the two compounds, which confirms the assignment.
Impurity profile
Typical checks include residual MT-II, Trp oxidation products, ring-opened forms (+18 Da), the L-Phe epimer and ester artefacts from sample preparation.
07
The C-terminus: PT-141 has a free carboxylic acid (–OH) where Melanotan II has an amide (–NH₂). The ring, core motif and N-terminal Nle are identical.
Their masses differ by only about 0.98 Da, and in a mixture the natural ¹³C isotope peak of MT-II lands about 0.02 Da from PT-141’s main peak. Ordinary resolution merges these signals. Pure samples, however, are easy to identify by their main peaks (m/z 1025.5 versus 1024.5).
It usually makes PT-141 elute a little earlier than MT-II in reversed-phase HPLC. The effect is small in acidic TFA conditions and larger at higher pH, where the acid is fully ionised.
Yes. Hydrolysis of MT-II’s C-terminal amide gives PT-141, so it is monitored as a degradation product in MT-II stability studies.
The two signals are about 0.02 Da apart at m/z 1025, so a resolving power of roughly 50,000 (FWHM) is needed to distinguish them and around 100,000 for baseline separation. Otherwise, rely on chromatographic separation.
No. Although bremelanotide is used in an approved medicine in some countries, the material listed here is supplied strictly as a reference compound for laboratory and analytical research and is not for human or animal use.
PT-141 Research Reference. (2026). Pure Peptides Labs Research Catalogue. Retrieved from https://purepeptideslabs.net/research-catalogue/melanocortin-research-peptides/pt-141/
Materials listed on this platform are supported by independent testing and published documentation for laboratory research contexts.
