PT-141 | 20mg | 3ml

PT-141 (bremelanotide) is a synthetic seven-residue peptide closed into a macrocycle by a side-chain lactam bridge, supplied here as a pre-mixed solution in a 20mg vial for in-vitro laboratory research.

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Two things about this listing differ from a lyophilized vial of the same compound: the material arrives already in solution, and the molecule sits one small chemical edit away from a close structural relative, which makes identity verification less trivial than it first appears. Both are worth understanding before you compare suppliers.

Structural identity

A single terminal group separates it from the parent scaffold

PT-141 is built on the same cyclic backbone as Melanotan II, its structural parent and, for the purposes of this page, nothing more. The distinguishing edit sits at the C-terminus: where the parent carries a primary carboxamide, PT-141 carries a free carboxylic acid — a nitrogen-for-oxygen exchange with the loss of a hydrogen. That is the whole structural difference, and it has three analytical consequences a receiving lab should care about.

Mass. Swapping an amide for an acid changes the average mass by roughly one dalton — about 1025.2 for PT-141 against about 1024.2 for the amide form. On a low-resolution instrument, or on a report quoting mass only to the nearest unit, those two isotope envelopes overlap badly. A COA line reading "MS confirms ~1025" is not, on its own, a confirmation of identity. Ask for theoretical and observed masses printed side by side, at resolution sufficient to separate them.

Charge. The free acid is deprotonated at neutral pH, so the molecule carries one more negative charge than the amide form. That shows up in ion-exchange behavior and in the isoelectric point, and typically shifts retention slightly under identical reversed-phase conditions. Retention time alone is weak evidence; retention time against a reference standard run in the same sequence is much stronger.

Ring integrity. The lactam is formed by condensing the aspartate side-chain carboxyl with the lysine side-chain amine, so the cyclic peptide is 18 daltons lighter than its linear precursor. Uncyclized linear material is the impurity to look for, visible both as a distinct chromatographic peak and as a +18 mass; cyclodimers appear at approximately twice the monomer mass. Separately, the phenylalanine residue is in the D-configuration, and epimerization to the L-configuration produces a diastereomer of identical mass that mass spectrometry cannot resolve at all. Only chromatography, or dedicated chiral analysis, separates it — the single strongest argument for insisting on a full chromatogram rather than a summary purity number.

Receptor selectivity as a research subject

The melanocortin family comprises five G-protein-coupled receptors, MC1R through MC5R. Published work on melanocortin ligands is a body of in-vitro and animal laboratory research under examination, and this page draws no conclusions from it. The methodological point is the transferable one: in-vitro reports of how a ligand distributes its binding affinity across those subtypes depend heavily on assay format, cell line, receptor expression level and readout, and the values are not portable between systems. Treat subtype numbers as properties of a particular assay, not of the molecule in the abstract.

Pre-mixed solution against lyophilized powder

Neither format is universally better. The trade is that a solution removes a step where a lab introduces its own error, and accepts in exchange a shorter stability window and real sensitivity to how the package travelled. Ask any overseas supplier what packaging and temperature control it uses, and inspect on arrival.

What a Certificate of Analysis should contain

Documentation for this catalogue is published at /coa. Whoever you buy from, a COA is only worth reading if it carries all of the following.

• A lot or batch number matching the vial in your hand, plus an analysis date. A document with no lot linkage is a marketing asset, not a record.
• A reversed-phase HPLC chromatogram, not just a percentage. Note the detection wavelength: 214 nm reports the peptide bond and sees far more than 280 nm, which reports only aromatic residues.
• Purity stated with its definition: area percent, at which wavelength, integrated over what window.
• Mass confirmation with theoretical and observed values, and enough resolution to be meaningful given the one-dalton issue above.
• The identity of the testing laboratory. An unnamed lab, or one document reused verbatim across lots, is a red flag.
• For solutions: appearance, container and closure description, and any statement on filtration.
• Where applicable, residual solvent, counterion content and bioburden or endotoxin results.

Ask for that document before ordering rather than after, and ask whether the raw chromatograms behind it can be produced. A supplier that answers plainly is a better bet than one quoting a higher purity figure with nothing behind it.

Checks on receipt, storage and handling

Record the condition of the package on arrival before anything else: external temperature indicator if present, seal and septum integrity, clarity and color of the solution, and any visible particulate or haze. Keep the vial cold and dark; both tryptophan and histidine are oxidation-sensitive, and light exposure is a slow, cumulative and entirely avoidable stressor. Minimize freeze–thaw cycling, and let a chilled vial equilibrate before opening so condensation does not form inside the closure. Log lot numbers against your own analytical results, so a future anomaly can be traced to a batch rather than argued about.

Origin and testing

This material is synthesized in the UK and verified in US laboratories. The split is the point: synthesis and independent analysis are not performed by the same party, which is what makes the analysis worth anything. The Certificate of Analysis for this product is available at /coa.