PT-141 (Bremelanotide)

What PT-141 is, structurally

PT-141 is a cyclic peptide of seven residues. The product page describes it as arranged in a ring and derived from Melanotan II, and both statements have consequences that can be worked out from the specifications the site already carries.

The relationship to Melanotan II is exact, and worth setting out because both are sold here. Melanotan II is C50H69N15O9 at 1024.18; this is C50H68N14O10 at 1025.17. The difference is one nitrogen and one hydrogen removed and one oxygen added, precisely a C-terminal amide replaced by a free acid. The two are the same cyclic scaffold terminating differently.

That produces a hazard worth stating plainly. The two products are separated by 0.98 daltons, while a single deamidation adds about one, so the difference between two distinct products is smaller than the shift of a common degradation reaction. A low-resolution mass measurement will not reliably tell them apart, and a deamidated impurity of one is close to isobaric with the other. Chromatography separates them properly, since an amide and a free acid differ in charge and therefore retention.

The ring is the other defining feature. A cyclised peptide is conformationally constrained in a way a linear one is not, which generally gives sharper chromatographic behaviour. It also introduces a degradation route linear peptides lack: hydrolysis of the bond closing the ring. That adds a water molecule, making the product eighteen daltons heavier, which is a large and unambiguous shift at this mass.

There is no sulfur in the formula, so no disulfide chemistry, no thiol oxidation and no methionine sulfoxide. Fourteen nitrogens across seven residues indicates substantial basic side-chain content, so the molecule should carry net positive charge at neutral pH despite the free acid terminus.

Reconstitution and handling

Sterile water is the default solvent. The molecule carries several charged groups and should dissolve readily.

Add diluent slowly down the inside wall of the vial, swirl gently, and let the solution clarify. Do not vortex; it introduces foam without dissolving anything faster.

Buffer pH deserves more thought here than on a linear peptide. Both extremes accelerate hydrolysis of the ring-closing bond, so near-neutral conditions protect the cyclic structure as well as suiting the peptide generally.

No reducing agent is needed, since the formula contains no sulfur.

Adsorption deserves the usual allowance at low concentration, with low-protein-binding tubes reducing measured loss. A net positive molecule has more affinity for negatively charged glass than for polypropylene.

This is laboratory preparation chemistry, not dosing, administration, or protocol guidance of any kind.

Storage and stability

Store the lyophilized powder at -20 degrees C, sealed, desiccated and protected from light, and let a cold vial warm before opening. Reconstituted, hold at 2 to 8 degrees C, or aliquot and freeze rather than thawing repeatedly.

Ring opening is the liability specific to this molecule and the one to design storage around. It requires water, so the dry solid is well protected while solutions are not, and both acidic and alkaline pH accelerate it. The eighteen-dalton addition is readily detected, making it a tractable route to monitor.

With no sulfur present, the oxidation routes that dominate many peptides are closed, and air exposure matters correspondingly less.

Deamidation remains possible at any asparagine or glutamine, adding about one dalton, which on this compound moves the mass toward that of the related product above. Without a sequence this page will not name positions.

These are storage conditions for the material, not dosing or administration guidance.

How PT-141 is tested

Reversed-phase HPLC establishes chromatographic purity as area percent, and here it does more work than usual: it is what distinguishes this material from the closely related product it derives from, which mass alone does not do reliably.

Detection should be assumed near 214 nm, though aromatic content may make 280 nm usable.

Area percent is a chromatographic measure and not peptide content by weight. Solid-phase synthesis delivers a salt, and on a basic peptide of a thousand daltons the counter-ion is a meaningful proportional share of the vial contents.

Mass spectrometry confirms identity against 1025.17, with two results worth anticipating. A figure near 1043 indicates a ring-opened molecule. A figure near 1024.2 is not a light impurity but the amidated analogue, a different product. Since the two differ by less than one dalton, instrument resolution should be established before drawing any conclusion from a near-miss.

These describe general methodology, not a claim about any particular batch.

Handling FAQ

How is this distinguished from Melanotan II? Not reliably by mass alone. The two differ by 0.98 daltons, being the amide and free acid of the same cyclic scaffold. That gap is smaller than a single deamidation's shift, so chromatography, where the differing charge changes retention, is the dependable separation.

What does a result eighteen daltons high indicate? Ring opening. Hydrolysis of the bond that closes the cycle adds a water molecule, giving a linear molecule eighteen daltons heavier. Near-neutral pH and dry storage limit it.

Full specifications for PT-141.