Quick answer
Buy PT-141 from a source that publishes HPLC purity, mass-spec identity near 1,025 Da and a lot-matched CoA.
- Molecule: cyclic lactam heptapeptide, C50H68N14O10, about 1,025 g/mol
- Check: +18 Da (uncyclized) and dimer signals are absent; lot number matches the vial
- V8 listing: 10 mg lyophilized vial, ≥98% HPLC purity per the product page
- Records: batch documents on the CoA page; stock math in the calculator
For laboratory research use only; not for human consumption.
Why PT-141 sourcing is a documentation problem
PT-141 (bremelanotide) is a small cyclic peptide, and its size doesn't make sourcing it any less demanding. A synthetic heptapeptide still has to be synthesized cleanly, purified correctly and verified by mass spectrometry before it is reliable in receptor-binding or cAMP signaling work. It is easy to find PT-141 listed cheaply from dozens of vendors, so purity documentation is really the only thing separating a usable research sample from one that will confound your data. The compound itself is introduced in what is PT-141 and in more depth in the PT-141 research overview.
Structurally, PT-141 is the lactam-cyclized melanocortin analog Ac-Nle-cyclo[Asp-His-D-Phe-Arg-Trp-Lys]-OH (the sequence given on the PT-141 product page). The ring is closed by an amide bond between the aspartate side chain and the lysine side chain. It is closely related to the cyclic α-MSH analogs discussed in melanotan II research, differing in having a free carboxylic acid at the C-terminus rather than the amide, and it acts at melanocortin receptors, with MC4R as the principal target in research models.
What to evaluate when buying PT-141
| Criterion | What a good answer looks like | Red flag |
|---|---|---|
| Measured purity | A stated percentage (e.g. ≥98% by HPLC) with the method named; see HPLC purity analysis | A bare claim printed on a label |
| Batch-specific CoA | Document tied to the exact lot; see how to read a CoA | A generic spec sheet reused across runs |
| Independent testing | Outside-lab confirmation; see third-party testing explained | Internal-only claims |
| Molecular identity | Mass spectrometry matching about 1,025 g/mol (C50H68N14O10) and a cyclic structure | Only an HPLC retention time near the expected value |
| Net peptide content | Reported separately from gross weight | Label mass treated as pure peptide |
| Reconstitution and storage guidance | Honest, with clear Research-Use-Only labeling | Vague or absent |
| Shipping | Handling that recognizes a heat- and light-sensitive lyophilized peptide as a sensitive reagent | No handling information |
Why price alone isn't a quality signal
Synthesizing a cyclic peptide like PT-141 involves forming and confirming an intramolecular ring closure, then purifying away linear or mis-cyclized byproducts that can be structurally close enough to survive a sloppy HPLC method. That work costs real money. A price that is dramatically below the rest of the market for a stated ≥98% purity grade is a reason to ask harder questions, not a reason to skip them, especially when the same supplier can't produce a lot-specific COA on request.
The specific impurities worth knowing about explain why. The linear, uncyclized precursor has the same constituent atoms plus the elements of water (about 18 Da more), and dimers formed by intermolecular ring closure have roughly double the mass. Both are distinguishable by mass spectrometry, but only if the supplier actually looks. An HPLC-only report may show them as small shoulders that integrate into the main peak under a fast gradient.
What the identity data should show
A mass spectrum for PT-141 should show an observed molecular weight within instrument tolerance of the theoretical value (about 1,025 Da), and the report should print both numbers side by side. The cyclic lactam is the feature to confirm: the open-chain precursor is 18 Da heavier, so a clear +18 Da signal indicates incomplete cyclization. The tryptophan residue is prone to oxidation, so +16 Da species are worth looking for too. Background on why both HPLC and mass spec are needed is in HPLC vs. mass spectrometry, and the analytical walk-through is in PT-141 purity testing.
| Observed vs. ~1,025 Da | Likely meaning |
|---|---|
| Within instrument tolerance | Consistent with the cyclic target |
| About +18 Da | Open-chain (uncyclized) precursor |
| About +16 Da | Oxidized tryptophan or similar |
| About 2× the mass | Dimer from intermolecular ring closure |
Questions worth asking a supplier directly
Before ordering, it's reasonable to ask whether the Certificate of Analysis is generated per batch or reused across runs, whether HPLC and mass spec testing were performed in-house or by an independent lab, and whether the vial's labeled mass reflects net peptide content or gross lyophilized weight. A supplier with nothing to hide will answer plainly and can produce documentation on request instead of pointing back to marketing copy. Our general checklist is in choosing a peptide supplier.
Why net content matters for the lab: lyophilized vials include counter-ion mass and residual moisture, so a label that doesn't separate those from active peptide overstates concentration in every downstream calculation. For a 10 mg vial reconstituted in 2 mL, the nominal concentration is 5 mg/mL, and the true peptide concentration is lower by the net-content fraction. The calculator handles the arithmetic once you have that fraction.
Handling and storage once it arrives
The product page specifies refrigeration at 2–8 °C, −20 °C for long-term storage, and protection from heat, moisture and light. General principles are in storing research peptides, and why the dry form is the stable one is explained in what is lyophilization. On receipt, record the lot number, check it against the certificate, and inspect that the seal is intact and the cake is uniform before storing the vial.
How lot quality shows up in a receptor assay
PT-141 is typically examined in melanocortin-receptor work such as binding competition and cAMP accumulation in cells expressing MC4R. In those formats an impurity matters in two directions. A more potent contaminant, for instance another melanocortin-active peptide, inflates apparent potency, while an inactive impurity or an over-estimated peptide content shifts the concentration-response curve to the right. Either way, an EC50 or Ki value reported from an unverified lot is hard to compare with anyone else's.
Practical steps that reduce the risk include running a reference lot alongside a new one in the same plate, recording lot numbers in the notebook entry for each experiment, and keeping a retained aliquot so a surprising result can be re-tested against the original material. None of these steps replaces upfront documentation, but they make any discrepancy traceable.
Common sourcing mistakes
- Accepting a percentage without a chromatogram. A number with no trace behind it cannot be reviewed.
- Treating retention time as identity. Different peptides can elute together; mass is the identity check.
- Ignoring the lot number. A real certificate for a different lot says nothing about the vial in hand.
- Calculating from label mass. Net content below 100% means the true concentration is lower than the nominal figure.
- Storing at room temperature "for a few days". Cumulative warm time is what generates degradation peaks later.
Where to buy documented PT-141
V8 Peptides supplies PT-141 (bremelanotide) as a 10 mg lyophilized vial listed at ≥98% purity by HPLC, with a batch-specific certificate of analysis for each lot. The vial is available on the PT-141 (Bremelanotide) product page. For background on the compound itself, see the PT-141 research overview, and for a broader rundown of what separates a documented supplier from an undocumented one, see the supplier checklist. Published batch records are listed on the CoA page. Comparable guides exist for related melanocortin compounds, for example where to buy melanotan II.
Frequently asked questions
What should a PT-141 certificate of analysis include?
A lot number matching the vial, HPLC purity with the method, a mass spectrum with observed and theoretical mass, the test date and the testing laboratory.
Why is the cyclic structure important when buying?
Cyclization is a separate synthetic step with its own byproducts. Linear precursor and dimers can be structurally close to the target, so identity should be confirmed by mass, not by HPLC retention alone.
What mass should PT-141 show?
About 1,025 g/mol for C50H68N14O10. A signal roughly 18 Da higher suggests uncyclized precursor.
Is a very low price a bad sign?
It is a prompt to ask more questions, not a verdict. Ring closure and purification cost money, so a very low price paired with no lot-specific documentation is the combination to be wary of.
Why does net peptide content matter?
Counter-ions and water make up part of a lyophilized vial's mass. If the label mass is treated as pure peptide, every concentration calculated from it is slightly too high.
What size does V8 list?
The product page lists a 10 mg vial at ≥98% HPLC purity.
How should PT-141 be stored?
Per the product page: 2–8 °C, −20 °C for long-term storage, away from heat, moisture and light.
Where can I learn about the compound before ordering?
The PT-141 research overview covers the mechanism and model systems, and what is PT-141 is a plain-language introduction.
Research Use Only. For laboratory research use only; not for human consumption. This article is research education, not usage guidance.
References
- Understanding HPLC purity (V8 Peptides)
- HPLC vs. mass spectrometry (V8 Peptides)
- How to read a certificate of analysis (V8 Peptides)
- Third-party testing explained (V8 Peptides)
- PT-141 purity testing (V8 Peptides)
- PT-141 research overview (V8 Peptides)
- PT-141 (Bremelanotide) research vial (V8 Peptides product page)