Buying Semax in the USA: the 16-Dalton Peak to Expect

Semax gets described as an ACTH(4–10) analog. That is near enough for a catalog line and slightly wrong in a way that matters once you start reading certificates, because the residues it did not keep are the ones that would otherwise dominate how it is measured.

The sequence, exactly

Met-Glu-His-Phe-Pro-Gly-Pro. The first four — methionine, glutamate, histidine, phenylalanine — correspond to positions 4 through 7 of adrenocorticotropic hormone. Positions 8, 9 and 10 of ACTH are arginine, tryptophan and glycine, and Semax has none of them. Pro-Gly-Pro sits there instead.

The accurate description, then, is an ACTH(4–7) fragment with a synthetic C-terminal extension. It comes from the ACTH(4–10) region, which is where the looser phrasing originates, but three of those residues were swapped out rather than carried over.

What the tail is for

Pro-Gly-Pro is there to stabilize, not to bind. Proline’s side chain loops back onto its own backbone nitrogen, locking the local geometry and making that bond awkward for most exopeptidases to process. A short peptide without such protection is chewed in from its ends quickly; with it, the molecule lasts long enough to be worth studying.

The same three residues are bolted onto an entirely different parent in Selank, which is tuftsin plus Pro-Gly-Pro. Two unrelated sequences, one stabilization strategy — the tail is shared engineering, not a family resemblance. Selank and its Pro-Gly-Pro tail covers the other half of the pair.

Dropping the tryptophan changes how it is measured

Tryptophan is the strongest chromophore in the standard amino acid set, and losing ACTH’s Trp-9 has consequences for measurement rather than for anything else.

Detection first. At 280 nm only tryptophan, tyrosine and, weakly, phenylalanine absorb. Semax keeps one phenylalanine and nothing else aromatic, so its response at that wavelength is poor — and any impurity without aromatics is poorer still. Purity for this sequence belongs at 214 nm or 220 nm, where the peptide bond absorbs and everything peptidic shows up. A high purity figure quoted at 280 nm on this compound is a statement about the wavelength, not about the material.

Second, tryptophan is itself among the most oxidation-prone residues, so its absence removes a common degradation route. Semax has a different one.

The methionine and the 16-dalton peak

Position one is methionine, and its thioether sulfur oxidizes readily to the sulfoxide. That reaction adds exactly one oxygen atom, and one oxygen weighs 16 daltons.

This is the single most useful thing to look for on a Semax mass spectrum. A peak 16 Da above the expected mass is oxidized material: a different compound from the one on the label, produced by air, light or peroxide contamination in a solvent rather than by a fault in synthesis. It can form after manufacture, so a certificate showing none is no guarantee of none later.

In practice that makes air exposure the variable worth controlling — few vial entries, powder kept dry and dark, and nothing left standing open once reconstituted. How peptides degrade and aliquoting reconstituted peptides cover the handling.

Identity figures

Molecular formula C37H51N9O10S — the sulfur is the methionine — mass about 813.9 g/mol, CAS registry number 80714-61-0. Those are the values to match across listings, because the naming is inconsistent and registry numbers are not. The calculated mass is also the reference point for the oxidation check: 813.9 is the compound, 829.9 is the sulfoxide.

On a column

Semax carries a glutamate and a histidine on a small, largely polar backbone, and it comes off a reversed-phase column early. The oxidized form is more polar than the parent and elutes earlier still, which is why oxidation on this compound tends to appear as a front shoulder on the main peak rather than as a cleanly separated impurity. A shallow gradient resolves it; a steep one buries it. Reading an HPLC chromatogram shows what that looks like, and what a third-party peptide test actually measures covers which figures decide whether a number means anything.

What to look for on a certificate

The detection wavelength. Measured mass against 813.9, with attention to anything 16 Da higher. And net peptide content, which accounts for counter-ion and residual water and tells you how much peptide is actually in the vial — see net peptide content. Every report we hold is published in full on the certificates of analysis page.

Names you will see

Semax, the sequence MEHFPGP, and occasionally ACTH(4–7)-PGP, which is the most accurate of the three. Anything written as ACTH(4–10) is naming the region it derives from rather than what it is made of.

What we supply

Semax is stocked as a 10mg vial, lyophilized, with identity data published on the product page. Supplied for laboratory research use only. We publish no dosing information, no protocols involving human or veterinary subjects, and no statements about what the compound does in a person — and we do not provide them on request.