Buying Selank or Semax in the USA: Same Design, Different Head

Pairing Selank with Semax is conventional, and the convention rests on a shared origin and a shared design trick rather than on any shared target. Each is a synthetic elongation of a fragment that occurs naturally, and both came out of the same research program.

Where each molecule came from

Selank starts from tuftsin, a tetrapeptide with the sequence Thr-Lys-Pro-Arg. Tuftsin exists as a proteolytically released fragment of the immunoglobulin G heavy chain, residues 289–292. Add the extension and the result is a heptapeptide. The tuftsin literature itself goes back to the early 1970s and is built around phagocyte function, reporting binding at receptors on macrophages and neutrophils together with effects on phagocytosis and chemotaxis. One caution belongs with that body of work: the receptor assignments in it are less settled than the citation count suggests, and the proposed targets have moved around over the decades. Literature: PubMed and tuftsin.

Semax starts further up the same kind of chain, from ACTH(4-10), a heptapeptide reading Met-Glu-His-Phe-Arg-Trp-Gly that is a fragment of adrenocorticotropic hormone. Because the fragment omits the C-terminal region of ACTH that drives adrenal steroidogenesis, the steroidogenic activity of the intact hormone does not come with it. What the fragment does retain is the His-Phe-Arg-Trp melanocortin core, which is why any discussion of it has to sit alongside the melanocortin receptor subtypes rather than dismissing that connection. Then comes a detail routinely passed over: Semax cuts back further still, to ACTH(4-7), before the extension is appended, and that truncation takes out part of the core motif, with real consequences for what melanocortin receptor engagement can reasonably be expected. Literature: PubMed.

Why both end in Pro-Gly-Pro

The appended C-terminal tripeptide is the deliberate part of both designs. Termini rich in proline hold up against exopeptidase attack, so bolting Pro-Gly-Pro onto a fragment that would otherwise disappear quickly buys a great deal of time, and the same maneuver shows up repeatedly across peptides from that program.

The residue choice follows from proline’s geometry. Its side chain curls back to the backbone nitrogen, which strips out the amide hydrogen and pins the phi angle, leaving the residue conformationally restricted. Two things follow from that, and both are being used deliberately.

None of that comes free, because an extension is itself a structural alteration. A heptapeptide is not a tetrapeptide with a better half-life; it is another compound. To say that Selank recapitulates tuftsin pharmacology, or that Semax recapitulates the pharmacology of ACTH(4-10), is to state a hypothesis, and the published record does not close the question.

A literature that has been replicated thinly

Better to put this bluntly than tactfully. A large share of the primary work is in Russian and is indexed patchily in the databases most people search, so an empty PubMed result does not mean an empty literature, though it does mean the accessible slice is a skewed sample. Authorship is concentrated as well, much of it tracing to a handful of connected groups; that is not an accusation, but it is a reason independent replication counts for extra wherever it turns up and a reason to notice where it has not. Mechanistic accounts differ between papers on the same compound, and where two proposals contradict one another no more than one survives, which makes any design that presupposes a particular mechanism fragile. Finally, a good deal of the in vivo work rests on rodent behavioral endpoints, sensitive to handling, housing and experimenter effects, and carrying a replication problem that belongs to the whole field rather than to these compounds specifically.

The workable response is to make mechanism the thing under study instead of the backdrop. An experiment that interrogates a proposed target head-on is worth more here than one that takes the target for granted and reads out a downstream phenotype.

Working with them at the bench

Design first. Run the parent fragment as a comparator, meaning tuftsin against Selank and ACTH(4-10) or ACTH(4-7) against Semax, because without it nothing can be attributed to the extension rather than the parent, and that attribution is the whole open question. Run Pro-Gly-Pro on its own too: it is cheap, it excludes the possibility that the extension is doing something by itself, and almost nobody bothers. Verify the stability claim in your own hands, since the half-life advantage is the entire rationale for these molecules and one LC-MS time course in your buffer at your serum concentration turns an assumption into data. For Semax, put a melanocortin receptor panel in rather than treating it as exotic, given that partial core motif. And blind and randomize any behavioral readout, because given the replication history nothing unblinded will convince a reader.

Characterization next. Confirm the mass against the stated sequence: both are short enough that the calculated value is unambiguous, which makes the check a strong one, and establish whether the C terminus is free acid or amide, a difference of 18 Da or 1 Da that produces a different molecule either way. Get RP-HPLC purity with the gradient, column and wavelength all stated; short hydrophilic peptides come off early, where resolution is at its worst and where buffer and injection artifacts live, so the trace matters more than the figure — see reading an HPLC chromatogram. Ask for net peptide content, since both carry a lot of charge for their length, which makes the counterion contribution proportionally heavy and leaves gross vial mass overstating peptide mass appreciably; see net peptide content explained. Check Semax for oxidation, because its methionine and tryptophan both oxidize and the signature is a plus-16 satellite on the MS trace, covered in deletion, truncation and oxidation impurities. And establish the counterion, because residual trifluoroacetate is active in certain cell assays and a good deal of the relevant work is cell-based; see counterions and salt form.

Handling last. Both go into water or a near-neutral buffer without difficulty, and since neither sequence contains cysteine there is no disulfide scrambling to worry about and reducing agents in the buffer are harmless. Semax is the one needing light and oxygen discipline on account of that methionine and tryptophan: amber glass or foil, headspace kept small, bench time kept short. Selank has neither residue and forgives more. Aliquot when you reconstitute, because short peptides still lose material to freeze–thaw cycles, and adsorption to plastic at low working concentrations is a genuine loss that leaves no trace. Track how old the solution is, not how old the vial is. Documentation practice sits in keeping a peptide inventory, with conditions covered in storage and stability and degradation by light, oxygen and temperature.

Terms used here

Products: Selank 5mg, Selank 10mg, Semax 10mg, listed under neuropeptides.

ExoLabz supplies compounds for laboratory research use only. Nothing on this page is medical advice or a suggestion of human or veterinary use. The third-party reports we have obtained are published on our certificates of analysis page.