Semax vs Selank: A Research Comparison

Semax and Selank get shelved together and mentioned in the same breath, which makes it easy to assume they are variations on one theme. They are not. They are two distinct molecules with different parent compounds and different places in the research literature. What they share is a country of origin and a structural quirk at one end of the chain. This post compares them on identity: what each molecule is, and how each is supplied. It does not rank them, because an identity comparison has no winner.

What the two actually share

Both are synthetic heptapeptides developed in Russia.

Each is seven amino acids long, and both trace back to the Institute of Molecular Genetics at the Russian Academy of Sciences, which is why they so often appear side by side. They were built the same way, too: take a shorter natural peptide, then extend it at the C-terminus with a proline-glycine-proline tail. That terminal Pro-Gly-Pro is the shared design feature, and it is why the two look like cousins on paper even though their active portions come from unrelated proteins.

That is roughly where the resemblance ends.

Two different parent molecules

The clearest difference is lineage. Semax descends from a pituitary hormone fragment, Selank from an antibody-derived peptide.

Semax is an ACTH-related peptide. Its front end corresponds to a short stretch of ACTH(4-10), the adrenocorticotropic hormone fragment, capped with the Pro-Gly-Pro tail. So the borrowed portion of Semax comes from a hormone of the pituitary axis.

Selank is a tuftsin analog. Tuftsin is a natural tetrapeptide, Thr-Lys-Pro-Arg, that occurs as a fragment of the immunoglobulin G heavy chain. Selank is that tuftsin sequence carrying the same Pro-Gly-Pro tail. Its borrowed portion comes from an antibody protein, not a hormone.

Same construction method, unrelated source material. If you remember one distinction between these two, make it this one.

The identity data

Here is the label-level chemistry, the part that does not change between suppliers.

Semax - CAS number: 80714-61-0 - Sequence: Met-Glu-His-Phe-Pro-Gly-Pro (MEHFPGP) - Molecular formula: C37H51N9O10S - Molecular weight: 813.93 g/mol - Class: ACTH(4-10)-related heptapeptide

Selank - CAS number: 129954-34-3 - Sequence: Thr-Lys-Pro-Arg-Pro-Gly-Pro (TKPRPGP) - Molecular formula: C33H57N11O9 - Molecular weight: 751.88 g/mol - Class: tuftsin analog heptapeptide

One small tell is worth noting. Semax's formula carries a sulfur atom, the S at the end of C37H51N9O10S, which comes from its methionine residue. Selank has none. If a certificate for either compound reports a molecular weight far from the figures above, or lists the wrong terminal residues, that is the first sign to slow down and match the CAS number before trusting anything else on the page. Names get transcribed loosely. CAS numbers do not.

Which research fields each shows up in

The two are studied in different research contexts, and naming those contexts is as far as this section goes.

Semax appears in neuroscience research literature. Selank appears in neuropeptide and behavioural research literature. Those are subject areas, not findings. Saying that a compound has been studied within a field states what researchers have looked at, and nothing about what any study reported.

I am not going to summarize outcomes here, and it is worth being blunt about why. This category has a hard line between describing what a molecule is and describing what anyone claims it does, and a comparison post stays on the identity side of that line. If you want the primary sources, the sequences and CAS numbers above are the search terms that pull the real papers.

How each is supplied and handled

Both ship the same way: lyophilized, then kept cold and out of light.

Semax and Selank each arrive as a freeze-dried solid. Store either at -20C and protect it from light, keep the vial sealed until you are working with it, and handle it as the laboratory reagent it is. There is no meaningful difference between the two at the bench, and none of this is preparation guidance. It is storage, nothing more.

How the lots are tested

Every lot of each compound ships with its own third-party certificate of analysis. Purity is measured by HPLC read at 220 nm, the standard wavelength for the peptide bond, and identity is confirmed by mass spectrometry, which verifies that the molecule matches the label rather than only measuring how clean the sample is. Lots go to independent labs including independent third-party laboratories, and the full PDF is published on each batch page.

You can see both products here: [Semax](/products/semax) and [Selank](/products/selank). If you have not read one of these certificates before, there is a walkthrough on [how to read a peptide COA](/research/how-to-read-a-peptide-coa) using two real reports, plus a [checklist for sourcing research peptides in Canada](/research/buying-research-peptides-canada). Kits Peptides ships tracked from Vancouver, BC across Canada.

*All products are supplied as laboratory reagents for research use only. Not for human or veterinary use.*