Semax and Selank are usually listed side by side, and for once the adjacency reflects something real. Both came out of the same Russian research programme, both are short fragments of a larger endogenous molecule, and both use the same structural modification to survive long enough to be studied.
What each derives from
Semax is based on a fragment of adrenocorticotropic hormone, specifically residues 4 to 10, often written ACTH(4-10). The parent hormone's endocrine activity resides elsewhere in the sequence, so this fragment is studied without that activity.
Selank is based on tuftsin, a four-residue fragment derived from the heavy chain of immunoglobulin G. Tuftsin itself has a documented literature in immune signalling.
So: one from a pituitary hormone, one from an antibody. Different origins, same design problem.
The Pro-Gly-Pro tail
Both compounds append the same three residues, proline-glycine-proline, to the parent fragment. This is the structural signature of the series and it is there for a specific reason: short peptides are rapidly degraded by aminopeptidases and carboxypeptidases, and proline residues are poor substrates for many of these enzymes.
Flanking the active fragment with prolines therefore slows enzymatic clearance without altering the fragment itself. It is an elegant and economical piece of design, and recognising it explains why two compounds from unrelated parent molecules look structurally similar at one end.
Variants in the catalog
Both appear in more than one form, and the suffixes are not decorative.
- An amidate form has the C-terminal carboxyl replaced by an amide. This removes a negative charge and typically increases resistance to carboxypeptidases, and it changes the molecular weight by roughly one dalton, which a good mass spectrometry result resolves.
- N-acetyl forms carry an acetyl group on the N-terminus, blocking aminopeptidase attack at that end.
- Catalog entries such as N-acetyl semax amidate carry both modifications, protecting both termini.
These are genuinely different molecules with different masses, not packaging variants. A certificate should name which one the lot is, and the observed mass should match that specific form.
Why they are so often liquid
Both compounds appear widely as liquid sprays rather than lyophilised powder. A liquid presentation shifts the stability questions: a solution has no reconstitution step but also no lyophilised shelf life, so the supported period is shorter and storage conditions matter more continuously.
For a spray the relevant documentation also changes. Concentration per millilitre and total volume replace vial mass as the figures that matter, and a certificate should state both rather than a mass alone.
Research context
Published work on both compounds is concentrated in the Russian-language literature and examines neuropeptide signalling, with Semax studied in models of neurotrophic factor expression and Selank in anxiolytic and immune signalling models. The practical limitation for an English-language reader is access to and translation of that record, which is worth acknowledging when assessing what is actually established.
Material is supplied strictly for laboratory research.
Shared research tradition, different molecules
A common origin in one research programme explains why these compounds are discussed together and says nothing about whether they behave alike. Each has its own sequence, its own mass and its own handling profile, and a certificate for one establishes nothing about the other.
The practical implication is narrow: order and document them separately, even when they arrive together, because a shared lot number across two compounds is a documentation error rather than a convenience.
Handling, read from the sequence
Both are short peptides, which changes which failure modes apply. Aggregation needs enough chain to form a hydrophobic face and is largely absent at this length. What replaces it is susceptibility at the chain ends, since a short peptide is almost entirely ends.
Why some peptides resist dissolving covers the solubility side, and how peptides degrade covers which residues carry a liability in a given sequence.
Why short sequences change the handling questions
At this length the failure modes that dominate longer peptides largely disappear. Aggregation needs enough chain to present a hydrophobic face, and a seven-residue peptide does not have one. What replaces it is exposure at the termini, because a short chain is almost entirely termini with no protected interior.
That shifts the practical advice. Guidance written for structured peptides, about avoiding unfolding and surface denaturation, is aimed at a problem these compounds do not have. Why some peptides resist dissolving covers what does apply at this length.
Documentation notes for this pair
Both are well-defined short sequences with straightforward masses, which makes the certificate arithmetic easy and therefore worth doing. Summing the residue masses and adding one water should land on the published figure, and a gap that nothing accounts for is a question rather than a rounding issue.
Where the two are supplied together in any combined format, the content line reports one component rather than the total, which is the ordinary blend reading covered in blends and single vials.
Why these two are often stocked together
Because they are studied in overlapping contexts and are frequently ordered by the same buyers, not because they are chemically similar. Stocking them together is a procurement convenience that should not become a documentation shortcut: separate lots, separate certificates, separate records.
One further note on documentation. Because both compounds are short and well defined, a certificate for either should reconcile cleanly against the published formula, and a gap that nothing explains is unusual enough to be worth raising with the supplier before the material is used rather than afterwards.
For where this sits among the other molecules a catalogue carries, not everything in a peptide catalog is a peptide covers how the classes differ.