Semax and Selank: A Comparative Research Reference

Semax and Selank: A Comparative Research Reference

Semax and Selank are frequently catalogued together and are often supplied as a paired set, which can create the impression that they are structurally related. They are not. The two are short synthetic peptides derived from entirely different parent molecules, and beyond a similar chain length they have little in common chemically. This reference sets out what distinguishes them.

The scope here is chemical identity and analytical verification. No use of any kind is described.

Semax: an ACTH fragment analogue

Semax derives from adrenocorticotropic hormone, specifically the fragment spanning residues four to seven of that parent sequence, conventionally written ACTH(4-7). To this fragment a Pro-Gly-Pro tripeptide extension is added, producing the seven-residue sequence Met-Glu-His-Phe-Pro-Gly-Pro.

The molecular weight is approximately 813.9 g/mol. The Pro-Gly-Pro extension is the defining synthetic modification: proline residues impose significant conformational constraint on a peptide backbone because the side chain is covalently bonded back to the backbone nitrogen, restricting rotation. A sequence terminating in Pro-Gly-Pro therefore has quite different conformational properties from the bare ACTH(4-7) fragment.

The sequence contains a methionine at the N-terminus and a histidine — the former oxidation-susceptible, the latter contributing to the molecule’s ionisation behaviour across pH ranges.

Selank: a tuftsin analogue

Selank derives from an entirely different parent. Its basis is tuftsin, a naturally occurring tetrapeptide with the sequence Thr-Lys-Pro-Arg. As with Semax, a Pro-Gly-Pro extension is appended, giving the seven-residue sequence Thr-Lys-Pro-Arg-Pro-Gly-Pro and a molecular weight of approximately 751.87 g/mol.

The shared Pro-Gly-Pro motif is the only meaningful structural commonality between the two compounds, and it reflects a common design approach rather than any relationship between the parent molecules. ACTH and tuftsin are unrelated in origin, sequence and structure.

Selank carries no methionine and no tryptophan, and is therefore considerably less oxidation-susceptible than Semax. It does carry lysine and arginine, giving it a distinctly basic character.

Analytical contrasts

The compositional differences translate directly into different analytical expectations for the two compounds.

• Semax contains phenylalanine and histidine, giving useful ultraviolet absorbance behaviour; Selank contains no aromatic residues at all, which makes detection at 280 nm impractical and places greater reliance on 214 nm peptide-bond absorbance.
• Semax carries an oxidation-susceptible N-terminal methionine; Selank does not, so their expected impurity profiles differ in character.
• Selank’s lysine and arginine content makes it more strongly basic than Semax, affecting reversed-phase retention and ionisation efficiency.
• The two differ by approximately 62 g/mol, readily distinguished by mass spectrometry.

The absence of aromatic residues in Selank is worth dwelling on, because it is a genuine practical constraint. Many routine peptide analysis methods assume a chromophore is present. For Selank, purity determination depends on detection at the lower wavelength where the peptide bond absorbs, a region in which many solvents and buffers also absorb strongly. Method conditions must be selected accordingly.

Presentation and verification

Both compounds are supplied as lyophilised powders in sealed glass research vials, typically specified at greater than 98% purity by HPLC with identity confirmed by LC-MS. At roughly 750 to 815 g/mol both sit comfortably within the range where mass spectrometry gives clean, easily interpreted results.

The Pro-Gly-Pro motif

Since the same three-residue extension appears in both compounds, it is worth understanding what it does structurally. Proline is unique among the twenty standard amino acids in that its side chain loops back and bonds to its own backbone nitrogen, forming a five-membered ring. That ring removes the nitrogen-hydrogen normally available for hydrogen bonding and severely restricts rotation around the adjacent backbone bond.

The practical effect is that proline residues impose conformational rigidity. A peptide containing proline has far fewer accessible conformations than one of equivalent length without it, and sequences rich in proline tend to adopt extended rather than folded arrangements.

Glycine is the opposite case. With only a hydrogen atom as its side chain it is the smallest and most conformationally permissive residue, able to adopt backbone angles unavailable to any other amino acid. A Pro-Gly-Pro sequence therefore alternates severe constraint with unusual freedom — a deliberate structural motif rather than an arbitrary addition.

For analytical work, proline-containing peptides can exhibit cis-trans isomerism about the bond preceding the proline. Where interconversion between the two forms is slow relative to the chromatographic timescale, this can produce peak broadening or even apparent peak splitting that is not an impurity at all. Recognising this artefact for what it is matters when interpreting a chromatogram of either compound.

Storage-relevant chemistry

The compositional difference between the two translates into different storage sensitivities. Semax carries an N-terminal methionine, which oxidises to the sulfoxide on exposure to oxygen, adding 16 g/mol and producing a detectable additional species. Selank has no such residue and is correspondingly less sensitive to oxygen.

Both are supplied lyophilised in sealed vials, where the absence of water suppresses hydrolysis and low molecular mobility slows other degradation routes. Moisture ingress through a compromised seal is the shared concern; oxidation is a Semax-specific one.

Corix Labs supplies Semax 10mg and Selank 10mg as separate catalogue items, and as a paired set of two separate vials. Being supplied together does not make them a blend: each remains an individually characterised single-compound preparation with its own batch documentation.

Corix Labs publishes analytical documentation for its research materials. See third-party testing reports for further detail on how batches are verified.

All materials referenced in this article are supplied strictly for laboratory research and analytical use. They are not licensed medicines and are not sold for use in humans or animals. Nothing in this article constitutes guidance on administration, preparation for use, or any clinical, therapeutic, diagnostic or cosmetic application.