Glow Peptides · Research Journal

Semax vs Selank: Two Sequences, One Shared Tail

Glow Peptides Research Team · 2026-09-24

For research use only. Not for human or veterinary use, diagnostic use, or in any therapeutic application.

Semax and Selank are both seven-residue research peptides that share the same last three residues, Pro-Gly-Pro, but come from completely unrelated parent molecules. Most comparisons of the pair stop at the level of their names, and the two sequences side by side say something more specific: each was built by attaching Pro-Gly-Pro to a short fragment of something else. This page sets out what is actually in a vial of research-grade Semax and a vial of research-grade Selank: both sequences, both formulas, both masses, the tail they share, and how differently the two behave analytically despite that shared ending.

The two sequences, and the thing they have in common

Semax is Met-Glu-His-Phe-Pro-Gly-Pro, written MEHFPGP. PubChem holds it as CID 9811102 with molecular formula C₃₇H₅₁N₉O₁₀S and a molecular weight of about 813.9.

Selank is Thr-Lys-Pro-Arg-Pro-Gly-Pro, written TKPRPGP. PubChem holds it as CID 11765600 with molecular formula C₃₃H₅₇N₁₁O₉ and a molecular weight of about 751.9.

Line the two up and the last three residues are identical:

The first four residues have nothing in common. The last three are the same tripeptide, Pro-Gly-Pro, in both compounds.

Where the first four residues of each come from

Semax. PubChem's own canonical title for the compound is "ACTH (4-7), Pro-Gly-Pro-", which is the whole construction written out. ACTH, adrenocorticotropic hormone, is a 39-residue peptide encoded as part of the larger POMC precursor, held under UniProt accession P01189, where corticotropin runs from precursor residue 138 to 176 and begins Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly. Counting into that chain, ACTH residues 4 to 7 are Met-Glu-His-Phe, which is exactly the first four residues of Semax.

Selank. Its first four residues, Thr-Lys-Pro-Arg, are tuftsin, a four-residue peptide held as PubChem CID 156080 with a molecular weight of about 500.6. Tuftsin is a fragment of an immunoglobulin heavy chain, which has no relationship at all to ACTH.

So the two compounds were built the same way out of entirely different starting material: take a short natural fragment, add Pro-Gly-Pro to the C-terminal end. Two unrelated parents, one shared tail.

What the shared tail does, analytically

Pro-Gly-Pro is not a neutral ending. Two prolines in three residues gives both compounds a particular chromatographic character, because proline is the one amino acid whose backbone nitrogen sits inside a ring. The peptide bond in front of a proline can adopt either a cis or a trans arrangement and can interconvert slowly, which on a chromatogram shows up as broader peaks and sometimes as split or shouldered peaks from a single pure species.

Selank shows this more than Semax, because Selank has a third proline at position 3. Three prolines out of seven residues is forty-three percent, which is high even for a short peptide. Semax has two, both inside the shared tail.

Where the two diverge

The shared tail makes them look similar. The first four residues make them behave quite differently.

Sulfur, and therefore oxidation. Semax has a methionine at position 1, which is the only sulfur in the molecule and the reason its formula carries an S. Methionine oxidizes readily to the sulfoxide, adding sixteen mass units, so a Semax lot can carry an oxidized species at about 829.9. That form is more polar than the parent and elutes earlier on a reversed-phase column. Selank contains no cysteine and no methionine, so its formula has no sulfur at all and neither oxidation to a sulfoxide nor disulfide formation is possible.

Net charge, in opposite directions. Selank carries a lysine at position 2 and an arginine at position 4, both strongly basic, giving it a substantial positive charge at neutral pH. Semax carries a glutamate at position 2, which is acidic, and a histidine at position 3, whose side chain is only weakly basic and is largely uncharged at neutral pH. So Selank runs positive and Semax runs closer to neutral or slightly negative. Retention on a reversed-phase column and the charge states seen in mass spectrometry differ accordingly, and a method developed for one is not automatically right for the other.

Neither has a 280 nanometer handle, and this is where Semax misleads. Semax contains a phenylalanine at position 4 and a histidine at position 3, so it is tempting to assume it has aromatic absorbance available. It does not, at least not usefully. Absorbance at 280 nanometers comes almost entirely from tryptophan and tyrosine. Phenylalanine's ring absorbs at shorter wavelengths and far more weakly, and histidine contributes essentially nothing at 280. Semax has no tryptophan and no tyrosine, and neither does Selank. Both therefore have to be detected on the peptide backbone near 214 to 220 nanometers.

They are only 62 mass units apart. 813.9 against 751.9 is a much smaller gap than separates most pairs of compounds, so if the two were ever handled in the same workflow, mass alone gives less margin than it would elsewhere. What a purity figure does and does not capture is unpacked in what a purity percentage means and what it misses, and the techniques themselves in how HPLC and mass spectrometry verify peptide purity.

Specification summary

Reading a certificate for either compound

The general discipline is unchanged: a lot number that matches the vial in front of you, the issuing laboratory named rather than described, the analytical method stated alongside the purity figure, a report date belonging to that batch, and an accession number that lets the report be looked up at the laboratory rather than confirmed by the seller. The field-by-field walkthrough is in how to read a peptide certificate of analysis.

Three fields are worth extra attention across these two.

On Semax, any mass sixteen units high. A figure near 829.9 rather than 813.9 is the methionine sulfoxide, not the target compound and not a rounding artifact. Because the methionine sits at position 1, exposed at the end of the chain, this is a real handling and storage variable as well as a synthesis one.

On both, the detection wavelength. It should read in the 214 to 220 nanometer range. A certificate reporting 280 nanometer detection is describing a measurement that cannot work as stated for either of these peptides, because neither contains tryptophan or tyrosine.

On both, peak shape. Two prolines in Semax and three in Selank mean broad or slightly shouldered peaks can come from a single pure species rather than from an impurity. A chromatogram that looks less sharp than one from a proline-free peptide of similar length is not on its own evidence of a problem.

Published lot records for this catalog are listed on the quality page, each with its own certificate. Lot document availability varies by compound and by batch, so check the specific lot rather than assuming coverage.

What this page deliberately does not cover

Most pages returned for this comparison move quickly to what each compound may do, which of the two may suit a given purpose, how much to take and how to take it. Those subjects are absent here on purpose, and saying so is more useful than leaving the gap unexplained.

Glow Peptides supplies cataloged materials for laboratory research only. Nothing here is a drug, a supplement, a cosmetic or a medical device, and nothing here is intended for human or veterinary use. Dosing, administration, reconstitution, comparison of effects and protocol questions are outside what a supplier of research materials should answer. What is in scope, and what this page covers, is composition, structure, strength, purity, analytical method, handling and documentation.

Frequently asked questions

What do Semax and Selank have in common structurally?

Their last three residues. Semax is Met-Glu-His-Phe-Pro-Gly-Pro and Selank is Thr-Lys-Pro-Arg-Pro-Gly-Pro, so both are seven residues long and both end in the same tripeptide, Pro-Gly-Pro. Their first four residues have nothing in common: Semax opens with ACTH residues 4 to 7 and Selank opens with tuftsin. Both compounds were built the same way, by attaching Pro-Gly-Pro to a short natural fragment, but from entirely unrelated starting material.

What are the sequences and molecular weights of Semax and Selank?

Semax is Met-Glu-His-Phe-Pro-Gly-Pro, written MEHFPGP, with molecular formula C37H51N9O10S and a molecular weight of about 813.9, held as PubChem CID 9811102. Selank is Thr-Lys-Pro-Arg-Pro-Gly-Pro, written TKPRPGP, with molecular formula C33H57N11O9 and a molecular weight of about 751.9, held as PubChem CID 11765600. The two differ by about 62 mass units.

Is Semax a fragment of ACTH?

Only partly, and the distinction matters. Semax's first four residues, Met-Glu-His-Phe, are an exact excerpt of ACTH: they are residues 4 to 7 of the 39-residue hormone, verified against UniProt P01189. PubChem's own title for the compound is "ACTH (4-7), Pro-Gly-Pro-". But the last three residues, Pro-Gly-Pro, are added rather than excerpted, so the whole molecule is an analog of ACTH(4-10) rather than a fragment of ACTH. ACTH(4-10) itself is Met-Glu-His-Phe-Arg-Trp-Gly, and Semax replaces its final three residues.

Does Selank contain tuftsin?

Yes. Selank's first four residues, Thr-Lys-Pro-Arg, are tuftsin, a four-residue peptide held as PubChem CID 156080 with a molecular weight of about 500.6. Tuftsin is a fragment of an immunoglobulin heavy chain. Selank is tuftsin with Pro-Gly-Pro attached to the C-terminal end, which is the same construction pattern used for Semax but applied to a completely different parent molecule.

Can Semax be detected at 280 nanometers because it contains phenylalanine?

No. Absorbance at 280 nanometers comes almost entirely from tryptophan and tyrosine. Phenylalanine's aromatic ring absorbs at shorter wavelengths and far more weakly, and histidine contributes essentially nothing at 280. Semax has a phenylalanine and a histidine but no tryptophan and no tyrosine, so it has no useful 280 nanometer absorbance. Selank has no aromatic residue at all. Both peptides therefore have to be detected on the peptide backbone near 214 to 220 nanometers.

Which of the two can oxidize?

Semax. It carries a methionine at position 1, the only sulfur in the molecule and the reason its formula includes an S. Methionine oxidizes readily to the sulfoxide, adding sixteen mass units, so a lot can carry an oxidized species at about 829.9, which is more polar than the parent and elutes earlier on a reversed-phase column. Selank contains no cysteine and no methionine and no sulfur at all, so neither sulfoxide formation nor disulfide formation is possible for it.

Why does this page not compare what Semax and Selank do?

Because Glow Peptides supplies cataloged materials for laboratory research only. The products are not drugs, supplements, cosmetics or medical devices, and they are not intended for human or veterinary use. Comparing effects, and giving dosing, administration, reconstitution or protocol guidance, are outside what a research materials supplier should provide, so they are deliberately absent rather than accidentally missing. What is covered here is composition, structure, strength, purity, analytical method, handling and documentation.

Sources

• PubChem, National Library of Medicine. Semax, CID 9811102, including the canonical title "ACTH (4-7), Pro-Gly-Pro-". Read 22 September 2026.
• PubChem, National Library of Medicine. Selank, CID 11765600. Read 22 September 2026.
• PubChem, National Library of Medicine. Tuftsin, CID 156080. Read 22 September 2026.
• UniProt. Pro-opiomelanocortin, accession P01189. Corticotropin runs from precursor residue 138 to 176, a 39-residue chain beginning Ser-Tyr-Ser-Met-Glu-His-Phe-Arg-Trp-Gly, from which ACTH residues 4 to 7 are Met-Glu-His-Phe and ACTH residues 4 to 10 are Met-Glu-His-Phe-Arg-Trp-Gly. Read 22 September 2026.
• Both molecular weights independently recomputed from their sequences using standard average residue masses, with water added for the chain termini. Both recomputed figures agree with the database values.

For research use only. Not for human or veterinary use, diagnostic use, or any therapeutic application.

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