Semax vs Selank: Comparing Two Synthetic Peptide Structures

Key takeaways

• Semax is a synthetic heptapeptide derived from a fragment (ACTH 4-10) of adrenocorticotropic hormone, sequence Met-Glu-His-Phe-Pro-Gly-Pro, molecular formula C37H51N9O10S, monoisotopic mass ~813.9 g/mol.
• Selank is a synthetic heptapeptide analog of the endogenous tetrapeptide tuftsin, sequence Thr-Lys-Pro-Arg-Pro-Gly-Pro, molecular formula C33H57N11O9, mass ~751.9 g/mol.
• Both peptides append a C-terminal Pro-Gly-Pro (PGP) tripeptide to their parent fragment — a deliberate stabilizing motif that resists peptidase cleavage.
• Their parent molecules differ entirely: Semax descends from a melanocortin/ACTH lineage, while Selank descends from the immunopeptide tuftsin.
• Both are lyophilized powders requiring reconstitution in bacteriostatic water and cold storage; every Ascend Bio Labs batch ships with a public, batch-ID-linked Certificate of Analysis (HPLC + LC-MS).

Semax and Selank are frequently mentioned in the same breath because they are both short synthetic peptides built on the same structural trick: take a biologically interesting parent fragment and bolt a C-terminal proline-glycine-proline (Pro-Gly-Pro) tail onto it. That shared design pattern can make them look like siblings on a product page. Structurally, however, they descend from two completely unrelated parent molecules.

This article compares the two strictly as molecules — amino-acid sequence, molecular weight, peptide class, and handling characteristics. It makes no claims about what either compound does in any organism; these are research-use-only chemicals, and the only honest comparison is a structural one. If you handle these in a laboratory, the structural and storage details below — plus the batch Certificate of Analysis — are what actually matter.

Two heptapeptides, two different parents

Both Semax and Selank are heptapeptides — seven amino-acid residues each. That is the most superficial similarity, and it is where the parallels begin and end at the sequence level.

Semax originates from the melanocortin family. Its core is the 4-10 fragment of adrenocorticotropic hormone (ACTH 4-10), a naturally occurring sequence within the much larger ACTH polypeptide. Semax takes a shortened, modified version of that fragment and stabilizes it.

Selank originates from a completely different lineage: it is a synthetic analog of tuftsin, an endogenous tetrapeptide (Thr-Lys-Pro-Arg) that is a fragment of the immunoglobulin heavy chain. Selank extends tuftsin's four residues with the same Pro-Gly-Pro stabilizing tail.

So while both are seven residues long and both end in Pro-Gly-Pro, one is an ACTH-fragment derivative and the other is a tuftsin analog. They are not chemically related beyond the shared C-terminal motif.

• Semax parent: ACTH 4-10 fragment (melanocortin/adrenocorticotropic hormone lineage)
• Selank parent: tuftsin (an immunoglobulin-derived tetrapeptide)
• Shared design element: a C-terminal Pro-Gly-Pro (PGP) tripeptide appended for peptidase resistance

Sequence and molecular weight side by side

The clearest way to distinguish the two is by their primary sequence and mass. Semax is written Met-Glu-His-Phe-Pro-Gly-Pro (MEHFPGP). Selank is written Thr-Lys-Pro-Arg-Pro-Gly-Pro (TKPRPGP). Read the last three residues of each and you can see the conserved Pro-Gly-Pro tail; read the first four and you see how unrelated the parent fragments are.

Molecular weight is the second discriminator. Semax (C37H51N9O10S) carries a sulfur atom from its N-terminal methionine and a higher count of aromatic and acidic residues (histidine, phenylalanine, glutamic acid), giving it a monoisotopic mass near 813.9 g/mol. Selank (C33H57N11O9) is sulfur-free and more basic, owing to its lysine and arginine residues, with a mass near 751.9 g/mol. On an LC-MS trace, that mass difference is one of the simplest identity confirmations a lab can read.

For batch identity work, the molecular formula and target mass are exactly the data points an LC-MS report confirms — which is why a Certificate of Analysis that pairs HPLC purity with LC-MS molecular confirmation is the meaningful artifact for either peptide.

Why the Pro-Gly-Pro tail matters structurally

The most interesting shared feature is the C-terminal Pro-Gly-Pro motif. Proline is a conformationally rigid amino acid — its side chain loops back to the backbone nitrogen, forming a ring that constrains the peptide bond. Placing prolines around the terminus is a well-known structural strategy for slowing enzymatic degradation, because many peptidases struggle to cleave bonds adjacent to proline.

Both Semax and Selank were designed by appending this PGP tail to a parent fragment that, on its own, would be far more labile. The motif is purely a structural and stability consideration here: we are describing the chemistry of the molecule, not any physiological effect. The takeaway for a comparison is that two structurally unrelated parents converged on the same terminal stabilizer.

If you work with other short synthetic peptides, this proline-stabilization pattern recurs across the field — it is one of the recurring motifs you will see when reading sequences alongside guides like What Is Epithalon? Tetrapeptide Sequence and Molecular Weight.

Reconstitution and storage (research handling)

Both Semax and Selank ship as lyophilized (freeze-dried) white-to-off-white powders. As with most research peptides, the powder is reconstituted in the laboratory using bacteriostatic water (or another appropriate solvent per the protocol), swirled rather than shaken vigorously, and kept cold once in solution. Neither peptide should be exposed to repeated freeze-thaw cycles, which can degrade the molecule and show up as reduced purity on a follow-up HPLC run.

The lyophilized powder is the most stable form and is generally stored frozen and protected from light and moisture; reconstituted solution is far less stable and is typically refrigerated and used within a short window. Because both are small peptides, oxidation (especially of Semax's methionine residue) and microbial contamination are the two handling risks worth controlling for. General cold-chain practice for either compound is covered in Peptide Storage and Cold-Chain Handling: A Research Reference.

• Lyophilized powder: store frozen, away from light and humidity — most stable form
• Reconstitute with bacteriostatic water per protocol; swirl gently, do not shake hard
• Refrigerate reconstituted solution; avoid repeated freeze-thaw cycles
• Semax's methionine residue is oxidation-sensitive — minimize air and light exposure

How Ascend Bio Labs documents identity and purity

For two peptides that differ by parent molecule, formula, and mass, the documentation that confirms which molecule is actually in the vial is what separates a verifiable supplier from a label. Ascend Bio Labs tests every batch through independent third-party laboratories using HPLC for purity and LC-MS for molecular identity, and publishes a per-batch Certificate of Analysis. Each vial carries a unique batch ID that links directly to its own COA, so the sequence and target mass on the report can be checked against the molecule you expect.

All synthesis, testing, storage, and shipping are US-domestic, with no overseas transshipment, and orders ship insulated and tracked. The point of a structural comparison like this one is to know exactly what each molecule is; the point of a public COA is to confirm that the vial in front of you matches.

Several other US-domestic suppliers also publish testing data, and the table below states only what each vendor publicly lists. For anything a vendor does not publicly state, verify directly with that vendor rather than assuming.

Related research notes

• What Is Epithalon? Tetrapeptide Sequence and Molecular Weight
• What Is MOTS-c? Mitochondrial-Derived Peptide Structure
• Peptide Storage and Cold-Chain Handling: A Research Reference
• GH Secretagogue Research Peptides: A Structural Overview