Semax and Selank look like sibling molecules. Both are seven-residue peptides, both end in the same Pro-Gly-Pro tail, and both came out of the same Moscow laboratory. The resemblance is real but shallow. They descend from completely different parent peptides, they are studied for opposite reasons (one for focus, one for calm), and they do not even store the same way, because only one of them carries an oxidation-prone methionine. This comparison covers what each molecule is, where the kinship is genuine and where it is convergent engineering, and the handling difference that the most common storage question gets wrong.
Two peptides from one research program
Both Semax and Selank were developed at the Institute of Molecular Genetics of the Russian Academy of Sciences, beginning in the 1980s, as part of a program to turn short, rapidly-degraded regulatory peptides into stable, usable research compounds. Both are heptapeptides. Both are administered intranasally in the published literature. Both share an identical C-terminal Pro-Gly-Pro motif that is the program's structural signature. From there, they diverge.
What Semax is
- Sequence:
Met-Glu-His-Phe-Pro-Gly-Pro(MEHFPGP) - Molecular formula: C₃₇H₅₁N₉O₁₀S
- Average mass: 813.93 g/mol
- Parent: An analog of the ACTH(4-10) fragment of adrenocorticotropic hormone. The bioactive core is the ACTH(4-7) tetrapeptide Met-Glu-His-Phe; Semax appends Pro-Gly-Pro to it for stability.
The name is literal: "Semax" is shorthand for the seven amino acids. Its parent, ACTH(4-10), is a neuropeptide fragment, and Semax is studied in cognitive, attention, and neuroprotection paradigms — the stroke and optic-nerve literature is where much of the Russian clinical work sits. In animal models it rapidly raises the expression of brain-derived neurotrophic factor (BDNF) and its receptor TrkB in the hippocampus, which is the most-cited piece of its mechanism. The fuller mechanism is not fully established, so BDNF elevation is best described as a documented effect rather than a complete explanation.
What Selank is
- Sequence:
Thr-Lys-Pro-Arg-Pro-Gly-Pro(TKPRPGP) - Molecular formula: C₃₃H₅₇N₁₁O₉
- Average mass: 751.89 g/mol
- CAS: 129954-34-3
- Parent: An analog of tuftsin, the immunomodulatory tetrapeptide Thr-Lys-Pro-Arg derived from the heavy chain of immunoglobulin G. Selank appends the same Pro-Gly-Pro tail.
Selank's parent comes from the immune system, not the brain, and its research profile is anxiolytic and immunomodulatory rather than stimulating. Its best-characterized mechanism is inhibition of the enzymes that degrade enkephalins: by slowing enkephalinase, Selank prolongs endogenous enkephalin signaling without binding opioid receptors directly. It also influences serotonin metabolism and, in at least one ethanol-withdrawal model, normalized rather than simply raised hippocampal BDNF. Gene-expression work has linked it to GABAergic neurotransmission as well. Russia has registered it as an anxiolytic.
The shared Pro-Gly-Pro tail
The Pro-Gly-Pro motif is why either molecule is usable at all. The native parent peptides are degraded within minutes in plasma — ACTH(4-10) is a known substrate for peptidases, and free tuftsin is similarly short-lived. Capping the C-terminus with Pro-Gly-Pro, a "glyproline" unit rich in proteolysis-resistant proline, slows that degradation by roughly an order of magnitude. For Semax, one study measured a half-life above an hour against rat forebrain membrane peptidases, with Pro-Gly-Pro surviving as the dominant fragment.
A useful subtlety: the plasma half-life and the duration of effect are decoupled. Both peptides clear from blood quickly even with the Pro-Gly-Pro tail, yet their BDNF-mediated central effects persist for hours after the compound is gone. The tail buys enough survival time to reach the target; the downstream neurotrophic signaling outlasts the molecule itself.
| Property | Semax | Selank |
|---|---|---|
| Sequence | MEHFPGP | TKPRPGP |
| Parent peptide | ACTH(4-10) / ACTH(4-7) | Tuftsin (IgG fragment) |
| Parent biology | Neuroendocrine | Immune |
| Average mass | 813.93 g/mol | 751.89 g/mol |
| Best-characterized mechanism | BDNF / TrkB upregulation | Enkephalinase inhibition |
| Research lean | Cognition, focus, neuroprotection | Anxiolytic, immunomodulatory |
| Oxidation-prone residue | Methionine (position 1) | None |
Do Semax and Selank store the same way?
No. They look like twins, but only Semax carries methionine, and that one residue changes the storage rules. The methionine at position 1 oxidizes to methionine sulfoxide on exposure to air and light, and the oxidized form has reduced activity. Selank has no methionine, no cysteine, and no aromatic photolysis liability, so it is the more forgiving of the two on the bench.
Stability and handling differences
Neither peptide contains cysteine, so disulfide scrambling is not a concern for either. Both follow the standard lyophilized-storage envelope: most stable sealed and frozen at −20°C, refrigerated for shorter holds, reconstituted with bacteriostatic water and used within a few weeks refrigerated. The mechanical rules in our reconstitution guide apply to both — diluent against the glass wall, swirl rather than shake.
The practical asymmetry is oxidation. Semax warrants the extra air-and-light discipline that the broader storage and stability framework recommends for methionine-bearing peptides: minimal headspace exposure, foil-wrapping or amber vials, prompt refrigeration. Selank tolerates ordinary handling better. Treating the two identically is the most common handling mistake, and it always costs Semax, never Selank.
Common errors to avoid:
- Storing Semax like Selank. Semax's methionine oxidizes invisibly; it needs the air and light protection that oxidation-robust Selank does not require.
- Reading "they're both PGP heptapeptides" as "they're interchangeable." Different parents, different mechanisms, different research questions.
- Shaking either vial to speed dissolution. Both aggregate at the air-water interface. Swirl only.
- Assuming plasma half-life equals duration of effect. Both clear quickly; the BDNF-mediated effects outlast the molecule.
- Confusing the modified variants with the base peptides. N-acetyl Semax amidate and N-acetyl Selank are distinct compounds with their own properties.
Modified variants worth knowing
Both peptides have stabilized analogs in circulation that are easy to confuse with the parent. N-acetyl Semax amidate adds an N-terminal acetyl group and a C-terminal amide on top of the Pro-Gly-Pro tail, engineered for longer activity; the N-terminal acetylation also changes how the peptide coordinates metals, which matters because native Semax is a high-affinity copper(II) ligand. N-acetyl Selank exists in the same vein. When reading a certificate of analysis, the sequence and observed mass, not the name on the label, are what confirm which molecule is in the vial. Our CoA guide covers how to make that check, and every lot we release is in the open CoA library.
Where to find primary literature
PubMed indexes both under their names; the mechanistic overlap is captured in work like the 2001 report that both Semax and Selank inhibit the enkephalin-degrading enzymes of human serum, which ties the two compounds to a shared pathway despite their different parents. Search "Semax BDNF" for the neurotrophic work and "Selank anxiolytic enkephalin" for the anxiety and immunomodulation literature. HelixCore stocks both as 10 mg lyophilized vials: Semax and Selank, each pairing with bacteriostatic water for reconstitution.
