Semax vs Selank is a comparison of two synthetic heptapeptides built on the same design. Both are seven amino acids long and both end in the same stabilizing Pro-Gly-Pro tail. They differ in the four residues at the front: Semax carries a fragment of adrenocorticotropic hormone (ACTH), and Selank carries tuftsin, a fragment of an immune protein.

That single difference in parent sequence explains almost everything else. It gives the two molecules different formulas, different charge, and different chemistry, and it has pointed the published research on each in a different direction.

This article compares the two molecules side by side: their structure, their shared C-terminal tail, and what laboratory studies have measured about each. At Janera Science, both compounds are supplied with lot-specific documentation, so the comparison closes with how a researcher can tell them apart on paper.

Semax vs Selank at a Glance

Semax and Selank share a length, a C-terminal tail, and a research programme, and differ in their parent sequence and molecular identity. The table below sets out the core data for each, drawn from PubChem's compound records (PubChem, 2026; PubChem, 2026).

Read across the rows and the pattern is clear. The last three columns of each sequence are identical, and every difference in the table traces back to the first four residues.

Where Semax and Selank Come From

Both peptides were developed at the Institute of Molecular Genetics of the Russian Academy of Sciences in Moscow. Selank was developed there as a synthetic analogue of tuftsin, and Semax came from the same institute (Wikipedia, 2026).

The shared origin is visible in the literature, where many of the same research groups and authors appear on studies of both molecules. The parent molecules they were built from, however, come from entirely different corners of biology.

Semax: built on a hormone fragment

Adrenocorticotropic hormone is a 39-amino-acid signalling peptide. Semax keeps residues 4 to 7 of it, the sequence Met-Glu-His-Phe, and drops the residues that follow. Our full Semax chemistry article walks through that parent fragment in detail.

Selank: built on an immune fragment

Tuftsin is a natural four-residue peptide, Thr-Lys-Pro-Arg, located in the Fc domain of the heavy chain of immunoglobulin G (Wikipedia, 2026). Selank keeps all four tuftsin residues intact. The Selank chemistry article covers tuftsin's structure and history.

The Shared Design: The Pro-Gly-Pro Extension

Semax and Selank both end in the same proline-glycine-proline tripeptide, a sequence described in the literature as an active factor of resistance to the biodegradation of peptide drugs (Genes, 2022). Short natural fragments are generally cleared quickly by peptidases, the enzymes that cleave peptide bonds, and the tail is there to slow that process.

Proline is the key residue. Its side chain loops back and bonds to its own backbone nitrogen, forming a rigid ring that many peptidases cannot fit around easily. Two prolines near the end of the chain create steric hindrance, a physical crowding that slows the enzymes that would otherwise trim the peptide from that end.

Semax has been studied for stability directly. In the presence of plasma membranes from the basal nuclei of the rat forebrain, Semax showed a half-life longer than one hour. The authors identified dipeptidylaminopeptidases as the main enzymes involved, cleaving Semax successively to the pentapeptide HFPGP and then the tripeptide PGP (Bioorganicheskaia Khimiia, 2004). In both products, the Pro-Gly-Pro end remains intact.

Both molecules pair a short natural fragment with the same tail. What differs is the fragment.

How the Two Sequences Differ Chemically

The first four residues give Semax and Selank opposite character: Semax carries an acidic residue and a sulphur atom, while Selank carries two strongly basic residues. Those differences change how each molecule behaves in solution and what it can bind.

• Charge. Selank's lysine and arginine are both positively charged at neutral pH, so the molecule carries a net charge of about +2. Semax has one glutamic acid, which is negatively charged, and no lysine or arginine, so its net charge is slightly negative, close to -1.

• Sulphur and metal binding. Semax's N-terminal methionine contributes the single sulphur atom in its formula. Semax forms complexes with copper(II) and zinc(II) ions, and acetylating its free N-terminal amino group changes the geometry of the copper complex (Journal of Inorganic Biochemistry, 2016). Selank has no methionine.

• Proline content. Selank contains three prolines, one inside the tuftsin core and two in the tail. Semax contains two, both in the tail.

• Mass. Semax has four more carbons, one more oxygen and one sulphur, while Selank has six more hydrogens and two more nitrogens. Net, Semax is heavier by 62.0 g/mol.

These are the properties a chemist reads straight from the sequence. If the three-letter and one-letter residue codes are new to you, our guide to peptide nomenclature explains how they work.

What Published Research Has Measured: Semax vs Selank

Few laboratory studies test Semax and Selank in the same experiment; most examine one peptide at a time, in different models. The sections below separate the direct comparison from the single-peptide work, so that results from different models are never read as a head-to-head result. All of it is laboratory research in serum, cell, or animal models.

The direct comparison: enkephalin-degrading enzymes

The clearest side-by-side measurement comes from a 2001 study of enzymes in human serum that break down enkephalins, short regulatory peptides. Both heptapeptides inhibited these enzymes in a concentration-dependent way. Semax reached half-maximal inhibition (IC50) at 10 micromolar and Selank at 20 micromolar, both stronger than the reference inhibitor puromycin at 10 millimolar (Bioorganicheskaia Khimiia, 2001).

The same study tested fragments of each molecule. The pentapeptide fragments also inhibited the enzymes, while the tri-, tetra- and hexapeptide fragments did not. These enzymes also degrade other regulatory peptides, so the authors proposed this inhibition as one mechanism behind the activity of both compounds.

A shared line of research: BDNF in the rat hippocampus

Both peptides have been studied for their relationship to brain-derived neurotrophic factor (BDNF), a protein that supports nerve cell function, in the same brain region of the same species.

• Semax: a single administration raised BDNF protein in rat hippocampus by up to 1.4-fold, with a 1.6-fold rise in tyrosine phosphorylation of its receptor TrkB, and higher BDNF and trkB messenger RNA (Brain Research, 2006).

• Selank: intranasal administration was reported to regulate BDNF expression in rat hippocampus in vivo (Doklady Biological Sciences, 2008).

The two studies come from overlapping research groups but used separate protocols. They show that both molecules have been linked to the same neurotrophic system. They do not rank one above the other.

Research specific to Semax

Beyond the shared work above, the Semax literature includes neurochemistry and metal coordination studies.

• Monoamine systems. In rodents, Semax increased the serotonin metabolite 5-HIAA in the striatum by 25 percent in tissue two hours after administration, while leaving dopamine levels unchanged on its own (Neurochemical Research, 2005).

• Gene expression in an ischemia model. A genome-wide study of rat brain cortex after focal ischemia found Semax mainly altered the expression of immune-system genes, with further changes in genes linked to vascular development (BMC Genomics, 2014).

• Copper and zinc coordination. Semax forms a copper(II) complex with a four-nitrogen geometry. Acetylating its N-terminus changed that geometry, and the acetylated form did not protect a neuroblastoma cell line against copper toxicity, which the authors attribute to the loss of the free amino group (Journal of Inorganic Biochemistry, 2016).

Research specific to Selank

Beyond the shared work above, the Selank literature includes gene expression studies in immune and neurotransmitter systems.

• Inflammation-related genes. In mouse spleen, a study of Selank and two of its fragments found significant changes in 34 of the 84 inflammation-related genes measured. The immune-development gene Bcl6 changed in response to each of the three peptides (Regulatory Peptides, 2011).

• GABA-related genes. In rat frontal cortex, a study that gave rats either Selank or GABA reported expression changes in 45 of 84 neurotransmission-related genes one hour after administration and in 22 genes after three hours, counting both compounds together. At one hour, the changes caused by Selank correlated with those caused by GABA, and the authors proposed allosteric modulation of the GABA system as one possible mechanism (Frontiers in Pharmacology, 2016).

How the Two Literatures Differ

The published work on each peptide has clustered around different themes, and those themes parallel the parent molecules: a hormone fragment for Semax and an immune fragment for Selank. Both literatures share enkephalin-degrading enzyme and BDNF work.

The Semax studies covered here add neurotransmitter systems, an ischemia-model transcriptome, and copper and zinc coordination chemistry. The Selank studies add inflammation-related gene activity in immune tissue and GABA-related gene expression in the brain.

The shared Pro-Gly-Pro tail is common to both molecules. The front four residues are what make them different subjects.

Telling Semax and Selank Apart on a Certificate of Analysis

Because the two peptides share a length and a tail, the lot-specific documentation is what confirms which compound is in a vial and how pure it is. A researcher comparing the two should check that each certificate names the correct sequence and matches the lot number printed on the vial.

High-performance liquid chromatography (HPLC) separates a sample into its components and reports purity as the share of the main peak. Janera Science's lot-specific certificates report HPLC purity and assay, plus heavy metal screening, endotoxin and sterility results for each batch. You can compare the posted certificates for Semax and Selank on our lab results page, and our guide to reading a COA explains each field.

Semax and Selank are both available from Janera Science, alongside our wider range of nootropic peptides for research. Every lot is independently tested before release, and our How Janera Tests page sets out what each test covers.

Frequently Asked Questions

What is the difference between Semax and Selank?

Semax and Selank are both synthetic heptapeptides ending in Pro-Gly-Pro, but they are built on different parent sequences. Semax carries the ACTH(4-7) fragment Met-Glu-His-Phe, and Selank carries the tuftsin sequence Thr-Lys-Pro-Arg. That gives them different formulas, charge and chemistry.

Are Semax and Selank related?

Yes. Both were developed at the Institute of Molecular Genetics of the Russian Academy of Sciences (Wikipedia, 2026). Both also end in the same Pro-Gly-Pro tail, a sequence associated in the literature with resistance to biodegradation (Genes, 2022).

Which is heavier, Semax or Selank?

Semax is heavier. Its molecular weight is 813.9 g/mol against 751.9 g/mol for Selank, a difference of 62.0 g/mol (PubChem, 2026; PubChem, 2026). Semax also contains one sulphur atom, from its methionine residue, while Selank contains none.

Have Semax and Selank been compared in the same study?

Yes. The clearest example is a 2001 study of enkephalin-degrading enzymes in human serum. Semax inhibited the enzymes with an IC50 of 10 micromolar and Selank with an IC50 of 20 micromolar (Bioorganicheskaia Khimiia, 2001). Most other published work studies the two peptides separately, in different models.

What does Pro-Gly-Pro do in Semax and Selank?

Pro-Gly-Pro is a three-residue tail at the C-terminus of both peptides, described in the literature as an active factor of resistance to biodegradation (Genes, 2022). In one laboratory study, Semax had a half-life longer than one hour in the presence of rat brain membranes (Bioorganicheskaia Khimiia, 2004).

How can a researcher confirm which peptide is in a vial?

Check that the lot-specific certificate of analysis names the correct sequence and that its lot number matches the vial. Janera Science posts lot-specific certificates for both compounds, reporting HPLC purity and assay, heavy metals, endotoxin and sterility.

Key Takeaways

• Semax and Selank are both synthetic heptapeptides ending in Pro-Gly-Pro, built on different parent sequences: ACTH(4-7) for Semax and tuftsin for Selank.

• Their molecular identities differ: C37H51N9O10S at 813.9 g/mol for Semax and C33H57N11O9 at 751.9 g/mol for Selank (PubChem, 2026; PubChem, 2026).

• Selank is basic and Semax is slightly acidic: Selank carries lysine and arginine, while Semax carries glutamic acid and the methionine that gives it its sulphur atom.

• The clearest direct comparison measured Semax at an IC50 of 10 micromolar and Selank at 20 micromolar against enkephalin-degrading enzymes in human serum (Bioorganicheskaia Khimiia, 2001).

• The two literatures overlap and then diverge: both include enzyme-inhibition and BDNF work, with monoamine and metal-binding studies for Semax and immune and GABA-related gene expression for Selank.

All Janera Science compounds are supplied strictly for in-vitro laboratory research use only. They are not for human or veterinary use and have not been evaluated by the FDA.