Selank and Semax are two synthetic research peptides that are often catalogued side by side and, because both are short synthetic chains that close with the same Pro-Gly-Pro motif, are sometimes confused for one another. They are distinct reference materials with different registry numbers, different parent sequences, and different molecular formulas. This page compares the two strictly on verified compound identity, so a laboratory can confirm which material it holds without relying on the name alone.
How the two compounds differ at a glance
At the level of identity the two separate cleanly. Selank is catalogued as a synthetic analogue of the naturally occurring peptide tuftsin, while Semax is described in the literature as an analogue of a fragment of the adrenocorticotropic peptide known as ACTH; this classification is a literature category and not a statement of any outcome. Each carries its own CAS registry number, its own molecular formula, and a different set of four residues at the start of the chain.
| Identity attribute | Selank | Semax |
|---|---|---|
| Peptide class | Synthetic heptapeptide | Synthetic heptapeptide |
| Literature category | Analogue of tuftsin | Analogue of an ACTH fragment |
| CAS registry number | 129954-34-3 | 80714-61-0 |
| Single-letter sequence | TKPRPGP | MEHFPGP |
Compound identity and CAS registry
Selank
Selank is registered under CAS number 129954-34-3. It is a synthetic heptapeptide, meaning a chain of seven amino-acid residues, and is catalogued as a synthetic analogue of the naturally occurring peptide tuftsin.
Semax
Semax is registered under CAS number 80714-61-0; a secondary registry number, 4037-01-8, also appears in some databases. It is a synthetic heptapeptide and is described in the literature as an analogue of the ACTH(4-10) fragment of the adrenocorticotropic peptide ACTH. This mechanistic classification is a literature category and not a statement of any outcome.
Molecular formula, molar mass, and structure
Selank
Selank has the molecular formula C33H57N11O9 and an average molar mass of approximately 751.9 g/mol (751.887 g/mol). It contains no sulfur and is the more nitrogen-rich of the two, carrying eleven nitrogen atoms.
Semax
Semax has the molecular formula C37H51N9O10S and an average molar mass of approximately 813.9 g/mol (813.93 g/mol). A single sulfur atom, shown as the S in its formula and contributed by its methionine residue, is a quick structural tell that sets it apart from Selank. The two differ in molar mass by roughly 62 daltons.
Amino-acid sequence
Selank has the sequence Thr-Lys-Pro-Arg-Pro-Gly-Pro, written TKPRPGP in single-letter code. Semax has the sequence Met-Glu-His-Phe-Pro-Gly-Pro, written MEHFPGP. The two share the same three residues at the C-terminal end of the chain, Pro-Gly-Pro, which is part of why the names are sometimes compared, but they diverge completely across the first four residues, and that is where their separate identities are established.
Confirming identity and purity by LC-MS
Because the two peptides differ both in molar mass, by roughly 62 daltons, and in elemental composition, liquid chromatography-mass spectrometry (LC-MS) distinguishes them without ambiguity. The chromatographic step resolves the intact peptide, and the mass spectrometer reads the measured mass against the formula expected for each sequence, so a material labelled Selank cannot be mistaken for one labelled Semax. Every batch catalogued at livvmore is characterised by the USC Alfred E. Mann Multi-Omics Mass Spectrometry Core, which confirms the identity of the compound and reports its measured purity.
Which one is catalogued at livvmore
Both compounds are catalogued as research reference materials. Selank is listed on the Selank product page, with its full identity record on the Selank research profile. Semax is listed on the Semax product page, with its identity record on the Semax research profile. Each product page carries the batch-specific certificate of analysis for the material in stock.
For laboratory research use only. Not for human or veterinary use.
Frequently asked questions
Are Selank and Semax the same compound?
No. They are separate compounds with separate CAS registry numbers (129954-34-3 for Selank, 80714-61-0 for Semax), different molecular formulas, and different amino-acid sequences. They are related only in that both are short synthetic heptapeptides.
Why are the two so often compared?
Both are synthetic heptapeptides, and both sequences close with the same C-terminal Pro-Gly-Pro motif, so they are frequently catalogued together and listed near one another. The comparison is one of naming and family, not of identical structure.
What is the clearest structural difference between them?
Semax contains a single sulfur atom, reflected as the S in its formula C37H51N9O10S and contributed by its methionine residue, whereas Selank (C33H57N11O9) contains no sulfur and carries more nitrogen. They also differ in molar mass by about 62 daltons and derive from different parent categories, tuftsin for Selank and an ACTH fragment for Semax.
How is the identity of each confirmed?
Identity and purity are confirmed by LC-MS at the USC Alfred E. Mann Multi-Omics Mass Spectrometry Core, which measures the intact peptide mass against the formula expected for each sequence and reports the result on a batch certificate of analysis.
What are the two CAS registry numbers?
Selank is registered under CAS 129954-34-3. Semax is registered under CAS 80714-61-0, with a secondary number of 4037-01-8 seen in some databases.