Selank vs Semax: Two Related Peptides and How to QC Them

Selank and Semax are easy to confuse on a shelf and easy to tell apart on paper, once you see how they were built. Each takes a short fragment of a naturally occurring peptide and extends it with the same three-residue tail. For labs sourcing Selank in Canada, often alongside Semax in the same order, that shared design explains most of what matters analytically: why their chromatograms can look untidy, which degradation product belongs to which compound, and why a mass that seems wrong may simply describe a modified form. This article walks through both molecules for QC and purchasing staff.

Selank in Canada, and Semax beside it: identity data

Both are heptapeptides with free termini in their plain forms. The core data:

The pair were developed within a single Russian research programme, and much of their literature is found in Russian-language sources. As with any compound whose record is concentrated in one network of groups, independent characterisation of the material you actually receive carries extra weight.

Endogenous heads, a shared tail

Read each sequence from its N-terminus. Selank opens with threonine, lysine, proline and arginine, the natural tetrapeptide known as tuftsin. Semax opens with methionine, glutamate, histidine and phenylalanine, matching positions 4 to 7 of adrenocorticotropic hormone. Each head is then followed by the same tripeptide, proline-glycine-proline.

The reasoning behind the design is straightforward. Short natural fragments are broken down quickly by peptidases, and a proline-rich C-terminal extension makes the chain a poorer substrate for many of those enzymes. Seen this way, the names stop being arbitrary: Selank is tuftsin with a PGP tail, and Semax is ACTH(4-7) with the same tail.

Proline and the chromatogram

That tail has a side effect in the analytical lab. Peptide bonds preceding proline can adopt cis or trans configurations, and the switch between them is slow on the timescale of an HPLC run. A molecule with several prolines can therefore exist as more than one slowly exchanging form during separation, giving a broadened, shouldered or even split main peak.

Selank, with three prolines, is more prone to this than Semax with two. This behaviour belongs to the peptide itself and does not signal an impurity, and a method optimised for a proline-free peptide may make good material look worse than it is. Raising the column temperature often speeds interconversion enough to sharpen the peak, and mass spectrometry across the peak confirms whether a shoulder is the same molecule or something else.

Detection wavelength also deserves a note. Neither sequence contains tryptophan or tyrosine; Semax has phenylalanine and histidine, which absorb only weakly near 280 nm, and Selank has no aromatic residue at all. Purity for both should be measured at a low-UV wavelength, around 214 nm.

Chemistry that sets them apart

Beyond the shared tail, the two heads behave very differently:

• Semax and methionine oxidation. The sulfur in the Semax formula belongs to its N-terminal methionine, which oxidises readily to the sulfoxide. That product sits 16 Da above the parent, near 829.9 g/mol. A near-neighbour peak with that mass is the expected degradation product, not a mystery contaminant, and it grows faster in solution and in the presence of air than in a sealed, dry vial.
• Selank and basicity. Selank has no sulfur and no comparable oxidation liability. Its lysine and arginine, plus the free N-terminus, make it strongly basic, which affects retention, counter-ion content and adsorption to surfaces rather than chemical stability.
• Charge balance in Semax. With glutamate offsetting the N-terminal charge, Semax is far less basic than Selank, so the two can behave quite differently when dissolved and chromatographed under the same conditions.

Modified forms: the 42-dalton and 1-dalton questions

Both peptides also circulate in modified versions, and catalogue listings do not always say so. The two changes to watch for:

1. N-terminal acetylation adds about 42 Da. N-acetyl Selank sits near 793.9 g/mol and N-acetyl Semax near 855.9 g/mol.
2. C-terminal amidation replaces the free acid with an amide and lowers the mass by just under 1 Da.

A certificate showing a mass about 42 Da away from your reference value is much more likely to be describing the acetylated form than a faulty batch. If two documents seem to conflict, first work out which form each one refers to, and put the intended form, with its mass, on your purchase order.

Counter-ions add a further layer. Both peptides are isolated as salts, commonly acetate or trifluoroacetate, and Selank’s higher basic content means it can carry more counter-ion mass per molecule. Net peptide content and HPLC purity answer different questions, so compare like with like across suppliers.

Receiving and logging a mixed order

Two white lyophilised heptapeptides from the same category are exactly the situation where labels alone are not enough. When both arrive together:

• Check each certificate’s mass against the plain or modified form you ordered.
• Match cap and crimp colour to the correct certificate and record the pairing, so a Selank vial can never be logged against a Semax document.
• Note vial size. Selank 5 mg and Selank 10 mg are the same compound in different quantities, while Semax 10 mg is a separate line.
• Store both cold, dry and dark, and treat opened Semax solutions as the more time-sensitive of the two.
• For a long series of experiments, request a single lot per compound to keep results comparable.

Bulk Peptides products are third-party tested for purity by HPLC, and certificates are published for some products on the certificates of analysis page. Orders ship from within Canada, and with mix-and-match volume pricing every vial in the cart counts toward the volume break, so Selank, Semax and other neuropeptides such as DSIP can be combined in one order.

Selank, Semax and the other compounds mentioned are sold for in-vitro laboratory research only. None of them is intended for use in people or animals, and this article offers no guidance on any such use.