Selank vs Semax: Peptide Structures Compared

Selank and Semax are two synthetic heptapeptides built on the same plan: a four-residue fragment of a larger natural peptide, extended at the C-terminus by the tripeptide Pro-Gly-Pro. Selank, Thr-Lys-Pro-Arg-Pro-Gly-Pro, starts from tuftsin. Semax, Met-Glu-His-Phe-Pro-Gly-Pro, starts from the fragment ACTH(4–7). Beyond the shared tail the two chains have nothing in common, and that difference in the first four residues gives them different formulas, masses, charges and chromatographic behaviour.

This page sets the two molecules side by side: the sequences aligned residue by residue, a computed comparison table, the arithmetic behind their formulas, and what the differences mean for HPLC and mass spectrometry. Each compound also has its own page: what is Selank and what is Semax.

The two sequences, aligned

Both peptides are written N-terminus to C-terminus, with a free amine at the start and a free carboxylic acid at the end: H-Thr-Lys-Pro-Arg-Pro-Gly-Pro-OH and H-Met-Glu-His-Phe-Pro-Gly-Pro-OH. Aligned position by position, the split between the variable head and the shared tail is plain to see.

In one-letter code the contrast is just as clear: TKPRPGP against MEHFPGP. Every residue in both chains is in the L-configuration apart from glycine, which is achiral. Neither peptide contains cysteine or tryptophan, and neither carries an acetyl, amide or other terminal modification. The notation is covered in peptide sequence notation.

Side by side: the computed figures

The table below is computed from the catalogue sequence of each peptide, using the same residue data as our peptide reference data page.

Each figure follows from the sequence alone. The formulas and masses are sums of residue contributions plus one water for the free termini; the net charge comes from the ionisable groups at pH 7; and the extinction coefficient at 280 nm is zero for both, because neither chain contains tryptophan, tyrosine or cystine, the groups that absorb at that wavelength.

Two parent fragments: tuftsin and ACTH(4–7)

The first four residues of Selank, Thr-Lys-Pro-Arg, are the complete sequence of tuftsin, a tetrapeptide with the formula C21H40N8O6. The first four residues of Semax, Met-Glu-His-Phe, are residues 4 to 7 of ACTH, a fragment written ACTH(4–7) with the formula C25H34N6O7S. The bracketed numbers give positions in the parent chain, not in Semax. Semax is often described as an analogue of the longer fragment ACTH(4–10), with Pro-Gly-Pro in place of the residues that follow Phe.

The two heads are very different chemically. Tuftsin contributes two strongly basic side chains, the amine of lysine and the guanidinium group of arginine, plus a proline. ACTH(4–7) contributes an acidic side chain, glutamic acid, a weakly basic imidazole in histidine, a hydrophobic aromatic ring in phenylalanine and a thioether in methionine.

The shared Pro-Gly-Pro extension

Pro-Gly-Pro, C12H19N3O4, closes both chains. Joining it to a four-residue head forms one new peptide bond and releases one molecule of water, so each formula is head plus tail minus H2O.

The shared tail gives both peptides a proline-rich C-terminus. Proline's side chain loops back onto its own backbone nitrogen, so a prolyl residue has no amide hydrogen and restricts the conformations nearby. The peptide bonds that precede a proline are also the ones most able to adopt the cis form, so both molecules exist in solution as mixtures of prolyl isomers. Selank, with a third proline at position 3, has three such bonds; Semax has two.

Formula and mass, line by line

The formulas differ by C4H−6N−2OS. Semax gains four carbons, an oxygen and a sulfur, and loses six hydrogens and two nitrogens. The extra carbons and the sulfur come from methionine and phenylalanine; the missing nitrogens reflect the absence of lysine and arginine, whose side chains carry one and three nitrogen atoms.

The nitrogen count matters beyond the formula. Net peptide content is sometimes estimated from elemental nitrogen, and the calculation needs the nitrogen share of the exact molecule: 20.5% by mass for Selank and 15.5% for Semax. A figure computed for one peptide does not transfer to the other.

Charge and what it means for the solid

Near pH 7 the two molecules sit on opposite sides of neutral. Selank carries positive charges on its N-terminal amine, lysine and arginine against one negative charge at the C-terminus, for a net charge of about +2. Semax carries a positive charge on its N-terminal amine against negative charges on glutamic acid and the C-terminus, with its histidine mostly neutral, for a net charge of about −0.8.

Both are isolated from purification as salts, with counter-ions such as acetate or trifluoroacetate paired with their positive groups. Selank, with three basic groups, pairs with more counter-ions per molecule than Semax at the acidic pH of purification, which affects how much of the powder's gross weight is peptide. The arithmetic is explained in peptide purity vs net peptide content.

How the two behave in HPLC

Reversed-phase HPLC separates molecules by how strongly they interact with a hydrophobic stationary phase. Selank is small, highly polar and strongly basic, with no aromatic side chain, so it is weakly retained and elutes early. Semax carries a phenylalanine ring and a methionine thioether, both hydrophobic, so under the same gradient it is expected to elute later.

Detection differs too. Neither peptide absorbs usefully at 280 nm, so both are measured in the peptide-bond region at 210 to 220 nm. Semax's phenylalanine adds a weak band near 257 nm; Selank has no aromatic ring at all. Both can show broadened peaks or shoulders where cis and trans prolyl forms interconvert during the run, a property of the molecules rather than a sign of a second compound. More on the method is in HPLC and peptide purity.

How the two behave in mass spectrometry

At these masses electrospray gives mainly singly and doubly charged ions, so identity can be read almost directly from the spectrum. The roughly 62-unit gap between the two [M+H]+ ions means there is no risk of confusing one compound with the other.

Tandem mass spectrometry adds a neat structural check. Fragment ions that contain the C-terminus, the y ions, are identical for the last three residues of both peptides: the y3 ion for Pro-Gly-Pro appears at m/z 270.145 in each. The b ions, which contain the N-terminus, differ from the first residue onwards. A spectrum that shows the shared y3 ion and the expected b ions confirms both the common tail and the distinct head. Mass spectrometry and peptide identity explains the b and y naming.

Typical related substances

The impurity profiles follow from the sequences. Semax's methionine can oxidise to the sulfoxide, adding 15.995 Da, a related substance Selank cannot form because it has no methionine, tryptophan or cysteine. Both peptides can carry the deletion sequences typical of solid-phase synthesis, each lighter than the parent by one residue mass: a missing proline, 97.053 Da, is possible in both, while a missing lysine, 128.095 Da, or arginine, 156.101 Da, is specific to Selank, and a missing phenylalanine, 147.068 Da, or glutamic acid, 129.043 Da, to Semax.

Supply and storage

Our Selank is supplied in a sealed 10 mg vial and our Semax in a sealed 30 mg vial, both at ≥99% purity (HPLC) as lyophilised powders. Store sealed vials at 2–8 °C for short-term storage, or at −20 °C and below for long-term storage, protected from light. For Semax, limiting contact with air matters more, because of its methionine. Storing lyophilised peptides covers the reasons.

Both compounds belong to the Neuropeptides research area. For a longer linear chain built on a similar proline-rich theme, see BPC-157, a fifteen-residue peptide with four prolines. Terms used on this page, such as monoisotopic mass, prolyl isomer and counter-ion, are defined in the glossary.