What Is Selank? Structure and Chemistry

Selank is a synthetic heptapeptide with the sequence Thr-Lys-Pro-Arg-Pro-Gly-Pro (TKPRPGP): the tetrapeptide tuftsin, Thr-Lys-Pro-Arg, extended at its C-terminus by the tripeptide Pro-Gly-Pro. It is a linear chain of standard amino acids with a free N-terminal amine and a free C-terminal acid. Its molecular formula is C33H57N11O9, its average molecular weight is 751.87 g/mol and its CAS number is 129954-34-3.

This page covers Selank as a molecule: the sequence residue by residue, how it relates to tuftsin, its formula, mass and charge, the acetylated variant, and how its identity and purity are confirmed by HPLC and mass spectrometry.

The Selank peptide sequence, residue by residue

Written in full, with both termini shown, the peptide is H-Thr-Lys-Pro-Arg-Pro-Gly-Pro-OH. The "H-" is the free amine at the N-terminus and the "-OH" the free carboxylic acid at the C-terminus. PubChem records the systematic name L-threonyl-L-lysyl-L-prolyl-L-arginyl-L-prolylglycyl-L-proline, which names the same seven residues in order. The conventions behind three-letter and one-letter codes are covered in peptide sequence notation.

Every residue except glycine is in the L-configuration. Threonine carries two stereocentres, (2S,3R) in PubChem's IUPAC name, so the molecule has seven defined stereocentres in all: two in threonine and one each in lysine, arginine and the three prolines. Selank contains no cysteine, methionine or tryptophan and no aromatic side chain at all.

Tuftsin and the Pro-Gly-Pro extension

The first four residues of Selank, Thr-Lys-Pro-Arg, are the complete sequence of tuftsin, a tetrapeptide recorded in PubChem under CID 156080 with the formula C21H40N8O6 and an average molecular weight of 500.6 g/mol. Selank is that sequence with a second, shorter block attached to the C-terminal arginine: the tripeptide Pro-Gly-Pro.

The formula confirms the relationship by simple arithmetic. Tuftsin, C21H40N8O6, plus Pro-Gly-Pro, C12H19N3O4, gives C33H59N11O10. Forming the peptide bond between the arginine carboxyl and the proline amine releases one molecule of water, H2O, which leaves C33H57N11O9, the formula PubChem lists for Selank.

The same design appears in Semax, the other heptapeptide in our Neuropeptides research area, where the Pro-Gly-Pro block extends a four-residue fragment of a different parent peptide. The two compounds share their last three residues and nothing else.

Molecular formula, molecular weight and CAS number

The monoisotopic mass is the sum of the seven residue masses plus one water for the two free termini, and it matches PubChem's computed value of 751.4341 Da. The average molecular weight depends slightly on the atomic-weight table used: 751.87 g/mol with the older standard atomic weights and 751.89 g/mol with the current IUPAC abridged values. Both round to PubChem's 751.9 g/mol; the difference reflects the reference table, not the molecule.

Charge, counter-ions and the solid form

Near pH 7, three groups carry a positive charge: the N-terminal amine, the side-chain amine of lysine and the guanidinium group of arginine. One group carries a negative charge, the C-terminal carboxylate. The net charge is therefore about +2, and with no acidic side chain anywhere in the sequence Selank is a basic peptide.

A basic peptide is isolated from purification as a salt, its positive groups paired with counter-ions such as acetate or trifluoroacetate. PubChem lists a diacetate form among Selank's synonyms. Counter-ions add weight to the powder without adding peptide. The table shows the arithmetic for two counter-ions and no water; real material is described by its certificate.

This is why net peptide content can sit below the gross weight of a lyophilised solid, as explained in peptide purity vs net peptide content.

Three prolines in seven residues

Proline is the one standard amino acid whose side chain loops back to bond with its own backbone nitrogen, forming a five-membered pyrrolidine ring. A prolyl residue therefore has no amide hydrogen, and it restricts the backbone conformations available around it. Selank has prolines at positions 3, 5 and 7, so almost half of the chain is proline.

The peptide bond that precedes a proline is also the one most able to adopt the cis form as well as the usual trans form, and Selank has three such bonds: Lys-Pro, Arg-Pro and Gly-Pro. In solution the molecule exists as a mixture of these prolyl isomers. When the cis and trans forms interconvert on the same timescale as a chromatographic separation, HPLC peaks can broaden or show a shoulder, a feature of the molecule rather than a second compound.

N-acetyl Selank, the acetylated variant

N-acetyl Selank is Selank with an acetyl group, CH3CO–, on the N-terminal amine of threonine, written Ac-Thr-Lys-Pro-Arg-Pro-Gly-Pro-OH. PubChem records it as a separate compound, CID 133082488, with the formula C35H59N11O10 and an average molecular weight of 793.9 g/mol.

The difference is C2H2O, 42.0106 Da, the standard mass shift of N-terminal acetylation. Capping the amine also removes one positive charge, so the acetylated peptide is less basic and is retained slightly longer in reversed-phase HPLC. Mass spectrometry distinguishes the two compounds at once. Our catalogue supplies Selank in its free-amine form, the compound registered under CAS 129954-34-3.

How Selank is analysed

Selank is small, polar and basic, with a PubChem computed XLogP of −6.1, so it elutes early in reversed-phase HPLC. An acidic ion-pairing agent such as trifluoroacetic acid in the mobile phase pairs with its positive groups, giving longer retention and a sharper peak. The fragment ions follow the standard b/y naming: b ions contain the N-terminus and y ions the C-terminus. Cleavage on the N-terminal side of proline is especially favoured in peptide MS/MS, so with three prolines Selank gives a clear set of fragments at those positions.

Selank contains no methionine, tryptophan or cysteine, the residues most prone to oxidation or disulfide scrambling, so the oxidised forms that dominate the impurity profile of many peptides are not a concern here. More detail on each method is in HPLC and peptide purity and mass spectrometry and peptide identity.

Certificates, specification and storage

Our Selank is supplied at ≥99% purity (HPLC) as a lyophilised powder in a sealed 10 mg vial. When reading a certificate for Selank, check that the sequence reads Thr-Lys-Pro-Arg-Pro-Gly-Pro with free termini, that the observed mass matches 751.4 Da or the ions above, that the HPLC method gives its detection wavelength, that the salt form is stated, and that the report carries a reference and an analysis date. How to read a peptide certificate of analysis walks through each line of a report.

Store the sealed vial at 2–8 °C for short-term storage, or at −20 °C and below for long-term storage. Protect it from light and avoid repeated freeze-thaw cycles. Storing lyophilised peptides explains why cold, dark and dry storage matters for a freeze-dried solid.

Selank belongs to the Neuropeptides research area, catalogued there with Semax. Terms used on this page, such as N-terminal acetylation, counter-ion and monoisotopic mass, are defined in the glossary.