What Is Selank? Structure, Chemistry and the Published Research

What Selank is

Selank is a synthetic peptide of seven residues with the sequence Thr-Lys-Pro-Arg-Pro-Gly-Pro.^[1] It is described as a synthetic analogue of tuftsin, the naturally occurring tetrapeptide Thr-Lys-Pro-Arg.^{[2, 3]} Selank is therefore tuftsin with a Pro-Gly-Pro tripeptide added at the C-terminus. Tuftsin itself is located in the Fc region of immunoglobulin G, at positions 289 to 292 of the CH2 domain.^[3]

A review of tuftsin and its analogues describes dozens of derivatives made over the years and reports the activities attributed to them in the literature.^[4] Selank is one entry in that large family. It shares a design feature with Semax, the compound described in a companion primer, namely the Pro-Gly-Pro tail.

Chemistry at a glance

The Sealed Peptides product lists the molecular formula C33H57N11O9 and a molecular weight of 751.9 Da. The average mass calculated from that formula is 751.89 g/mol, in agreement with the listing. The monoisotopic mass is 751.434, and a singly protonated ion would be expected near m/z 752.44.

The sequence contains two basic residues, lysine and arginine, and no acidic ones. It has no cysteine, methionine, tryptophan or tyrosine, so it has none of the residues most associated with disulfide chemistry or oxidation, and it contains three prolines. These are facts about the sequence and not predictions about behaviour; the way to learn how a given preparation behaves is to measure it.

Synthetic peptides carrying basic residues are normally supplied as salts, with an acid counter-ion bound to the positive charges. Which counter-ion is present, and how much of the powder it accounts for, is explained in TFA versus acetate counter-ions, and it is the reason a label mass and a peptide mass are different numbers.

What the published literature describes

The studies below are in-vitro or rodent experiments. They are summarized as description only.

Enzyme chemistry in vitro. Selank was reported to inhibit the enzymatic hydrolysis of enkephalin in plasma in a concentration-dependent way, with an IC50 of 15 µM, and to be more potent in that assay than the peptidase inhibitors bacitracin and puromycin.^[1]

Gene expression in mice. After a single exposure, expression of 34 of 84 genes involved in inflammation changed in mouse spleen, and the gene Bcl6 changed in response to Selank and to each of two fragments.^[5] A later time-course study followed four genes in mouse spleen and found a threefold fall in C3 mRNA within 30 minutes. The dipeptide Gly-Pro, a fragment of Selank, produced a similar change, and the authors suggested the fragment contributes to the effect of the whole peptide.^[6]

The tuftsin background. Tuftsin was reported to stimulate functions of polymorphonuclear leukocytes and macrophages, and attempts to raise antibodies against it failed in several species, which the authors attributed in part to its small size and to a lack of foreignness.^[3] That is a statement about the parent tetrapeptide, not about Selank.

What that literature cannot tell you

The papers cited here come from a small number of research groups and use rodent and in-vitro systems. They show that molecular readouts change under particular conditions; they do not show what Selank does elsewhere, and this page does not describe or endorse any use. The abstract of the enzyme study also contains clinical language about the compound. That language belongs to its authors and is deliberately not repeated here.

The Selank research overview lists the cited literature for the compound in the same descriptive way, and the solid-phase synthesis guide explains how a peptide of this length is made.

Checking the identity arithmetic

The sequence gives the expected mass without any instrument. Summing the seven residue masses (one Thr, one Lys, three Pro, one Arg and one Gly) and adding one water gives a monoisotopic mass of 751.434 Da and an average mass of 751.88 g/mol, which is what the peptide molecular weight estimator returns for the sequence TKPRPGP. A mass spectrum of an intact, correctly made peptide would be expected to show its main singly charged signal near m/z 752.44 and a doubly charged signal near m/z 376.72.

If an observed value is off by a fixed amount, the difference is informative, because common modifications and adducts have known masses; the mass spectrometry guide lists the usual ones. A certificate that quotes an expected and an observed mass is making exactly this comparison, and the guide to reading a certificate of analysis explains how to read one.

Analytical notes for the bench

The usual identity and purity questions apply: an observed m/z against the expected value, and chromatographic purity as share of total peak area. See mass spectrometry and peptide identity and reading an HPLC chromatogram.

Detection is the practical difficulty. Without tryptophan, tyrosine or cystine there is little absorbance at 280 nm,^[7] so measurements are made near 214 nm, where the peptide bond has a molar extinction coefficient of 923 M⁻¹ cm⁻¹ and a proline inside the chain absorbs about three times as strongly as a peptide bond.^[8] Selank has six peptide bonds and three prolines, all inside the chain. A signal at 214 nm is therefore mostly the backbone, which is why accurate concentration checks need care. See reconstitution math for how a measured value is compared with the arithmetic.