Selank: structure, nomenclature and handling

The sequence is threonine–lysine–proline–arginine–proline–glycine–proline, written TKPRPGP in single-letter code. It is a heptapeptide with a molecular weight of approximately 752 daltons.

The first four residues — threonine–lysine–proline–arginine — are the tetrapeptide tuftsin, itself a fragment of a larger protein: it corresponds to residues 289 through 292 of the heavy chain of immunoglobulin G, released from the parent by proteolysis rather than encoded as a standalone gene product. The remaining three residues are the appended proline–glycine–proline.

That tail does the same job here as it does in Semax. Proline at the amino side of a peptide bond makes that bond a poor substrate for most peptidases, so a proline–glycine–proline cap protects the carboxyl terminus from exopeptidase attack while leaving the fragment of interest chemically untouched. Two different fragments, one shared stabilising motif — a design convention rather than a coincidence.

The composition is strongly basic: a lysine and an arginine both carry positively charged side chains at neutral pH, which accounts for the molecule's high water solubility. Three of the seven residues are prolines, making this an unusually rigid short chain.

The material is catalogued as Selank, and in fragment notation as Thr-Lys-Pro-Arg-Pro-Gly-Pro or tuftsin-Pro-Gly-Pro. It carries CAS 129954-34-3. As with most short synthetic peptides, it is typically isolated as an acetate salt, which affects the mass weighed out without altering identity.

Tuftsin itself is a separate catalogue entity — the bare tetrapeptide, roughly 500 daltons, without the tail — and the two should not be treated as interchangeable listings. An N-acetylated variant of Selank also circulates, adding a defined 42-dalton increment, so the certificate's stated theoretical mass identifies which construct a batch is.

Mass spectrometry confirms the intact molecular mass near 752 daltons, distinguishing the full construct from the bare tetrapeptide by a wide margin and from the acetylated variant by a defined one.

High-performance liquid chromatography resolves the main peak from related substances and yields the purity figure. A seven-residue synthesis produces few deletion sequences, so the profile is usually simple. The sequence-specific consideration is proline-rich chemistry: sequences containing X-proline motifs are prone during synthesis to diketopiperazine formation, in which two residues cyclize and detach, and proline-containing chains can also present as slowly interconverting conformers, which show on a chromatogram as broadened or split peaks rather than as genuine impurities. Distinguishing a conformational artefact from a real related substance is a question the chromatogram answers and a headline percentage does not.

One analytical property follows from the composition: the sequence contains no aromatic residues, so it has essentially no absorbance at the 280-nanometre wavelength used to quantify most peptides by ultraviolet light. Detection relies on backbone absorbance near 214 nanometres instead, and any measurement built on 280-nanometre readings is uninformative for this compound.

HEEZ Research supplies Selank in a single format: a 10 mg lyophilized vial. There is no second strength in the current catalogue, so the format decision that arises with two-size products does not apply here.

The material is supplied as a lyophilized powder. Freeze-drying removes water and leaves the peptide as a solid, markedly more stable than the same peptide in solution, and stable enough at ambient temperature to ship without refrigeration in transit.

Reconstitution converts the dried cake into a working solution, and it is a standard laboratory step rather than a product specification. Which solvent is used, and at what final concentration, is determined by the protocol in force at the receiving facility.

What HEEZ specifies is the contents of the vial: lyophilized Selank in a single 10 mg format, stated on the label and on the batch certificate of analysis. Where a protocol is expressed in molar rather than mass terms, the additional figure it needs is the molecular weight, which the certificate records for the batch supplied.

A short, charged, proline-rich chain is about as undemanding as peptide handling gets. It dissolves rapidly in aqueous media, presents essentially no surface for aggregation, and requires no mechanical assistance. Vigorous agitation is unnecessary and avoided as a matter of routine.

Sealed lyophilized Selank is ambient-stable for the duration of transit, so no cold chain or insulated packaging is required for shipping. On receipt the vial goes to refrigeration and is kept out of direct light.

Reconstituted material is held refrigerated and used within the window the receiving facility's protocols specify, and repeated freeze-thaw cycling is avoided. Solutions are treated as short-lived relative to the lyophilized form.

By composition this is one of the more robust entries in the catalogue: no cysteine or methionine removes the common oxidation routes, no asparagine or glutamine removes deamidation, and no tryptophan removes the usual photosensitivity concern. The one residue-specific note is the threonine at the amino terminus, whose hydroxyl-bearing side chain makes N-terminal rearrangements possible in solution over extended periods — one more reason solutions are made up as needed rather than held.

The certificate records what an independent facility measured for one specific batch. Its value lies in being batch-specific, so the first check is that the lot number on the document matches the vial in hand.

The identity section compares observed molecular mass against a stated theoretical mass, which should sit near 752 daltons for the unmodified heptapeptide — not the bare tetrapeptide near 500, and not an acetylated variant 42 daltons heavier.

The purity figure is the HPLC main-peak area as a percentage of total peak area, and the attached chromatogram shows how that figure was arrived at. On a proline-rich sequence a broadened or shouldered main peak is worth interpreting rather than assuming: it may reflect conformers of the target rather than contamination, and the two have very different implications.

Peptide content, or net peptide content, is sometimes reported separately. It accounts for counter-ions and residual water, and it is the figure determining how much actual peptide a nominal 10 mg represents.