Buying Selank in the USA: Tuftsin With a Stabilising Tail
Selank is seven residues long and has a four-residue ancestor. Knowing which four came first, and what the remaining three are doing, accounts for most of what is distinctive about the compound — including one property that renders a carelessly produced purity certificate close to worthless.
Sequence, and its origin
Thr-Lys-Pro-Arg-Pro-Gly-Pro. The first four, Thr-Lys-Pro-Arg, are tuftsin: a naturally occurring tetrapeptide that is not synthesized as a peptide in its own right but liberated by enzymatic cleavage from the heavy chain of immunoglobulin G. The last three, Pro-Gly-Pro, are synthetic. Selank is an endogenous fragment wearing an engineered tail, which is a different kind of object from an analog that merely resembles something natural.
Why tuftsin needed the help
A free tetrapeptide in solution does not last. Exopeptidases work inward from both ends of a chain, and four residues is almost no chain to work through before nothing recognizable is left. Tuftsin’s brevity is precisely what makes it fragile, and that is the problem the Pro-Gly-Pro extension was built to solve.
What proline does
Proline is the one residue whose side chain loops back and bonds to its own backbone nitrogen. The ring removes the amide hydrogen and locks the local backbone into a narrow range of angles, and most proteases cannot fit it into an active site. A proline next to a scissile bond is an obstacle; a proline-glycine-proline run at the C-terminus is a wall.
So the tail is neither decoration nor a binding element. It is there so the molecule survives long enough to be studied. The same logic produced Semax, an ACTH fragment carrying the identical extension — two unrelated parent sequences, one stabilization strategy. Selank and Semax covers what the pair do and do not have in common, and peptide half-life covers the other modifications used toward the same end.
The measurement problem that rarely gets mentioned
Read the sequence again — threonine, lysine, proline, arginine, proline, glycine, proline. No tryptophan, no tyrosine, no phenylalanine anywhere in it.
That matters because of how purity gets measured. Peptide HPLC normally runs at 214 nm or 220 nm, where the peptide bond itself absorbs and everything peptidic is visible. Some certificates instead report 280 nm, where only aromatic side chains absorb. Selank has none, so at 280 nm the compound is nearly invisible — and so is most of what could be wrong with it.
A 99% figure obtained at 280 nm on an aromatic-free sequence is not a strong result. It is an artifact of picking a wavelength the analyte barely responds to. The wavelength is printed on the chromatogram, and on this compound it is the first thing to go looking for. What a third-party peptide test actually measures works through the remaining figures.
On a column
Selank carries a lysine and an arginine, both positively charged at working pH, on a backbone with no hydrophobic bulk whatsoever. It is a very polar molecule and comes off a reversed-phase column early — sometimes near enough to the void volume that only a shallow gradient will separate it from whatever co-elutes with it. That is one reason two laboratories can report different purity figures for one vial without either being wrong; why two certificates can disagree sets out the mechanism, and reading an HPLC chromatogram shows what an unresolved shoulder looks like.
Identity figures
Molecular formula C33H57N11O9, mass approximately 751.9 g/mol, CAS registry number 129954-34-3. Those three values are what to line up when comparing listings, because names get used loosely and registry numbers do not. Mass spectrometry settles identity to about a dalton, which is enough to tell the finished heptapeptide from a deletion sequence missing one of its prolines.
What a certificate should carry
Three things on this compound in particular. The detection wavelength, for the reason above. Measured mass against the calculated 751.9. And net peptide content, because purity describes the peptide fraction relative to other peptide-like material and says nothing about how much of the vial is peptide rather than counter-ion and residual water — see net peptide content. Every report we hold is published in full on the certificates of analysis page.
Handling
Selank is supplied lyophilized. It has no cysteine, so there is no disulfide to reduce and no free thiol to oxidize, which removes one common degradation route. What remains is ordinary discipline: bring the vial to room temperature before opening so moisture does not condense onto a hygroscopic powder, keep repeat entries few, and record the reconstitution date against the certificate for that material. Choosing a reconstitution solvent and aliquoting reconstituted peptides cover the routine.
Names you will see
Selank, TP-7, and the sequence TKPRPGP — all the same heptapeptide. Tuftsin on its own refers to the parent tetrapeptide, which is a different substance with a different registry number.
What we supply
Selank is stocked as a 5mg vial and a 10mg vial, lyophilized, with identity data published on each product page. Supplied for laboratory research use only. We publish no dosing information, no protocols involving human or veterinary subjects, and no statements about what the compound does in a person — and we do not provide them on request.