Selank: Tuftsin, the Pro-Gly-Pro Tail, and a Purity Figure Worth Checking
Selank is a seven-residue peptide with a four-residue ancestor. Understanding which four came first, and what the other three are doing there, explains most of what is distinctive about it — including one analytical property that makes a careless purity certificate almost meaningless.
The sequence, and where it comes from
Selank is Thr-Lys-Pro-Arg-Pro-Gly-Pro. The first four residues, Thr-Lys-Pro-Arg, are tuftsin — a naturally occurring tetrapeptide that is not synthesised as a peptide at all, but released by enzymatic cleavage from the heavy chain of immunoglobulin G. The last three, Pro-Gly-Pro, are a synthetic addition. Selank is therefore an endogenous fragment with an engineered tail, which is a different kind of object from an analog that merely resembles something natural.
Why tuftsin needed help
A free tetrapeptide in solution is a short-lived thing. Exopeptidases work inward from both ends of a peptide chain, and a four-residue sequence offers very little chain to work through before nothing recognisable is left. Tuftsin’s brevity is exactly what makes it fragile, and it is the problem the Pro-Gly-Pro extension was designed around.
What proline actually does
Proline is the one residue whose side chain loops back and bonds to its own backbone nitrogen. That ring removes the amide hydrogen and locks the local backbone into a restricted set of angles, and most proteases cannot accommodate it in their active site. A proline adjacent to a scissile bond is an obstacle; a proline-glycine-proline run at the C-terminus is a wall.
So the tail is not decoration and it is not there to change what the molecule binds. It is there so the molecule survives long enough to be studied. The same design logic produced Semax, which is an ACTH fragment carrying the identical Pro-Gly-Pro extension — two different parent sequences, one stabilisation strategy. Selank and Semax covers what the two have and do not have in common, and peptide half-life covers the other modifications used for the same purpose.
The analytical problem nobody mentions
Read the sequence again: threonine, lysine, proline, arginine, proline, glycine, proline. There is no tryptophan, no tyrosine and no phenylalanine anywhere in it.
That matters because of how purity is measured. Peptide HPLC is normally run at 214 nm or 220 nm, where the peptide bond itself absorbs and everything peptidic shows up. Some certificates instead report 280 nm, where only aromatic side chains absorb. Selank has none. At 280 nm this compound is very nearly invisible, and so is most of what could be wrong with it.
A 99% purity figure obtained at 280 nm on a sequence with no aromatic residues is not a strong result — it is an artefact of choosing a wavelength the analyte barely responds to. The wavelength is printed on the chromatogram, and on this compound it is the first thing to find. What a third-party peptide test actually measures works through the rest of the figures.
How it behaves on a column
Selank carries a lysine and an arginine, both positively charged at working pH, against a backbone with no hydrophobic bulk at all. It is a very polar molecule, and on a reversed-phase column it elutes early — sometimes close enough to the void volume that a shallow gradient is needed to separate it from anything co-eluting with it. This is one reason two laboratories can return different purity figures for the same 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
The molecular formula is C33H57N11O9 and the mass is roughly 751.9 g/mol, against CAS registry number 129954-34-3. Those three values are what to match when comparing one listing against another, because the names are used loosely and registry numbers are not. Mass spectrometry settles identity to about a dalton, which is enough to distinguish the finished heptapeptide from a deletion sequence missing one of its prolines.
What a certificate should show
Three things, on this compound in particular. The detection wavelength, for the reason above. The 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 lyophilised. It has no cysteine, so there is no disulfide to reduce and no free thiol to oxidise, which removes one common degradation route. What remains is the ordinary discipline: bring the vial to room temperature before opening it so moisture does not condense onto a hygroscopic powder, keep repeat entries to a minimum, and record the reconstitution date against the certificate for the material. Choosing a reconstitution solvent and aliquoting reconstituted peptides cover the routine.
How it is written
The compound appears as Selank, as TP-7, and as the sequence TKPRPGP. All three refer to the same heptapeptide. Tuftsin on its own refers to the parent tetrapeptide and is a different substance with a different registry number.
What we supply
Selank is stocked as a 5mg vial and a 10mg vial, lyophilised, with identity data published on each product page. It is 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.
