Selank

1 Identity

Synthetic linear heptapeptide. Residues 1-4 are the endogenous tetrapeptide tuftsin (Thr-Lys-Pro-Arg); residues 5-7 are a synthetic Pro-Gly-Pro C-terminal extension. Free-acid C-terminus, free N-terminal alpha-amine, no acylation, no amidation, no non-proteinogenic residue, no cysteine, no disulfide and no sulfur atom anywhere in the molecule. All seven residues proteinogenic and L-configured. Three prolines in seven residues, so the molecule carries cis/trans amide rotamers and can present as a broadened or split peak on a poorly developed reversed-phase method; that is a chromatographic property, not an impurity, and a method that cannot distinguish the two cases cannot release the product. and Selank acetate. CAS 129954-34-3 for the free base and CAS 2703745-90-6 for the 1:1 acetate: two registry numbers for one molecular entity, which is why a purchaser matching on CAS alone concludes they are different compounds. PubChem CID 11765600.

Sequence

H-Thr-Lys-Pro-Arg-Pro-Gly-Pro-OH; one-letter TKPRPGP, 7 residues. Free acid at the C-terminal proline, free alpha-amine at the N-terminal threonine. Supplied lyophilized as the acetate salt where the manufacturer performs a salt exchange and as the trifluoroacetate salt where it does not; the label rarely says which.

Molecular formula

C33H57N11O9 (free base). The complete absence of sulfur is an analytical asset: the isotope envelope of a genuine lot carries no 34S contribution at all.

Average mass

751.887 Da (C33H57N11O9) on IUPAC 2021 abridged conventional atomic weights (C 12.011, H 1.008, N 14.007, O 15.999). PubChem publishes 751.9 and a reagent catalog prints 751.87 for the same formula; 751.87 is that formula on the pre-2021 table, which returns 751.874. Neither figure is wrong, and the basis is stated on the face of the mass. The 1:1 acetate salt, C35H61N11O11, is 811.939 average, which is the figure a reagent catalog prints as 811.9 — and that is a formula weight for a defined salt stoichiometry, not a second value for the same molecule.

Monoisotopic mass

751.43407 Da neutral free base; [M+H]+ 752.44135; [M+2H]2+ 376.72431. A plus or minus 5 ppm identity window on the protonated ion is plus or minus 0.0038 Da, which is comfortably tighter than the 0.98 Da that separates this molecule from its C-terminal amide and the 41.05 Da that separates it from the N-acetylated amide. The 1:1 acetate formula unit C35H61N11O11 is 811.45520 monoisotopic, a number with a use in gravimetric calculation and no use in a mass spectrum, since acetate dissociates in solution and the observed ion is the protonated free base at 752.44.

Salt / variant note

Two different numbers are published under the bare name "Selank" by institutional suppliers and the gap is 60.05 Da. (1) Free base, C33H57N11O9, 751.887 average / 751.43407 monoisotopic, CAS 129954-34-3 — what PubChem and one reagent catalog mean by the name. (2) Acetate, C33H57N11O9 with C2H4O2, giving C35H61N11O11, 811.939 average / 811.45520 monoisotopic, CAS 2703745-90-6 — what another supplier means by it, sold at declared 1:1 stoichiometry with its own registry number and its own catalog molecular weight. A certificate reading 811.9 is not wrong, but it is not the free base, and net peptide content differs by 7.40 percent w/w between the two. (3) C-terminal amide, H-TKPRPGP-NH2, C33H58N12O8, 750.903 average / 750.45006 monoisotopic — 0.98 Da lighter and invisible to any nominal-mass instrument. (4) N-Acetyl Selank amide, Ac-TKPRPGP-NH2, C35H60N12O9, 792.940 average / 792.46062 monoisotopic, [M+H]+ 793.46790 — 41.05 Da heavier, separately cataloged, and the abbreviation "NASA" used for it collides with N-Acetyl Semax Amidate, which is a genuine and documented source of product confusion. (5) N-Acetyl Selank free acid, Ac-TKPRPGP-OH, C35H59N11O10, 793.924 average / 793.44464 monoisotopic — set against (4) that is a 0.98 Da separation, both are sold as "N-Acetyl Selank", and a certificate reading 793 identifies neither. (6) AdaSelank: structure not established, and no mass is stated here. Three candidate scaffolds give plain Selank plus adamantane-1-carbonyl, C44H71N11O10, 914.119 average / 913.53854 monoisotopic; N-acetyl-Selank-amide plus adamantane-1-carbonyl, C46H74N12O10, 955.172 average / 954.56509 monoisotopic; and N-acetyl-Selank free acid plus adamantane-1-carbonyl, C46H73N11O11, 956.156 average / 955.54910 monoisotopic. The 914.1 figure is the arithmetic of the first scaffold alone — parent peptide plus an increment from an assumed structure — and that same method produced a wrong number for Adamax, which turned out to be built on the acetylated and amidated scaffold. The scaffold question is worth 41-42 Da and must be answered by the manufacturer in writing before any figure is printed. (7) Tuftsin itself, TKPR, C21H40N8O6, 500.601 average / 500.30708 monoisotopic — the cheaper 4-mer core, 251.29 Da lighter. (8) des-Thr deletion sequence KPRPGP, C29H50N10O7, 650.782 average / 650.38639 monoisotopic, 101.11 Da lighter. (9) Trifluoroacetate at 1:1 is 13.17 percent w/w of the vial against 7.40 percent for acetate. Commercial direction matters: the routine substitution is plain Selank certified as N-Acetyl Selank, which shows as a 41-42 Da shortfall against the label.

2 Class & testing panel

Form

Single article

Testing panel

P1 — panel definition

3 Primary sources & evidence

The index reports the design and provenance of the literature, not a conclusion about effect.

Rodent behavioral and immunological studies and a modest in-vitro literature, together with a Russian-language clinical literature tied to the compound's Russian registration, most of it published in Russian neurology and psychiatry journals, frequently without randomization or placebo control and rarely reporting any analytical characterization of the test article. English-language PubMed indexing runs to a few dozen items, a substantial share of them reviews and translations rather than primary reports. No FDA-registered trial exists, and no completed controlled trial is registered on ClinicalTrials.gov under this name. There is a documented structure-activity point that bears on the analogue trade rather than on this molecule: tuftsin's structure-activity relationship is reviewed in Siebert and colleagues, Curr Med Chem 2017 (PMID 28745220), and the closest tested precedent for N-terminal acetylation in this compound family is Magri and colleagues (PMID 27586814, in vitro) on Semax, where the modification abolished a documented in-vitro activity. The citation index page carries the primary Russian reports with study-type badges recording language, design and sample size, and states plainly that no trial conducted to ICH GCP has been identified.

4 Storage & specification

Storage

Lyophilized white to off-white powder in amber Type I glass with a bromobutyl stopper, headspace displaced with nitrogen, one desiccant sachet per shipper. Labeled condition: store at -20 degrees C plus or minus 5 degrees C, desiccated, protected from light. The acetate salt is hygroscopic, so the label instructs that vials be equilibrated to room temperature before opening to prevent condensation onto the cake. Shipped on dry ice or in a validated shipper holding NMT 8 degrees C for transit up to 96 hours, with a single-use electronic temperature logger in every shipper and the logger record filed against the outbound order.

Shelf life

Provisional 24-month retest interval at -20 degrees C plus or minus 5 degrees C, desiccated. Provisional is meant literally: the interval is carried until this company's own stability program reports. That program places retain vials from every launch lot on 0, 3, 6, 12, 18, 24 and 36-month pulls at -20 degrees C with a parallel 12-month arm at 2-8 degrees C, assayed by the stability-indicating RP-HPLC method. Material reaching retest is re-assayed for purity, water and net peptide content and is either re-dated for a further 12 months or destroyed and recorded on the public failed-lot ledger.