Selank Peptide: Sequence, Tuftsin Origin, Peptidase Chemistry and Analysis

The Selank peptide is a synthetic heptapeptide, Thr-Lys-Pro-Arg-Pro-Gly-Pro (TKPRPGP), with the formula C33H57N11O9 and an average molar mass of about 751.9 g/mol. It is the natural tetrapeptide tuftsin (Thr-Lys-Pro-Arg) extended at the C-terminus by Pro-Gly-Pro, the same tripeptide that ends its sister peptide Semax. It was developed at the Institute of Molecular Genetics of the Russian Academy of Sciences in Moscow, and in blood plasma it is broken down mainly to the pentapeptide TKPRP, the tripeptide TKP and the dipeptides RP and GP [1].

Selank at a glance

Source: PubChem CID 11765600 for formula, mass and CAS; monoisotopic mass calculated from the formula; references 1 and 2.

Research materials: Vortex supplies Selank as a lyophilised research reagent: Selank peptide 10mg. Lab reports: /coa/.

What Selank is: tuftsin plus Pro-Gly-Pro

Tuftsin is a natural linear tetrapeptide, Thr-Lys-Pro-Arg [3, 4]. It was first described in 1970 as a sequence within the heavy chain of IgG. Its four residues carry two strongly basic side chains, lysine and arginine, which give it a net positive charge at neutral pH.

Selank keeps all four tuftsin residues and adds Pro-Gly-Pro to the C-terminus. That tail is the shared design feature of the peptides developed by the same Moscow institute: Semax joins Pro-Gly-Pro to the ACTH(4-7) fragment, and Selank joins it to tuftsin. The idea in both cases was to add a proline-rich end that peptidases cut less readily. How that design worked out for Semax is covered in our Semax peptide reference.

Chemically, Selank is plain. All seven residues are standard L-amino acids, both ends are unmodified, and there are no disulfides, cyclisation or unusual residues. Papers using the name Selank appear in PubMed from 2001. Earlier work called it a “heptapeptide” tuftsin analogue or used the code TP-7.

Selank structure: three prolines and their isomers

Three of Selank’s seven residues are proline, at positions 3, 5 and 7. The peptide bond on the N-terminal side of a proline (Lys2-Pro3, Arg4-Pro5 and Gly6-Pro7 in Selank) can sit in either the trans or the cis form, and the two interconvert slowly compared with other peptide bonds.

The tuftsin core shows this directly. In a 2D NMR study in DMSO-d6, D’Ursi and colleagues found two families of tuftsin conformations, one with a trans and one with a cis Lys-Pro bond [3]. The cis family was a mixture of extended structures. The trans isomer was a rigid, folded conformer, and conformational calculations pointed to a single structure with a Lys-Pro inverse gamma-turn [3]. A circular dichroism study in water and in organic solvents described a mixture of random coil and beta-turns [4].

No equivalent NMR study of the full Selank heptapeptide appeared in our PubMed search. With three X-Pro bonds, Selank can in principle populate up to eight cis/trans combinations in solution. This matters for analysis, covered below, because slow prolyl isomerisation can change how a peptide peak looks on HPLC.

Selank peptidase chemistry: how it is cut

The degradation papers on Selank come from the same Moscow groups that worked on Semax, and they used the same tool: peptides labelled evenly with tritium across every residue, so that every possible fragment can be traced by HPLC and counted.

Products in blood plasma

Zolotarev and colleagues used this approach to follow Selank and the tripeptide Pro-Gly-Pro in blood plasma [1]. The major Selank products were the pentapeptide TKPRP, the tripeptide TKP and the dipeptides RP and GP [1]. Read as a sequence, that pattern is two steps from the C-terminal end: loss of Gly-Pro gives TKPRP, and loss of Arg-Pro from TKPRP gives TKP. In the same paper, Semax incubated with nerve cells gave mainly the pentapeptide HFPGP and the tripeptide PGP, meaning it lost residues from the N-terminal end instead [1].

Inhibition of serum and plasma peptidases

Selank also acts on the enzymes that degrade other short peptides. Kost and colleagues tested Semax and Selank against enkephalin-degrading enzymes in human serum, a cell-free system [2]. Semax inhibited them with an IC50 of 10 µM and Selank with an IC50 of 20 µM, compared with 10 mM for puromycin. The pentapeptide fragments of both peptides also inhibited the enzymes, while the tri-, tetra- and hexapeptide fragments did not [2].

A follow-up study built an assay on tritium-labelled Leu-enkephalin in human plasma and separated the radioactive fragments by HPLC [5]. In that assay, aminopeptidases accounted for about 80% of the enkephalin-degrading activity, dipeptidyl carboxypeptidases about 10% and dipeptidyl aminopeptidases about 2%, and a carboxypeptidase pathway accounting for about 6% was newly described. Bestatin mainly inhibited the aminopeptidases and carboxypeptidases, while Selank was more specific for the carboxypeptidases and dipeptidyl carboxypeptidases [5].

Source: abstracts of references 1, 2 and 5, checked on PubMed.

Selank variants: N-acetyl Selank and the amidate

Two modified forms are sold under related names. Each is a different molecule from Selank, with its own formula and mass, and a lab report has to say which one was tested.

• N-acetyl Selank (Ac-TKPRPGP-OH). An acetyl group caps the N-terminal amine. It has its own PubChem record, CID 133082488, with the formula C35H59N11O10. Capping the amine removes one positive charge and blocks enzymes that need a free N-terminus.
• “N-acetyl Selank amidate”. The name implies acetylation at the N-terminus and amidation at the C-terminus (-CONH2 in place of -COOH). This form has no PubChem record. The formula below is calculated from that structure, not taken from a database.

Source: PubChem CIDs 11765600 and 133082488 for the first two formulas; masses calculated with standard atomic masses. Acetylation adds C2H2O (+42.011 Da); C-terminal amidation replaces OH with NH2 (−0.984 Da).

The acetylated and the acetylated-amidated forms differ by less than 1 Da, about the spacing of the natural ¹³C isotope peaks, so telling them apart needs adequate mass resolution and attention to the isotope pattern. Selank itself sits 42 Da below N-acetyl Selank, which any mass spectrometer separates easily. On reversed-phase HPLC, capping the N-terminal amine removes a charged group and shifts retention [6].

What the Selank literature looks like

A PubMed search on 24 October 2026 returned 68 records that mention Selank in the title or abstract, 59 of them with Selank in the title. Most published work is rodent pharmacology from a small number of Moscow groups, much of it in Russian-language journals or their English translations. The rest includes the degradation and peptidase-inhibition chemistry above, gene-expression measurements in cell lines and tissue samples, and reviews. This article does not summarise the rodent pharmacology. Its scope is identity, chemistry and molecule-level measurements.

One adjacent line of receptor chemistry concerns the tuftsin core rather than Selank. Von Wronski and colleagues found that tuftsin and the related peptide TKPPR bind selectively to neuropilin-1 and block the binding of vascular endothelial growth factor (VEGF) to that receptor in competition binding experiments [7]. Dimeric and tetrameric forms of TKPPR bound with much higher affinity [7]. That study did not test Selank, and a PubMed search for Selank and neuropilin returned no records, so whether the Pro-Gly-Pro extension changes this binding has not been measured.

Analytical verification of Selank

Identity by mass spectrometry

Electrospray ionisation transfers peptides into the gas phase intact and typically gives multiply charged ions [8]. Selank has three basic sites, the N-terminal amine, the lysine side chain and the arginine side chain, so in acidic LC-MS conditions it can carry up to three protons. Expect [M+H]⁺ at m/z 752.44, [M+2H]²⁺ at m/z 376.72 and [M+3H]³⁺ at m/z 251.49. Tandem MS breaks the backbone into b and y fragment ions. Cleavage on the N-terminal side of proline is typically enhanced, so y-ions beginning at Pro3, Pro5 and Pro7 (PRPGP, PGP and P) are expected to stand out and confirm the C-terminal half of the sequence.

Purity by HPLC

Reversed-phase HPLC with UV detection separates Selank from synthesis-related impurities such as deletion sequences, and from the acetylated variants. Column chemistry, organic solvent and N-terminal acetylation all shift peptide retention and selectivity [6]. Purity is reported as the main peak’s share of total peak area. It is a relative figure and does not measure water or counter-ions. Our explainer on HPLC peptide purity covers how the number is produced.

Proline isomers and peak shape

For peptides that contain proline, interconversion between the cis and trans forms of the prolyl bond during the run can distort peak shapes, and running the column at low temperature, from about −15 to 5 °C, separates the isomeric forms into distinct peaks [9]. With three X-Pro bonds, Selank is a candidate for this effect. A shoulder or broadened main peak whose shape changes with column temperature points to conformers of one molecule rather than a separate impurity, and MS across the peak settles it: conformers share the same mass.

Counter-ions and net content

Because Selank carries three positive charges at acidic pH, a synthetic lot is a salt, usually with acetate or trifluoroacetate as the counter-ion, and the powder also holds some water. These add mass to the powder without adding peptide. That is why a net content assay, which measures the peptide itself, is reported separately from HPLC purity.

Stability chemistry of lyophilised Selank

The main solid-state reactions of peptides are deamidation, peptide-bond cleavage, oxidation, the Maillard reaction, beta-elimination and aggregation, with temperature, moisture and the physical state of the solid as the main drivers [10]. Selank’s composition rules several of these down. It has no asparagine or glutamine, so deamidation is not a concern. It has none of the residues most prone to oxidation, which are methionine, cysteine, histidine, tryptophan and tyrosine [11]. That is a real difference from Semax, which carries a methionine and a histidine.

What remains is mostly hydrolysis and amine chemistry. Selank has two free amines, the N-terminus and the lysine side chain, that can take part in Maillard reactions if reducing sugars are present, and absorbed moisture speeds up both these reactions and peptide-bond cleavage [10]. Cold, dry storage of the sealed solid limits both. The background is in our explainer on lyophilised peptides.

Checking a lot of Selank

The Selank 10 mg report on the Vortex COA page covers lot PS07-SK10. It was issued by ILS Laboratories in San Diego to a wholesale partner facility for that lot and tested on 12 August 2026; the page labels it as a partner-facility report. It lists:

• Purity (HPLC): 99.38%
• Identity (HPLC): confirmed
• Net content: 10.30 mg in a vial labelled 10 mg
• Also listed: heavy metals (ICP-MS), sterility (PCR), endotoxin (USP <85>) and a contaminant screen

Identity on this report is by HPLC, which means the sample’s retention time matched a reference standard. That is a different test from LC-MS, which measures mass directly and is the method that separates Selank from N-acetyl Selank and the amidated form. The report carries a COA number and an access code for the lab’s verification portal, linked from the /coa/ page, and the figures across published reports are compiled at /lab-report-data/. For a field-by-field walk-through, see how to read a peptide COA.

Frequently asked questions

What is Selank?

Selank is a synthetic heptapeptide, Thr-Lys-Pro-Arg-Pro-Gly-Pro, with an average molar mass of about 751.9 g/mol (C33H57N11O9, CAS 129954-34-3, PubChem CID 11765600). It was developed at the Institute of Molecular Genetics of the Russian Academy of Sciences in Moscow. Both ends are unmodified.

How is Selank related to tuftsin and Semax?

Selank is tuftsin (Thr-Lys-Pro-Arg) with Pro-Gly-Pro added to its C-terminus. Semax uses the same Pro-Gly-Pro tail on a different core, the ACTH(4-7) fragment. In blood plasma, Selank is cut mainly from the C-terminal end, giving TKPRP and then TKP.

How does N-acetyl Selank differ from Selank?

N-acetyl Selank carries an acetyl group on the N-terminal amine (C35H59N11O10, PubChem CID 133082488), which adds 42.01 Da to the mass. “N-acetyl Selank amidate” is also amidated at the C-terminus and has no PubChem record. Each is a different molecule from Selank and needs its own identity data.

How is the identity of Selank confirmed?

LC-MS confirms identity by mass: Selank gives [M+H]+ at m/z 752.44, [M+2H]2+ at m/z 376.72 and [M+3H]3+ at m/z 251.49, and tandem MS confirms the sequence. HPLC identity compares retention time with a reference standard. Only a mass measurement separates Selank from its acetylated variants directly.

About this article: Written by the Vortex Research Editorial Team. Sources are peer-reviewed papers indexed in PubMed, chosen for degradation chemistry, peptidase inhibition, conformation, receptor binding and analytical methods. Each PMID was checked against PubMed for title, journal and year, and each sentence citing a paper was matched to its abstract. Identity data come from the PubChem records, quoted without linking. Variant masses were calculated from their formulas. Publication counts come from a PubMed search run on 24 October 2026.

Research materials: Selank peptide 10mg is supplied as a lyophilised research reagent. Published lab reports are at /coa/.

References

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2. Kost NV, Sokolov OIu, Gabaeva MV, Grivennikov IA, Andreeva LA, Miasoedov NF, Zozulia AA. [Semax and selank inhibit the enkephalin-degrading enzymes from human serum]. Bioorg Khim. 2001;27(3):180-183. PMID: 11443939. doi:10.1023/a:1011373002885
3. D’Ursi A, Pegna M, Amodeo P, Molinari H, Verdini A, Zetta L, Temussi PA. Solution conformation of tuftsin. Biochemistry. 1992;31(40):9581-9586. PMID: 1327119. doi:10.1021/bi00155a010
4. Siddiqui MZ, Sharma AK, Kumar S. Solution conformation of tuftsin. Int J Biol Macromol. 1996;19(2):99-102. PMID: 8842772. doi:10.1016/0141-8130(96)01108-7
5. Zolotarev IuA, Sokolov OIu, Kost NV, Vas’kovskiĭ BV, Miasoedov NF, Zozulia AA. [Leu-enkephalin homogeneously labeled with tritium in studying the Selank inhibiting effect on the enkephalin-degrading enzymes of human plasma]. Bioorg Khim. 2004;30(3):234-240. PMID: 15344652. doi:10.1023/b:rubi.0000030126.09208.c3
6. Mant CT, Cepeniene D, Hodges RS. Reversed-phase HPLC of peptides: assessing column and solvent selectivity on standard, polar-embedded and polar endcapped columns. J Sep Sci. 2010;33(19):3005-3021. PMID: 21038458. doi:10.1002/jssc.201000518
7. von Wronski MA, Raju N, Pillai R, et al. Tuftsin binds neuropilin-1 through a sequence similar to that encoded by exon 8 of vascular endothelial growth factor. J Biol Chem. 2006;281(9):5702-5710. PMID: 16371354. doi:10.1074/jbc.M511941200
8. Fenn JB, Mann M, Meng CK, Wong SF, Whitehouse CM. Electrospray ionization for mass spectrometry of large biomolecules. Science. 1989;246(4926):64-71. PMID: 2675315. doi:10.1126/science.2675315
9. Henderson DE, Mello JA. Physicochemical studies of biologically active peptides by low-temperature reversed-phase high-performance liquid chromatography. J Chromatogr. 1990;499:79-88. PMID: 2324223. doi:10.1016/s0021-9673(00)96965-6
10. Lai MC, Topp EM. Solid-state chemical stability of proteins and peptides. J Pharm Sci. 1999;88(5):489-500. PMID: 10229638. doi:10.1021/js980374e
11. Li S, Schöneich C, Borchardt RT. Chemical instability of protein pharmaceuticals: mechanisms of oxidation and strategies for stabilization. Biotechnol Bioeng. 1995;48(5):490-500. PMID: 18623513. doi:10.1002/bit.260480511

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