What mass should a Selank certificate report?

PubChem gives Selank an average molecular weight of 751.9 g/mol and a monoisotopic mass of 751.43 Da. A high-resolution result should land close to the monoisotopic figure. Because the peptide carries several basic groups, the raw spectrum may also show a doubly charged ion near half that value, which should convert back to the same neutral mass.

Is N-acetyl Selank the same as Selank for testing purposes?

No. Adding an acetyl group to the N-terminus changes the formula and the mass, so it is a different molecule for identity testing. The PubChem record for Selank describes the unmodified heptapeptide. Tell the laboratory which version the label claims, so the expected mass on the report matches the product you actually bought.

What would a des-Pro impurity look like on a Selank report?

It would appear as a separate peak with a mass lower by one proline residue. Selank has three prolines, and deletion sequences are a known class of synthesis impurity, so a chain missing one is a plausible by-product. HPLC alone shows the peak but not its identity, while LC-MS assigns it a mass and lets the report name it as a deletion.

Why does one Selank report list acetate and another trifluoroacetate?

Trifluoroacetate comes from synthesis and HPLC purification, and a 2025 study advises that replacing it with chloride or acetate should always be considered for synthetic peptides. The counter-ion does not change the Selank molecule, but it changes the weight of the salt, so two vials with the same label weight can hold different amounts of peptide.

One of the 30 compound testing profiles

Selank

Selank is a 7-amino-acid peptide (C33H57N11O9, average mass 751.9 g/mol); a test should confirm that mass, check that the three prolines and the Lys and Arg are all in place, and report how much of the weighed powder is counter-ion rather than peptide.

Selank is a synthetic heptapeptide, Thr-Lys-Pro-Arg-Pro-Gly-Pro, that the research literature describes as a peptide analogue of tuftsin (Konstantinopolsky et al., 2022).

Test your selank

Identity card

Formula: C33H57N11O9
Average mass: 751.9 g/mol
Monoisotopic mass: 751.43 Da
Length: 7 residues
Modifications: Free N-terminus; Free C-terminal acid
Also sold as: TKPRPGP, Thr-Lys-Pro-Arg-Pro-Gly-Pro
Sequence: TKPRPGP
Note: PubChem lists the biologic line notation H-Thr-Lys-Pro-Arg-Pro-Gly-Pro-OH. Products sold as N-acetyl Selank or with other end modifications are different molecules with different masses.

Source: Selank compound record (CID 11765600).

Check a certificate's mass against these values with the mass checker, prefilled for Selank.

What usually goes wrong with selank. Documented failure modes, with sources.

Counter-ions bound to Lys2, Arg4 and the N-terminus

Selank carries three basic sites: the free N-terminal amine, Lys2 and Arg4. Trifluoroacetate used in purification pairs with positively charged groups such as the N-terminus, arginine, histidine and lysine, so lyophilisation yields a peptide-TFA salt, and the salt forms introduce a weighting error because they differ in molecular weight (Erckes et al., 2025). A Selank vial can therefore be the right molecule at high purity and still hold noticeably less peptide than its label weight.

Deletion sequences in a proline-rich chain

Three of Selank's seven residues are proline. Synthesis-related impurities in peptide medicines include deletion sequences from inefficient Fmoc deprotection and insertion sequences from excess amino acid reagent (D'Hondt et al., 2014). A chain missing one proline (a des-Pro impurity) is a hexapeptide lighter by one proline residue. HPLC purity alone does not name such a peak; LC-MS gives it a mass.

Acetylated and modified versions under the same name

Selank is sold alongside modified versions such as N-acetyl Selank. The PubChem record describes only the unmodified heptapeptide with a free N-terminus and a free C-terminal acid (PubChem, Selank). An N-terminal acetyl group changes the formula and the mass, so the buyer and the laboratory need to agree which molecule the label claims before testing, or a correct result can read as a failure.

Shorter fragments of the chain

In a study using Selank labelled with tritium at every residue, the major products of its breakdown in blood plasma were the pentapeptide TKPRP, the tripeptide TKP and the dipeptides RP and GP (Zolotarev et al., 2006). Those were biological conditions, not a vial on a shelf, but they give the masses to look for: a vial peak matching TKPRP or TKP shows a cut chain, and only a method that assigns masses to peaks can show it.

Which tests matter most

LC-MSConfirms the 751.43 Da monoisotopic mass and assigns masses to neighbouring peaks, separating des-Pro deletions from acetylated variants and fragments.
HPLC-UVGives the purity figure and shows how many related peaks sit near the main Selank peak.
Counter-ion contentWith three basic sites on a small peptide, the counter-ion share of the powder decides how much actual Selank a milligram holds.
Amino acid analysisMeasures peptide content directly and checks the expected ratio of threonine, lysine, arginine, glycine and three prolines.

Reading selank results

Because peptide side chains can be basic, acidic or neutral, peptide drugs often occur as salts with different counter-ions, which affect structure and physicochemical properties (Sikora et al., 2020). For Selank, a report that gives purity but no counter-ion or net peptide content has not answered how much Selank is in the vial. See net peptide content and counterions.

Public Tested.ph results

No public selank records yet

Each public selank record will appear here with its identity, HPLC purity and measured amount against the label, once the first vial is tested.

FIG 13Amount against the label

Questions about testing selank

What mass should a Selank certificate report?: PubChem gives Selank an average molecular weight of 751.9 g/mol and a monoisotopic mass of 751.43 Da. A high-resolution result should land close to the monoisotopic figure. Because the peptide carries several basic groups, the raw spectrum may also show a doubly charged ion near half that value, which should convert back to the same neutral mass.
Is N-acetyl Selank the same as Selank for testing purposes?: No. Adding an acetyl group to the N-terminus changes the formula and the mass, so it is a different molecule for identity testing. The PubChem record for Selank describes the unmodified heptapeptide. Tell the laboratory which version the label claims, so the expected mass on the report matches the product you actually bought.
What would a des-Pro impurity look like on a Selank report?: It would appear as a separate peak with a mass lower by one proline residue. Selank has three prolines, and deletion sequences are a known class of synthesis impurity, so a chain missing one is a plausible by-product. HPLC alone shows the peak but not its identity, while LC-MS assigns it a mass and lets the report name it as a deletion.
Why does one Selank report list acetate and another trifluoroacetate?: Trifluoroacetate comes from synthesis and HPLC purification, and a 2025 study advises that replacing it with chloride or acetate should always be considered for synthetic peptides. The counter-ion does not change the Selank molecule, but it changes the weight of the salt, so two vials with the same label weight can hold different amounts of peptide.

Sources

Selank compound record (CID 11765600), PubChem, US National Library of Medicine
Konstantinopolsky MA et al. Selank, a peptide analog of tuftsin. Bull Exp Biol Med. 2022;173(6):730-733, Bulletin of Experimental Biology and Medicine (PubMed)
Erckes V et al. Towards a consensus for the analysis and exchange of TFA as a counterion in synthetic peptides. Pharmaceuticals. 2025;18(8):1163, Pharmaceuticals (PMC)
Sikora K et al. The role of counter-ions in peptides, an overview. Pharmaceuticals. 2020;13(12):442, Pharmaceuticals (PMC)
D'Hondt M et al. Related impurities in peptide medicines. J Pharm Biomed Anal. 2014;101:2-30, Journal of Pharmaceutical and Biomedical Analysis (PubMed)
Zolotarev IuA et al. Evenly tritium-labeled peptides and their in vivo and in vitro biodegradation. Bioorg Khim. 2006;32(2):183-91, Bioorganicheskaia Khimiia (PubMed)

Last reviewed 6 Oct 2026.