Semax, Selank and Their N-Acetyl Amidates: Sequences, Mass Differences, CAS and Salt Form

N-Acetyl Semax Amidate is Semax with two covalent end-group changes, an N-terminal acetyl and a C-terminal amide, which add C2H3N and raise the listed molecular weight from 813.90 to 854.98 g/mol. Both share the same seven residues, Met-Glu-His-Phe-Pro-Gly-Pro, and the formula moves from C37H51N9O10S to C39H54N10O10S. Semax acetate is a different thing: "acetate" names a counter-ion held in the salt, not a bond, so Semax acetate keeps the Semax formula and the Semax mass line (calculated [M+H]+ m/z 814.4). Selank and N-Acetyl Selank Amidate differ from each other in exactly the same way. Pepta Labs lists all four as separate 10 mg vials: Semax from $27.74, N-Acetyl Semax Amidate 10mg from $43.46, Selank from $33.24 and N-Acetyl Selank Amidate 10mg from $48.09. This page sets out the sequences, the mass arithmetic, the salt names and the registry numbers for each.

What is the difference between Semax and N-Acetyl Semax Amidate?

Semax and N-Acetyl Semax Amidate are not the same compound: Semax is H-Met-Glu-His-Phe-Pro-Gly-Pro-OH, C37H51N9O10S, 813.90 Da, CAS 80714-61-0, and N-Acetyl Semax Amidate is Ac-Met-Glu-His-Phe-Pro-Gly-Pro-NH2, C39H54N10O10S, 854.98 Da. The two share one residue sequence and differ only at the two ends: Semax has a free N-terminal amine and a free C-terminal carboxylic acid, and N-Acetyl Semax Amidate carries an N-terminal acetyl group and a C-terminal primary amide. Both changes are covalent, so the two are separate chemical entities with separate formulas, separate masses and separate listings; one is not a grade, salt or purer version of the other. Every figure in the table is either the value printed on the product listing (checked 2026-09-24) or calculated here from the formula and labelled as calculated.

Methionine is the only sulfur-containing residue in either sequence, which is why both formulas carry a single S. The one-letter code MEHFPGP and the H-/-OH and Ac-/-NH2 end markers follow the conventions set out in the peptide sequence notation guide.

How much heavier is N-Acetyl Semax Amidate, and where does the 41 come from?

N-Acetyl Semax Amidate is heavier than Semax by exactly C2H3N: +41.027 Da on monoisotopic masses and +41.05 g/mol on average masses (both calculated). The net change is the sum of two steps. Acetylation adds C2H2O (+42.011 Da) to the N-terminal nitrogen of methionine. Amidation swaps the C-terminal -OH for -NH2, which removes one oxygen and adds one nitrogen and one hydrogen (-0.984 Da). The table walks through the two steps in order.

The listing figures, 813.90 and 854.98, differ by 41.08 rather than 41.05. The extra 0.03 is rounding, not chemistry: calculated with standard atomic weights (C 12.011, H 1.008, N 14.007, O 15.999, S 32.06), C37H51N9O10S comes to 813.93 g/mol, and 813.90 and 813.93 both round to 813.9. For N-Acetyl Semax Amidate the listing value and the calculated value agree at 854.98.

Two quantities should not be mixed. An average molecular weight (813.90, 854.98) belongs beside a formula and is what a balance weighs; a monoisotopic mass (813.35, 854.37) plus the mass of a proton is what a mass spectrum shows as [M+H]+ (814.4, 855.4). Adding 42.01 and subtracting 0.98 from an average weight gives a figure that matches neither column. The expected-mass table applies the same calculation to other catalog compounds.

Is Semax acetate the same as N-acetyl Semax?

No. "Semax acetate" is Semax isolated as an acetate salt: the peptide is still H-Met-Glu-His-Phe-Pro-Gly-Pro-OH (C37H51N9O10S), and acetate ions (from acetic acid, C2H4O2) sit beside it as counter-ions paired with its protonated basic groups. "N-acetyl" means an acetyl group covalently bonded to the N-terminal nitrogen, which is part of the peptide's own formula. The two words share a root and nothing else.

The query "n-acetyl-semax acetate" therefore names a peptide with an acetylated N-terminus isolated as its acetate salt: two separate uses of the acetyl unit, one covalent and one ionic. Pepta Labs lists two Semax items only, Semax (free termini) and N-Acetyl Semax Amidate (both ends modified); the acetylated-only and amidated-only rows above are included because their masses are the ones to rule out on a report.

What does the counter-ion add to the weight of the powder?

A counter-ion adds weight to the solid but not to the peptide. Acetic acid is C2H4O2 (60.05 g/mol) and trifluoroacetic acid is C2HF3O2 (114.02 g/mol), so each counter-ion paired with one peptide molecule adds that much to the formula weight of the salt. How many counter-ions a salt carries depends on how many basic groups the peptide has and on the final purification step; the Semax and Selank listings state the salt family ("typically trifluoroacetate or acetate salt") but not a count, and the two N-acetyl amidate listings do not state a salt form. The figures below show one counter-ion per peptide molecule, as arithmetic only.

None of these salt weights appears on a mass spectrum. In the instrument the counter-ion separates from the peptide, so Semax acetate and Semax TFA salt both show the Semax ion at m/z 814.4, and only the covalent change to N-Acetyl Semax Amidate moves it to 855.4. The share of a vial's powder that is peptide rather than counter-ion and water is net peptide content, which is not reported per listing; the TFA vs acetate guide covers how the salt form changes that share.

Which salt names are used for Selank?

Selank appears under several names that all describe the same heptapeptide, H-Thr-Lys-Pro-Arg-Pro-Gly-Pro-OH (C33H57N11O9): Selank, TP-7, Selank acetate, Selank diacetate and Selank TFA salt. TP-7 is a research code; the acetate, diacetate and TFA names add the counter-ion of the salt, not a change to the peptide. The Pepta Labs listing gives the salt form as "typically supplied as trifluoroacetate or acetate salt (a diacetate form is also catalogued)".

The calculated 751.89 and the listed 751.87 both round to 751.9 g/mol; the 0.02 gap is rounding. Every salt name in the table shows the same peptide ion, m/z 752.4, because the counter-ions do not travel with the peptide in the mass spectrometer. Only the N-Acetyl Selank Amidate row carries a different peptide.

On sequence: the first four residues of Selank, Thr-Lys-Pro-Arg, are the tetrapeptide catalogued as tuftsin (C21H40N8O6, calculated 500.60 g/mol); Selank is that tetrapeptide extended at the C-terminus by Pro-Gly-Pro, and it carries its own CAS number, 129954-34-3.

What is the difference between Selank and N-Acetyl Selank Amidate?

Selank and N-Acetyl Selank Amidate are not the same compound: Selank is H-Thr-Lys-Pro-Arg-Pro-Gly-Pro-OH, C33H57N11O9, 751.87 Da, CAS 129954-34-3, and N-Acetyl Selank Amidate is Ac-Thr-Lys-Pro-Arg-Pro-Gly-Pro-NH2, C35H60N12O9, 792.94 Da. The two share the sequence Thr-Lys-Pro-Arg-Pro-Gly-Pro (one-letter TKPRPGP) and differ only at the ends: Selank has a free N-terminal amine and a free C-terminal acid, and N-Acetyl Selank Amidate is acetylated at threonine's N-terminal nitrogen and amidated at the C-terminal proline. The net change is the same +C2H3N as for the Semax pair.

Step by step (calculated): Selank, 751.4341 Da monoisotopic; plus the N-terminal acetyl (+C2H2O), 793.4446 Da; with the C-terminal -OH changed to -NH2, 792.4606 Da. The net shift is +41.0265 Da, and on average masses 751.89 becomes 792.94 g/mol (+41.05). With three basic groups in Selank and two in the amidate, both can form a doubly charged ion, so a report can print the [M+2H]2+ value instead of [M+H]+; the [M+3H]3+ values in the table are arithmetic for reports that list a higher charge state.

Which CAS number belongs to which listing?

CAS 80714-61-0 belongs to Semax and CAS 129954-34-3 belongs to Selank, each in its free-termini form (H-...-OH). A CAS registry number is tied to one exact structure, so neither number covers the acetylated, amidated forms, whose formulas differ from the parents by C2H3N.

The two N-acetyl amidate listings identify their compounds by sequence, formula and molecular weight, and the mass line on the analytical report ties the sampled production run to that formula. A listing elsewhere that prints 80714-61-0 against N-Acetyl Semax Amidate is using the Semax number for a molecule 41 Da heavier. A listing that prints a separate number for either amidate should be able to name the registry record it comes from; check that record's formula against C39H54N10O10S or C35H60N12O9 before relying on it. The CAS numbers and molecular weights reference and the registry discrepancies guide cover how such mismatches arise.

What should the mass line on a report show for each listing?

The mass line is the part of an analytical report that separates a parent peptide from its N-acetyl amidate, because each pair has the same residues and a purity percentage alone does not show which of the two was tested. Match the observed ion on the report to the row for the listing, allowing for the instrument's stated tolerance.

All values are calculated from the monoisotopic masses (C 12.000000, H 1.0078250, N 14.0030740, O 15.9949146, S 31.9720707) plus the mass of one or two protons (1.0072765) or one sodium ion; the observed value for the sampled production run is the one printed on the report.

Which ions point to the wrong compound?

• A report for the N-Acetyl Semax Amidate listing that shows [M+H]+ near 814.4 describes unmodified Semax.
• An ion near 856.4 describes Semax that is acetylated but not amidated (+42.01 with no -0.98).
• An ion near 813.4 describes Semax that is amidated but not acetylated. It sits about 1 m/z below Semax itself, so the report's resolution matters.
• A report for the N-Acetyl Selank Amidate listing that shows [M+H]+ near 752.4 describes unmodified Selank; an ion near 794.5 is acetylated-only Selank, and one near 751.5 is amidated-only Selank.
• For either Semax form, an extra ion 16 m/z above the expected value (near 830.4 for Semax, 871.4 for the amidate) corresponds to methionine sulfoxide, the same peptide with one added oxygen.
• A sodium adduct about 22 m/z above [M+H]+ is common in electrospray spectra and does not indicate a different compound.

What Pepta Labs represents for these four listings is "Independent third-party HPLC purity analysis and mass-spectrometry identity confirmation, commissioned through the purchasing group on a sampled production run from the source. Pepta Labs does not test in-house." "Sterility, endotoxin or LAL, potency, pharmaceutical-grade, cGMP, contaminant-panel and vial fill-quantity testing are not claimed." How to read every section of such a report is covered in the CoA verification guide, and what each instrument can and cannot show is in HPLC vs mass spectrometry.

What is the FDA listing status of Semax and Selank?

On the FDA compounding page that lists Category 2 substances and withdrawn nominations (content current as of April 22, 2026), Semax appears as "Semax (heptapeptide)" and Selank as "Selank acetate (TP-7)", both in the table of substances that were previously in Category 2 and whose nominations were withdrawn by the nominators. Neither N-acetyl amidate form is named on that page. The page concerns compounding pharmacies; the FDA Category 2 peptides page gives the status of every compound named in the September 2023 action, and peptide legal status in 2026 covers research-use labelling.

Where are Semax and Selank listed after the Science.bio closure?

Science.bio's storefront was open in a January 7, 2026 archive capture and showed a permanent-closure notice in a February 8, 2026 capture; on September 24, 2026 its site still showed only that notice. The Pepta Labs switching guide matches the Science.bio item names Semax and Selank by compound name to the Semax and Selank 10 mg listings; Pepta Labs is not affiliated with Science.bio and does not sell its inventory. The dated timeline is in Science.bio closed permanently, and the match for every closed vendor is on the switching page.

Where are price, storage, lot and report requests covered?

Each topic has one page that owns it, and this guide links there rather than repeating it.

• Price and packs: on each product page: Semax from $27.74, N-Acetyl Semax Amidate from $43.46, Selank from $33.24, N-Acetyl Selank Amidate from $48.09. Per-mg comparisons across the catalog are on the price per mg page.
• Storage of sealed vials: the peptide storage guide.
• Lot numbers: Vial labels identify the compound, quantity and research-use designation. They do not carry lot or batch numbers.
• Analytical report: Sent by email on request, before you order if you want to see it first, through the report request form or the panel on each product page. The quality page sets out what testing is and is not represented.
• What a research peptide listing is: the research peptides overview.

Frequently asked questions

Is N-Acetyl Semax Amidate the same as Semax?

No. N-Acetyl Semax Amidate (Ac-Met-Glu-His-Phe-Pro-Gly-Pro-NH2) carries two covalent end-group changes, an N-terminal acetyl and a C-terminal amide, so its formula is C39H54N10O10S and its listed molecular weight 854.98 Da, against C37H51N9O10S and 813.90 Da for Semax. The calculated [M+H]+ moves from m/z 814.4 to 855.4.

Is Semax acetate the same as N-acetyl Semax?

No. Semax acetate is Semax (C37H51N9O10S) isolated as an acetate salt, with acetate as a non-covalent counter-ion, so its peptide formula and its [M+H]+ of m/z 814.4 are those of Semax. N-acetyl Semax has an acetyl group bonded to the N-terminal nitrogen, which changes the peptide's own formula.

Which CAS number belongs to which listing?

CAS 80714-61-0 belongs to Semax and CAS 129954-34-3 to Selank, each with free termini. The N-Acetyl Semax Amidate and N-Acetyl Selank Amidate listings print no CAS number, and a listing that gives either amidate its parent's number is using a number for a molecule about 41 Da lighter.

Does N-Acetyl Selank Amidate have a CAS number?

No CAS number or PubChem record was located for N-Acetyl Selank Amidate (Ac-Thr-Lys-Pro-Arg-Pro-Gly-Pro-NH2, C35H60N12O9, 792.94 Da), and the listing prints none. Its identity rests on the formula, the molecular weight and the mass line on the analytical report, calculated [M+H]+ m/z 793.5.

Is N-Acetyl Selank Amidate the same as Selank?

No. N-Acetyl Selank Amidate (Ac-Thr-Lys-Pro-Arg-Pro-Gly-Pro-NH2) carries an N-terminal acetyl group and a C-terminal amide, two covalent changes, so its formula is C35H60N12O9 and its listed molecular weight 792.94 Da, against C33H57N11O9, 751.87 Da and CAS 129954-34-3 for Selank. The calculated [M+H]+ moves from m/z 752.4 to 793.5.

Is selank diacetate the same compound as Selank?

Yes, as a peptide. Selank diacetate is Selank (C33H57N11O9) isolated with two acetate counter-ions, a salt with a calculated formula weight of 871.99 g/mol against 751.89 g/mol for the free peptide. The peptide ion on a mass spectrum is the same, m/z 752.4.

What should the mass line on a report for each listing show?

Calculated [M+H]+ is m/z 814.4 for Semax, 855.4 for N-Acetyl Semax Amidate, 752.4 for Selank and 793.5 for N-Acetyl Selank Amidate. A report for the N-Acetyl Semax Amidate listing that shows 814.4 describes unmodified Semax, and one near 856.4 describes Semax that is acetylated but not amidated.

All products are supplied for laboratory research use only and are not for human or animal use.