N-acetyl Semax is Semax with an acetyl cap on its first amino acid, and the form PubChem records also has an amide at the other end. Semax weighs about 813.9; the capped form weighs about 855.0. A mass spectrum tells them apart easily. A purity figure cannot.
- Semax is Met-Glu-His-Phe-Pro-Gly-Pro, C37H51N9O10S, average mass 813.9 (PubChem CID 9811102).
- N-acetyl semax amidate is C39H54N10O10S, average mass 855.0 (PubChem CID 172638603).
- Acetylation adds C2H2O, about 42.01; C-terminal amidation changes the mass by about minus 0.98.
- The acetyl amide sits about 41.03 above Semax on the monoisotopic scale.
- Only a stated formula and an observed mass on the certificate show which form a vial holds.
Two names, two molecules
Two vials sit on a shelf. Both labels say Semax, one with "N-Acetyl" in front. They look identical: white powder, same size vial. The only way to know which is which is to measure the molecule, because the difference is a handful of atoms on the ends of the chain.
Semax is a seven-residue oligopeptide. Its PubChem record, CID 9811102, gives the formula C37H51N9O10S, an average molecular weight of 813.9 and a monoisotopic mass of 813.348. Read from the IUPAC name, the sequence is Met-Glu-His-Phe-Pro-Gly-Pro, with a free amino group on the methionine and a free carboxylic acid on the final proline.
N-acetyl Semax is the same seven residues with an acetyl group, CH3CO, capping that N-terminal amino group. The PubChem record usually reached under that name, CID 172638603, is listed with the synonyms "N-acetyl semax" and "N-acetyl semax amidate". Its IUPAC name shows two changes, not one: the acetamido group at the start and a carboxamide at the C-terminal proline. That detail matters for anyone reading a certificate, and most of this page is about it.
This guide covers structure and identity only. It does not discuss what either compound does.
- Read the stated sequence and end groups
- Match the formula: C37H51N9O10S or C39H54N10O10S
- Compare observed mass with 813.9 or 855.0
- Check ions near m/z 814.4 or 855.4
- Confirm the lot number matches the vial
What does the acetyl cap change chemically?
Acetylation replaces one hydrogen on the N-terminal amine with an acetyl group. The free amine, which carries a positive charge at neutral and acidic pH, becomes a neutral amide. Written in IUPAC-style shorthand, the peptide goes from H-Met-Glu-His-Phe-Pro-Gly-Pro-OH to Ac-Met-Glu-His-Phe-Pro-Gly-Pro-OH.
The net effect on the formula is an addition of C2H2O. On the monoisotopic scale that is 42.011; on the average scale, about 42.04. The sequence, the side chains and the peptide bonds are untouched. Losing the charged N-terminus makes the molecule a little less polar, so on a reversed-phase HPLC column the acetylated form would usually be expected to elute somewhat later than the parent under the same conditions. Retention differs between methods, so that is a direction, never a number to rely on.
C-terminal amidation is a separate change. The terminal carboxylic acid, -COOH, becomes a carboxamide, -CONH2. On the formula that swaps one oxygen for an NH, a net change of minus O plus N plus H, which is about minus 0.984 monoisotopic. It removes the negative charge the free acid carries at neutral pH.
The mass arithmetic
Because both edits are small and exact, the expected masses can be calculated from the Semax record without a reference sample. The table sets out the three forms a reader is likely to meet.
| Form | Formula | Average mass | Monoisotopic mass | Basis |
|---|---|---|---|---|
| Semax, free acid | C37H51N9O10S | 813.9 | 813.348 | PubChem CID 9811102 |
| N-acetyl Semax, free acid | C39H53N9O11S | about 855.9 | about 855.358 | Calculated: Semax plus C2H2O |
| N-acetyl Semax amide ("amidate") | C39H54N10O10S | 855.0 | 854.375 | PubChem CID 172638603 |
The step from Semax to the acetyl amide is about 41.03 on the monoisotopic scale, the acetyl gain of 42.01 minus the amide's 0.98. The acetylated free acid and the acetylated amide sit about 1 apart. That gap is small, but a mass spectrometer resolves it without trouble, and the isotope pattern would also shift, because the amide adds a nitrogen and removes an oxygen.
A certificate that states "N-acetyl Semax" and reports a mass near 855 has said which form is present. One that states the name with no mass has not. The guide to molecular formula and weight explains the difference between average and monoisotopic figures if the two columns are unfamiliar.
Why is the name alone not enough?
The PubChem synonym list shows the problem plainly: "N-acetyl semax" and "N-acetyl semax amidate" point at the same record, and that record is the doubly modified molecule. In supplier listings the two names are often used interchangeably, and sometimes "N-acetyl Semax" is used for material whose C-terminus was never amidated. The name on a label is a claim about the molecule. It is not evidence.
The practical consequence is that three different compounds can reasonably sit behind two names. Only the certificate, with a formula, an expected mass and an observed mass, closes the gap. If a document gives a formula of C39H54N10O10S, it is describing the amide. If it gives C39H53N9O11S, it is describing the acetylated free acid. If it gives no formula at all, ask for one.
For comparison, a reader looking at Semax itself, SKU XA5 at 5 mg or XA10 at 10 mg on the Semax product page, should expect a document written against C37H51N9O10S and 813.9. That page sells the unmodified peptide. It does not sell either N-acetyl form.
How does a certificate tell them apart?
Mass spectrometry does the work. Electrospray ionization typically shows a small peptide like this as its protonated ion and, often, a doubly charged ion as well. For Semax, the singly protonated ion would appear near m/z 814.4 and the doubly charged ion near 407.7. For the acetyl amide, the corresponding ions would appear near 855.4 and 428.2. Those values follow from the monoisotopic masses in the table plus the mass of one or two protons.
Chromatography supports the mass result. A good report attaches the HPLC trace, states the column and gradient, and names the wavelength. If a sample were a mix of acetylated and unacetylated material, the two would usually appear as separate peaks, and the mass of each peak would show which is which.
Fragmentation adds a further check. In tandem mass spectrometry, fragments that keep the N-terminus carry the extra 42 of the acetyl group, and fragments that keep the C-terminus carry the amide's shift. That places each modification on the correct end rather than just confirming the total. The Semax identity guide covers the oxidized methionine form, sixteen units heavier, which applies to all three molecules because each keeps the same methionine.
Purity is the result that cannot help here. A 99 percent HPLC area figure for N-acetyl Semax is equally consistent with any of the three forms, because it measures how much of the signal sits in one peak, not what that peak is.
Common errors when comparing the two
The most frequent error is reading the right document against the wrong target. A certificate for N-acetyl Semax amide that reports an observed mass near 855 is consistent with its label. The same number read by someone expecting Semax looks like a failure 41 units heavy. Before judging any result, write down which formula the label claims and calculate the expected mass from it.
A second error is mistaking an oxidized product for a modified one. Methionine sulfoxide adds 16 to whichever form is present. Semax plus 16 sits near 829.9 average, well away from 855, so the two are easy to separate by mass, but only if someone does the arithmetic.
A third is assuming acetylation can be seen by eye or by HPLC purity alone. It cannot. Both forms are white lyophilized powders, and both can show a single tall peak. The mass is the only routine measurement that separates them cleanly.
Reading the document for either compound
A short checklist covers most of it. The certificate should name the compound and give its sequence with any end groups written out, such as Ac- and -NH2. It should state a molecular formula and a theoretical mass matched to that formula. It should report an observed mass from a named method and attach the spectrum or at least the main ions. It should report HPLC purity with the method stated. And it should carry a lot number that matches the vial.
For Semax, the Semax certificate guide goes through each row, and the Semax storage guide covers the sealed lyophilized vial and laboratory solutions. Our certificates are published lot by lot on the certificate page; a product page shows results only once the certificate for that lot is published. Readers comparing two related neuropeptides can also see the Selank and Semax comparison.
FOR LABORATORY AND IN-VITRO RESEARCH USE ONLY. NOT FOR HUMAN OR ANIMAL CONSUMPTION.
What is Semax studied for?
Published research on Semax investigates the areas below, which is a different question from what Semax will do for anyone, a claim about a living system that nothing on this site is sold for.
What it is. A man-made peptide seven building blocks long, based on a fragment of ACTH.
What the research looks at. Animal research coming largely from a narrow set of research groups.
How it is thought to work. Built from a piece of adrenocorticotropic hormone with a short tail added for durability, and it does not carry the parent hormone’s effect on cortisol. How it works is not settled.
What is not established. No approved product in the United States and no official standard. As with Selank, the published record is concentrated and thinner than the volume of commercial writing suggests.
The full record, including the certificate for the lot in stock, is on the Semax product page.
Common questions
What is the difference between N-acetyl Semax and Semax?
How much heavier is N-acetyl Semax than Semax?
Is N-acetyl Semax amidate the same as N-acetyl Semax?
Can HPLC purity show which form is in the vial?
Published certificates for Semax
Every figure below is read from a report the laboratory issued for that lot; each page carries the PDF and the lab's own verification link.
More science guides
Sources
- PubChem Compound Summary for CID 9811102, Semax. Formula C37H51N9O10S, average mass 813.9, monoisotopic mass 813.348, and the IUPAC name from which the Met-Glu-His-Phe-Pro-Gly-Pro order is read.
- PubChem Compound Summary for CID 172638603, N-acetyl semax amidate. Formula C39H54N10O10S, average mass 855.0, monoisotopic mass 854.375; synonyms include both 'N-acetyl semax' and 'N-acetyl semax amidate', and the IUPAC name shows an acetamido N-terminus and a C-terminal carboxamide.
- IUPAC-IUB Joint Commission on Biochemical Nomenclature, 3AA-11 and 3AA-13: peptides. Defines a peptide, gives the 'fewer than about 10-20 residues' oligopeptide boundary, and the rough 50-residue point where authors start to say protein.
