What mass should a Semax certificate show?

PubChem lists Semax, filed as ACTH (4-7), Pro-Gly-Pro, with an average molecular weight of 813.9 g/mol and a monoisotopic mass of 813.35 Da. High-resolution reports usually compare against the monoisotopic value. A companion peak one oxygen atom heavier points to methionine oxidation; a main peak at a clearly different mass suggests a different sequence or an end-modified version.

Is N-acetyl Semax the same molecule as Semax?

No. Acetylating the N-terminus produces Ac-Semax, which researchers have studied as a separate compound with different chemical behaviour, including how it binds copper. It also has a different formula and mass. A certificate for one cannot confirm the other, so check which version the label names before asking for a test.

Is Semax the same as ACTH(4-10)?

No. Semax contains ACTH residues 4 to 7, Met-Glu-His-Phe, followed by a synthetic Pro-Gly-Pro tripeptide. ACTH(4-10) is a longer stretch of the natural hormone, so it is a different peptide with a different mass. Some papers call Semax an ACTH(4-10) analogue, which describes its origin, not a claim that the two molecules are identical.

One of the 30 compound testing profiles

Semax

Semax is a 7-amino-acid peptide (C37H51N9O10S, average mass 813.9 g/mol); a test should confirm that mass, show whether the methionine at position 1 has oxidised, and make sure the vial holds Semax rather than an acetylated variant or a different ACTH fragment.

Semax is a synthetic heptapeptide that joins residues 4 to 7 of adrenocorticotropic hormone (Met-Glu-His-Phe) to a C-terminal Pro-Gly-Pro tripeptide (Magrì et al., 2016).

Test your semax

Identity card

Formula: C37H51N9O10S
Average mass: 813.9 g/mol
Monoisotopic mass: 813.35 Da
Length: 7 residues
Modifications: Free N-terminus; Free C-terminal acid
Also sold as: MEHFPGP, Met-Glu-His-Phe-Pro-Gly-Pro, ACTH(4-7)-Pro-Gly-Pro
Sequence: MEHFPGP
Note: PubChem files Semax under the title "ACTH (4-7), Pro-Gly-Pro-" with the biologic line notation H-Met-Glu-His-Phe-Pro-Gly-Pro-OH. The single sulfur atom in the formula belongs to Met1.

Source: ACTH (4-7), Pro-Gly-Pro (Semax) compound record (CID 9811102).

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

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

Oxidation of methionine at position 1

Semax starts with methionine. Oxidation of amino acid side chains is one of the impurity classes found in peptide medicines (D'Hondt et al., 2014). In antibody work, methionine oxidation is called a particularly important chemical degradation pathway; a solvent-exposed methionine oxidised more readily than a buried one, the oxidised forms were resolved by LC-MS peptide mapping, and oxidation produced separate R and S sulfoxide isomers (Yagi-Utsumi et al., 2026). In Semax, Met1 sits at the free N-terminal end of a short chain, not inside a folded structure. Oxidised Semax is heavier by one oxygen atom, so a mass spectrometer separates it easily, while a UV purity trace shows only an unexplained peak.

Acetylated Semax sold under a similar name

N-terminally acetylated Semax (Ac-Semax) has been studied as a separate compound: acetylation changes how the peptide binds copper(II) and alters its chemical and biological properties (Magrì et al., 2016). The acetyl group also changes the formula, so its mass differs from the 813.9 g/mol listed for Semax in PubChem. A certificate is only useful if its expected mass matches the version printed on the vial.

Loss of terminal residues

With uniformly tritium-labelled Semax, splitting away of Met-Glu and Gly-Pro and the formation of pentapeptides were the predominant processes in the presence of brain cell cultures and membranes (Zolotarev et al., 2006), with the pentapeptide HFPGP and the tripeptide PGP the major products in the presence of nerve cells (Zolotarev et al., 2006, Bioorg Khim). These are biological conditions, but in a vial a peak whose mass matches HFPGP or PGP likewise shows a cut chain. LC-MS can name these fragments; UV purity alone cannot.

Which tests matter most

LC-MSSeparates intact Semax from its Met1-oxidised form and from fragments such as HFPGP by mass, and confirms the vial is not an acetylated variant.
HPLC-UVGives the purity figure; paired with masses, it shows whether small neighbouring peaks are oxidation products or synthesis impurities.
HRMSAccurate mass at the 813.35 Da monoisotopic value makes it easier to distinguish Semax from end-modified versions and oxidised forms.
Counter-ion contentTrifluoroacetate pairs with positively charged groups such as the free N-terminus and His3, adding weight that is not peptide.

Reading semax results

Some papers describe Semax as an analogue of ACTH(4-10) (Glazova et al., 2021; Magrì et al., 2016), but its sequence reproduces ACTH residues 4 to 7 followed by Pro-Gly-Pro. If a label or certificate names an ACTH fragment rather than Semax, the expected mass on the report should be the one for that exact sequence.

Public Tested.ph results

No public semax records yet

Each public semax 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 semax

What mass should a Semax certificate show?: PubChem lists Semax, filed as ACTH (4-7), Pro-Gly-Pro, with an average molecular weight of 813.9 g/mol and a monoisotopic mass of 813.35 Da. High-resolution reports usually compare against the monoisotopic value. A companion peak one oxygen atom heavier points to methionine oxidation; a main peak at a clearly different mass suggests a different sequence or an end-modified version.
Is N-acetyl Semax the same molecule as Semax?: No. Acetylating the N-terminus produces Ac-Semax, which researchers have studied as a separate compound with different chemical behaviour, including how it binds copper. It also has a different formula and mass. A certificate for one cannot confirm the other, so check which version the label names before asking for a test.
Is Semax the same as ACTH(4-10)?: No. Semax contains ACTH residues 4 to 7, Met-Glu-His-Phe, followed by a synthetic Pro-Gly-Pro tripeptide. ACTH(4-10) is a longer stretch of the natural hormone, so it is a different peptide with a different mass. Some papers call Semax an ACTH(4-10) analogue, which describes its origin, not a claim that the two molecules are identical.

Sources

ACTH (4-7), Pro-Gly-Pro (Semax) compound record (CID 9811102), PubChem, US National Library of Medicine
Magrì A et al. Influence of the N-terminus acetylation of Semax on copper(II) and zinc(II) coordination. J Inorg Biochem. 2016;164:59-69, Journal of Inorganic Biochemistry (PubMed)
Yagi-Utsumi M et al. Stereochemical and structural characterization of methionine oxidation in the IgG1 Fc region. Anal Chem. 2026;98(7):5316-5322, Analytical Chemistry (PubMed)
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 YA et al. Degradation of the ACTH(4-10) analog Semax in the presence of rat basal forebrain cell cultures and plasma membranes. Amino Acids. 2006;30(4):403-8, Amino Acids (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)
Glazova NY et al. Semax, synthetic ACTH(4-10) analogue, attenuates behavioural and neurochemical alterations following early-life fluvoxamine exposure in white rats. Neuropeptides. 2021;86:102114, Neuropeptides (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)

Last reviewed 6 Oct 2026.