An analyst running an LC-MS identity check on a Semax sample finds the expected signal for the intact peptide, but also a smaller companion 16 daltons heavier. On the UV trace, the main peak has a slight shoulder on its front edge. Neither observation means the sample is the wrong compound. Both point to one specific residue in the sequence, and understanding that residue makes Semax far easier to characterize.
Where the sequence comes from
Semax is the heptapeptide Met-Glu-His-Phe-Pro-Gly-Pro, often written MEHFPGP. It is usually described as an analog of the ACTH(4–10) region of adrenocorticotropic hormone, and that description needs a small correction.
| Position | ACTH residue | Semax residue |
|---|---|---|
| 4 | Met | Met |
| 5 | Glu | Glu |
| 6 | His | His |
| 7 | Phe | Phe |
| 8 | Arg | Pro |
| 9 | Trp | Gly |
| 10 | Gly | Pro |
Only the first four residues are shared with ACTH. The last three, Arg-Trp-Gly, are replaced by the synthetic tripeptide Pro-Gly-Pro. The most precise shorthand is therefore ACTH(4–7) with a Pro-Gly-Pro extension, sometimes written ACTH(4–7)-PGP. Calling it ACTH(4–10) names the region it was derived from rather than the residues it contains.
The role of Pro-Gly-Pro
The C-terminal tripeptide is a stabilizing element rather than a copy of the parent sequence. Proline’s ring constrains the local backbone and makes adjacent bonds poor substrates for many exopeptidases, which otherwise trim short linear peptides from their ends. In enzymatic stability studies, that is why the tail is of interest.
The same Pro-Gly-Pro tail appears on Selank, which is built on a completely different parent, the tetrapeptide tuftsin. The shared tail is a common engineering choice, not evidence that the two compounds are related in sequence. Profiles of each are available in what is Semax and what is Selank.
Methionine oxidation: the 16-dalton signal
Semax starts with methionine, and the sulfur in methionine’s thioether side chain oxidizes readily to methionine sulfoxide. That reaction adds a single oxygen atom, 16 Da. More aggressive oxidation can take it further to the sulfone, adding 32 Da in total, though the sulfoxide is the form most often seen.
A signal 16 Da above the expected mass is therefore the first thing to look for in a Semax mass spectrum. It is a distinct chemical species from the labeled compound. It can arise from exposure to air, light, or peroxides present as contaminants in solvents, and it can form after manufacture as well as during it. A clean result on one date does not guarantee the absence of oxidized material at a later date; oxidation is a property of the sample’s history, not only of its synthesis.
Identity values worth matching
- Molecular formula: C37H51N9O10S (the single sulfur atom belongs to the methionine).
- Average molecular weight: approximately 813.9 g/mol.
- Methionine sulfoxide form: approximately 829.9 g/mol.
- CAS registry number: 80714-61-0.
Product names vary widely between catalogs, while the registry number and formula do not, so these are the values to compare across listings. How measured masses are used to confirm identity is described in mass spectrometry and peptide identity.
What losing tryptophan does to detection
The substitution at position 9 has a practical consequence for measurement. Tryptophan is the strongest ultraviolet absorber among the standard amino acids, and Semax has none. Its only aromatic residue is phenylalanine, which absorbs weakly near 280 nm. A purity measurement made at 280 nm would see the peptide poorly and would barely see non-aromatic impurities at all, so a high figure at that wavelength says more about the detector setting than about the sample.
For this sequence, purity belongs at a low wavelength such as 214 or 220 nm, where the peptide bond itself absorbs and every peptide-containing species responds. Removing tryptophan also removes one of the residues most prone to oxidation, which leaves methionine as the main oxidative weak point.
What the chromatogram looks like
Semax is small and fairly polar, with glutamate and histidine side chains, so it elutes relatively early on a reverse-phase column. The sulfoxide is more polar than the parent and elutes a little earlier still. Because the two forms sit close together, oxidation often shows up as a leading shoulder on the main peak rather than a separate, baseline-resolved impurity. A shallow gradient across the elution window tends to pull the two apart; a steep gradient can hide the oxidized form inside the main peak. Reading shoulders like this is covered in how to read an HPLC chromatogram.
Why a co-eluting oxidized form distorts purity
If the sulfoxide is not resolved from the parent, its area is counted as part of the main peak, and the reported purity overstates how much unoxidized peptide is present. The error is invisible on the UV trace alone. There is a further subtlety: oxidizing the sulfur creates a new stereocenter, so methionine sulfoxide exists as two diastereomers. Under favorable conditions these can separate slightly from each other, which is why the oxidized material sometimes appears as a small doublet rather than a single shoulder. Either pattern is a reason to look more closely rather than to accept the headline figure.
A reading checklist for Semax data
- Confirm the detection wavelength is in the low-UV range.
- Look at the front edge of the main peak for a shoulder.
- If mass data are available, compare the observed mass with about 813.9 and check for a signal about 16 Da higher.
- Match the CAS number and formula rather than the product name.
Battle Born publishes an independent reverse-phase HPLC purity result for each product, including Semax, and matches each vial to that result by crimp and cap color. Mass confirmation, where a project requires it, is something a laboratory can commission separately.
Questions
Is Semax a fragment of ACTH?
Partly. Its first four residues match ACTH positions 4 to 7, but the remaining three are a synthetic Pro-Gly-Pro extension rather than ACTH sequence.
What does a +16 Da peak mean for Semax?
It indicates methionine sulfoxide, the product of adding one oxygen atom to the methionine side chain.
Why is 280 nm a poor wavelength for Semax purity?
Semax contains no tryptophan or tyrosine, only one phenylalanine, so its absorbance at 280 nm is weak and many impurities would be missed.
Where does the oxidized form elute?
Slightly before the parent peptide on reverse-phase HPLC, often as a shoulder on the front of the main peak.
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