What Is Semax? Structure and Chemistry

Semax is a synthetic heptapeptide with the sequence Met-Glu-His-Phe-Pro-Gly-Pro (MEHFPGP). Its first four residues are the fragment ACTH(4–7), Met-Glu-His-Phe, and its last three are the tripeptide Pro-Gly-Pro, added at the C-terminus. It is a linear chain of standard L-amino acids, apart from the achiral glycine, with a free N-terminal amine and a free C-terminal acid. Its molecular formula is C37H51N9O10S, its average molecular weight is 813.92 g/mol and its CAS number is 80714-61-0.

This page covers Semax as a molecule: its sequence residue by residue, how the two blocks combine into one formula, its mass and charge, the methionine that sets its impurity profile, and how identity and purity are confirmed by HPLC and mass spectrometry.

The Semax sequence, residue by residue

Written in full, with both termini shown, the peptide is H-Met-Glu-His-Phe-Pro-Gly-Pro-OH. The "H-" is the free amine at the N-terminus and the "-OH" the free carboxylic acid at the C-terminus. The systematic name, L-methionyl-L-α-glutamyl-L-histidyl-L-phenylalanyl-L-prolylglycyl-L-proline, names the same seven residues in order; "α-glutamyl" records that the glutamic acid is joined through its main-chain carboxyl rather than its side chain. The conventions behind these codes are covered in peptide sequence notation.

Six of the seven residues carry a stereocentre at the α-carbon, all in the L-configuration; glycine has none. Semax contains no cysteine, lysine, arginine, tryptophan or tyrosine. Its only aromatic side chains are the benzene ring of phenylalanine and the imidazole of histidine.

ACTH(4–7) and the Pro-Gly-Pro extension

The bracketed numbers in ACTH(4–7) follow the standard convention for peptide fragments: they give the positions of the residues in the parent chain, so the fragment runs from residue 4 to residue 7 of ACTH. In Semax those four residues occupy positions 1 to 4. Semax is often described as an analogue of the longer fragment ACTH(4–10), in which the residues after Phe are replaced by Pro-Gly-Pro, so the sequence keeps the N-terminal half of that fragment and ends in a proline-rich tripeptide instead.

The formula confirms the construction by simple arithmetic.

The same Pro-Gly-Pro block closes Selank, the other heptapeptide in our Neuropeptides research area, where it extends the tetrapeptide tuftsin. The two compounds share their last three residues and nothing else; Selank vs Semax sets the two structures side by side.

Molecular formula, molecular weight and CAS number

The monoisotopic mass is the sum of the seven residue masses plus one water for the two free termini. The average molecular weight depends slightly on the atomic-weight table used: 813.92 g/mol is the catalogue value, and the current IUPAC abridged atomic weights give 813.93 g/mol. The difference reflects the reference table, not the molecule.

Sulfur shapes the isotope pattern as well as the formula. Sulfur-34 makes up about 4% of natural sulfur, so the peak two mass units above the monoisotopic ion is a little taller than it would be for a sulfur-free peptide of the same size. A high-resolution spectrum that shows this pattern is a useful extra check on the formula.

Charge, counter-ions and the solid form

Near pH 7, Semax carries one positive charge, on the N-terminal amine, and two negative charges, on the glutamic acid side chain and the C-terminal carboxylate. The histidine imidazole, with a pKa near 6, is only partly protonated at neutral pH. The net result is a charge of about −0.8, which makes Semax a slightly acidic peptide, unlike most short catalogue peptides, which are basic.

At the acidic pH of a typical reversed-phase HPLC mobile phase, around pH 2, both carboxyl groups are largely protonated and neutral, while the amine and the imidazole are positively charged, so the molecule then carries a net charge of about +2. This change of charge with pH is why the mobile-phase acid has such a strong effect on how the peptide behaves on the column.

The supplied solid is isolated from purification with counter-ions paired to its positive groups, such as acetate or trifluoroacetate. Counter-ions add weight to the powder without adding peptide. This is why net peptide content can sit below the gross weight of a lyophilised solid, as explained in peptide purity vs net peptide content.

The N-terminal methionine and its sulfoxide

Methionine's thioether is the most easily oxidised group in the molecule. Oxidation adds one oxygen atom to the sulfur, giving methionine sulfoxide, Met(O), and a peptide 15.995 Da heavier: C37H51N9O11S, monoisotopic mass 829.3429 Da, [M+H]+ at m/z 830.350.

The sulfoxide is more polar than the thioether, so the oxidised peptide elutes slightly earlier than the parent in reversed-phase HPLC. Oxidation also creates a new stereocentre at the sulfur, so Met(O) exists as two diastereomers, which can appear as a closely spaced pair of small peaks. Further oxidation to the sulfone adds a second oxygen, 31.990 Da above the parent. These related substances are why methionine-containing peptides are stored sealed, cold and away from air.

Two prolines and the cis–trans question

Semax has prolines at positions 5 and 7. The peptide bond that precedes a proline is the one most able to adopt the cis form as well as the usual trans form, and Semax has two such bonds: Phe-Pro and Gly-Pro. In solution the molecule exists as a mixture of these prolyl isomers. When cis and trans forms interconvert on the same timescale as a chromatographic separation, HPLC peaks can broaden or show a shoulder, a feature of the molecule rather than a second compound. Column temperature changes the rate of interconversion, which is how a shoulder of this kind is told apart from a true impurity.

How Semax is analysed

Semax has no tryptophan or tyrosine, so it has no useful absorbance at 280 nm, and low-wavelength detection is the method of choice. With a phenylalanine and a methionine in the chain it is more retained in reversed-phase HPLC than a peptide of the same length built only from small polar residues, such as Selank.

In MS/MS the sequence ions follow the standard b/y naming: b ions contain the N-terminus and y ions the C-terminus. The y ion that holds the C-terminal Pro-Gly-Pro, at m/z 270.145, is the same for Semax and Selank, because both peptides end in that block, while the b ions differ from the first residue onwards.

More detail on each method is in HPLC and peptide purity and mass spectrometry and peptide identity.

Certificates, specification and storage

Our Semax is supplied at ≥99% purity (HPLC) as a lyophilised powder in a sealed 30 mg vial. When reading a certificate for Semax, check that the sequence reads Met-Glu-His-Phe-Pro-Gly-Pro with free termini, that the observed mass matches 813.3 Da or the ions above, that the HPLC method gives its detection wavelength, that the salt form is stated, and that the report carries a reference and an analysis date. How to read a peptide certificate of analysis walks through each line of a report.

Store the sealed vial at 2–8 °C for short-term storage, or at −20 °C and below for long-term storage. Protect it from light, keep it closed until use and avoid repeated freeze-thaw cycles. For a methionine-containing peptide, limiting contact with air matters more than usual. Storing lyophilised peptides explains why cold, dark and dry storage matters for a freeze-dried solid.

Semax belongs to the Neuropeptides research area, catalogued there with Selank. Elsewhere in the catalogue, PT-141 is another short heptapeptide, this one cyclic, and a useful contrast in how a seven-residue chain can be built. Terms used on this page, such as monoisotopic mass, counter-ion and related substance, are defined in the glossary.