Neuropeptides: UK Research Peptides

Structural features

Semax and Selank are both heptapeptides assembled on the same plan: a short fragment of a larger peptide, extended at the C-terminus by the tripeptide Pro-Gly-Pro. In Semax, H-Met-Glu-His-Phe-Pro-Gly-Pro-OH, the fragment is ACTH(4–7), Met-Glu-His-Phe. In Selank, H-Thr-Lys-Pro-Arg-Pro-Gly-Pro-OH, it is the tetrapeptide tuftsin, Thr-Lys-Pro-Arg. Both have a free N-terminal amine and a free C-terminal carboxylic acid, both are built only from standard amino acids, and neither carries a side-chain modification.

The Pro-Gly-Pro extension gives each sequence a proline-rich C-terminal end. Proline's side chain loops back onto its own backbone nitrogen, so a prolyl residue has no amide hydrogen and restricts the conformations nearby, and the peptide bonds that precede proline are the ones most able to adopt the cis form. Selank contains three prolines in seven residues and Semax two. Glycine, between the two prolines of the extension, is the only achiral residue in either sequence.

The two differ in charge. Selank carries a lysine and an arginine and no acidic side chain, so it is basic overall. Semax has a glutamic acid and a histidine but no lysine or arginine, and its only sulfur atom, in the N-terminal methionine, appears in its formula, C37H51N9O10S, against Selank's C33H57N11O9. Neither contains a cysteine, so there is no disulfide bridge to confirm in either peptide.

Identifiers at a glance

Both rows in the table show seven residues, so the molecular formula and the molecular weight carry the distinction: C37H51N9O10S and 813.92 g/mol for Semax, C33H57N11O9 and 751.87 g/mol for Selank. The single sulfur atom in the Semax formula comes from its N-terminal methionine, and Selank's formula has none. The price column gives the lowest cost per milligram for each peptide across the vials on sale, so the two can be read on the same footing.

Structure notes within this area

Residue-level detail sits on each product page. Within this area, read the structure notes on Selank and Semax.

How this research area is defined

The neuropeptides area is the most uniform in the catalogue. Both compounds are heptapeptides, both are linear chains with a free N-terminal amine and a free C-terminal acid, and both end in the same Pro-Gly-Pro tripeptide. The defining structure is therefore a design principle rather than a single motif: a short fragment of a larger peptide, extended at the C-terminus by the same three residues, with no ring, conjugate or side-chain modification anywhere in either chain.

That shared design makes the differences easy to locate. The first four residues of each sequence carry everything that distinguishes the two: Met-Glu-His-Phe in Semax and Thr-Lys-Pro-Arg in Selank. Those four positions set the charge, the sulfur content and the aromatic content of each peptide, while the last three are identical. Because the two have the same length and the same C-terminal block, any difference between them in retention or in mass traces back to those first four positions.

Both peptides are short enough to write out in full, and each product page gives the complete sequence in three-letter notation with its free termini. Read together, the two sequences show the area's design at a glance: four distinguishing residues at the N-terminal end, followed by the shared Pro-Gly-Pro block that defines the class on this hub. A researcher working with either peptide can therefore place the other in a moment, by its formula, its molecular weight and those four positions.

Analytical considerations

At 751.87 and 813.92 g/mol, both peptides sit in the range where electrospray mass spectrometry shows mainly singly and doubly charged ions, so identity can be read almost directly from the spectrum against the expected mass. Each chain contains histidine or arginine, so positive-ion electrospray gives a strong signal for both. With a free N-terminal amine and a free C-terminal acid, each also carries both a positive and a negative terminal charge at neutral pH. Their different compositions also separate them in reversed-phase HPLC: Selank, basic and without an aromatic side chain, is weakly retained, while Semax, with its phenylalanine and methionine, would be expected to elute later under the same gradient.

Each has a characteristic point to watch. The methionine of Semax can oxidise to the sulfoxide, which adds 16 Da and is a typical related substance, and its phenylalanine and histidine give it weak absorbance above 250 nm. Selank has no methionine, tryptophan or cysteine, so it offers no easy site for oxidation, and because it has no aromatic side chain its absorbance comes almost entirely from the peptide bonds, which makes low-wavelength detection around 214 nm the practical choice. Proline-rich chains of this kind can also give broadened peaks where cis and trans prolyl forms interconvert during the separation.

Further reading

• What Is Selank? Structure and Chemistry
• Peptide Sequence Notation: Reading an Amino Acid Sequence
• Mass Spectrometry and Peptide Identity
• HPLC and Peptide Purity: How It Is Measured
• How to Store Lyophilised Peptides
• Peptide chemistry glossary