AC-Semax-NH2 research: what terminal capping changes

The Semax family is one of the clearest worked examples in peptide chemistry of a single idea applied three times: protect the termini, keep the active core intact, and watch the half-life extend. Understanding what each modification does — and, importantly, what it does not do — is what allows work to move between the analogues without silently breaking.

Where the sequence comes from

Adrenocorticotropic hormone is a 39-residue peptide. Its corticotropic activity — the stimulation of the adrenal cortex it is named for — requires the N-terminal region including residues 1 to 24. Its behavioural and neurotrophic effects, however, were localised to a much shorter fragment: ACTH(4-10), and eventually to ACTH(4-7), the tetrapeptide Met-Glu-His-Phe.

That separation is the whole point. A fragment that carries neurotrophic activity without adrenal stimulation is a far cleaner research tool than the parent hormone, because an observed effect can be attributed to one pathway rather than confounded by a second.

The problem was that a free tetrapeptide is cleared almost immediately. Aminopeptidases attack the N-terminus, carboxypeptidases the C-terminus, and a four-residue peptide has very little to hide behind.

Modification one: the Pro-Gly-Pro tail

Semax is ACTH(4-7) extended at the C-terminus with Pro-Gly-Pro, giving the heptapeptide Met-Glu-His-Phe-Pro-Gly-Pro. The extension is a protease block. Proline-rich C-termini resist carboxypeptidase cleavage — the enzyme cannot readily accommodate proline in its active site — so the tail acts as a sacrificial shield for the active tetrapeptide sitting behind it.

This is the compound with the bulk of the published literature behind it, most of it Russian-language and much of it in cerebral ischaemia models. When a paper says Semax, this is what it means.

Modification two: capping both ends

N-Acetyl Semax Amidate — also written AC-Semax-NH2 or NA-Semax-amidate — takes the same logic to both termini. An acetyl group caps the N-terminal amine; the C-terminal carboxyl is converted to a primary amide.

Acetylation blocks aminopeptidase attack. Amidation blocks the carboxypeptidase route that the Pro-Gly-Pro tail only partially defends. Together they extend the molecule's working life considerably.

The part that gets missed

Neither modification is pharmacophoric. The acetyl group and the terminal amide are not doing receptor chemistry; they are keeping enzymes away from a sequence that would otherwise be dismantled. The active core is the same in both compounds.

But there is a second-order effect that is easy to overlook. Capping both termini removes both terminal charges, and that changes the molecule's overall polarity. The capped analogue is measurably more lipophilic than the parent. That shows up in three places that matter practically:

1. Retention time. The capped analogue elutes later on a reversed-phase gradient. If you transfer a method without re-qualifying it, you may not find your peak where you expect it.
2. Solubility. It dissolves more slowly in purely aqueous diluent. Extend the standing time rather than warming or vortexing.
3. Distribution. Reduced polarity changes how the molecule partitions in a preparation, which is relevant wherever a membrane is involved.

Naming, and the confusion it causes

This compound is sold and cited under at least four names: N-acetyl semax amidate, NA-Semax-amidate, AC-Semax-NH2, and occasionally just Semax amidate. All refer to Ac-MEHFPGP-NH2. Meanwhile, plain Semax is frequently sold under descriptions that imply terminal modification it does not have.

The consequence for anyone reproducing published work is real: if a paper reports an effect and you buy a compound under a name you assume matches, you may be running a different molecule. The defence is straightforward — check the stated sequence and the mass on the certificate, not the trade name. Ac-MEHFPGP-NH2 at 855.0 g/mol is unambiguous in a way that a product title is not.

Handling

Both compounds contain a single methionine, which oxidises on air exposure in solution. Expect a small early-eluting peak to grow in an ageing preparation; that is methionine sulfoxide, not contamination.

• Reconstitute against the vial wall and let the powder dissolve unaided; if dissolution is slow, wait rather than warming.
• Twenty-one days at 2–8 °C in solution for the capped analogue.
• Adsorption to plastic is modest but real at low concentrations — use glass or low-binding plasticware for dilute working solutions.
• Confirm which analogue any published protocol used before transferring conditions across.

What we supply

Ryzen stocks both the unmodified heptapeptide and the doubly capped analogue, released at not less than 99.0% area-percent by reversed-phase HPLC with identity confirmed by mass spectrometry. Both certificates state the full modified sequence explicitly rather than relying on the trade name, because as set out above, the trade name is the least reliable thing on the label.