Selank and Semax: the same tail on two different peptides

A shared design, two different molecules

Selank and Semax come from the same research tradition and use the same structural trick: a short active fragment with Pro-Gly-Pro appended to the C-terminus to blunt enzymatic trimming. Beyond that shared tail they have nothing in common.

Both are heptapeptides. Both end in the same three residues. Their parent fragments come from unrelated molecules — one from an immunologically active fragment, the other from a pituitary hormone — and their analytical behaviour follows the front of the chain, not the tail.

The difference that shows on a chromatogram

Read the first residue of each. Semax begins with methionine; Selank begins with threonine and contains no methionine, cysteine or tryptophan anywhere.

That single difference splits their degradation profiles:

• Semax has an oxidation route. Methionine oxidises readily, the oxidised species is 16 mass units heavier and more polar, and it elutes earlier — so early peaks are the first thing to check on that compound's chromatogram.
• Selank does not. With no oxidisable side chain, its impurity profile is dominated by synthesis-related species: deletion sequences from the three prolines, which couple less efficiently than most residues and sit at positions 3, 5 and 7 of a seven-residue chain.

Two peptides with the same tail, the same length and the same design rationale, and you can predict which region of each chromatogram to examine before seeing either one. That is the general value of reading a sequence: it tells you what to look for. See how to read an HPLC chromatogram.

What identity has to confirm

For any fragment analogue defined by an addition, the identity result should correspond to the complete seven-residue molecule rather than the shorter parent fragment. Those differ by the mass of the appended tail — far beyond any method's tolerance — so the check is unambiguous where a mass is stated. Where identity is given as HPLC-RTM, the report should name the reference standard.

Storage of the lyophilized solid

Supplied as a freeze-dried solid, governed by temperature and moisture. With no oxidisable residues, an intact seal is doing less work for this compound than for its methionine-bearing relative — the ordinary considerations apply without special additions.

Stability is a documented property rather than an assumed one: a laboratory establishes it by holding material under defined conditions and re-analysing at intervals. Where a retest date is stated, the question worth asking is what data supports it. See storing lyophilized research materials.

Scope of this documentation

This page concerns material identification and analytical documentation for laboratory research. It does not recommend personal, human, clinical, therapeutic or veterinary applications. Published research findings do not establish the properties of Arctic stock; use the applicable lot record and its stated methods and limitations.

Related documentation