Semax and Selank are frequently discussed together because both are short synthetic peptides that originated from Russian research programs and both are engineered as stabilized analogs of endogenous regulatory peptides. Despite this shared design philosophy, they descend from entirely different parent molecules. Semax is a synthetic heptapeptide modeled on the ACTH(4-10) fragment of adrenocorticotropic hormone (ACTH), extended with a C-terminal Pro-Gly-Pro motif that is added specifically to slow enzymatic degradation of the core sequence. Selank is a synthetic heptapeptide modeled on the immunomodulatory tetrapeptide tuftsin, likewise extended with a Pro-Gly-Pro tripeptide tail for the same stabilizing purpose. In classification terms, both are non-glycosylated linear oligopeptides rather than complex biologics, and both are commonly cataloged as regulatory-peptide research analogs.
Structurally, the two compounds share a common architectural motif — a short bioactive "message" segment fused to a proline-rich terminal "stabilizer" segment — but differ in the amino-acid composition of that message segment. Semax's reported sequence, Met-Glu-His-Phe-Pro-Gly-Pro, carries a methionine, glutamate, histidine, and phenylalanine core reflecting its melanocortin-fragment lineage. Selank's reported sequence, Thr-Lys-Pro-Arg-Pro-Gly-Pro, carries the threonine-lysine-proline-arginine core reflecting its tuftsin lineage, giving it a more basic (positively charged) character owing to the lysine and arginine residues. Both terminate in the Pro-Gly-Pro triad, which is the deliberate structural signature shared across this family of stabilized short peptides and the feature that distinguishes each analog from its unmodified biological parent. Both are studied in neuropeptide and CNS-oriented in-vitro and preclinical research contexts, but that association describes the research area only, not any outcome.
From a molecular-profile standpoint, both are small linear peptides in the heptapeptide size class, well below the mass range of protein biologics, and both lack disulfide bridges, cyclization, or post-translational modification. Their proline-rich C-termini contribute conformational rigidity and reduced susceptibility to aminopeptidase cleavage relative to their parent peptides. The principal chemical contrast is charge and polarity: Selank's arginine/lysine content renders it comparatively basic, while Semax's histidine/glutamate composition gives it a different ionization profile across pH ranges — a distinction relevant to reconstitution chemistry, buffer selection, and analytical separation (for example, retention behavior in reversed-phase HPLC or migration in ion-exchange methods).
In physical format, materials in this category are typically supplied as lyophilized (freeze-dried) powders sealed in vials for laboratory handling, a presentation chosen for the solid-state stability of short peptides prior to reconstitution. As a matter of reconstitution chemistry only, lyophilized peptides of this class are generally dissolved in an appropriate sterile aqueous or buffered solvent for in-vitro work, with cold storage of the dry powder favored to preserve integrity and the reconstituted solution handled under refrigerated, protected-from-light conditions to limit hydrolysis and oxidation (Semax's methionine residue, for instance, is a site of potential oxidative sensitivity worth noting in stability planning). Analytical identity and purity for both are commonly confirmed by mass spectrometry and HPLC. All descriptions here are for research-use-only characterization and imply no physiological effect.