Semax vs. Selank: Nootropic Peptide Structural Comparison

Semax and Selank are two synthetic heptapeptides originally developed at the Institute of Molecular Genetics of the Russian Academy of Sciences. Both are classified as nootropic peptides in the research literature and are derived from endogenous regulatory peptides — Semax from the adrenocorticotropic hormone (ACTH) fragment, and Selank from the immunomodulatory tetrapeptide tuftsin. Despite their shared origin in Russian peptide research, the two compounds differ substantially in amino acid sequence, parent peptide lineage, molecular weight, and physicochemical properties.

This article provides a purely structural and analytical comparison of Semax and Selank as research compounds. It does not address pharmacological activity, clinical applications, or dosage considerations.

Semax is a synthetic analog of the ACTH(4-7) fragment (Met-Glu-His-Phe) with the C-terminal addition of the tripeptide Pro-Gly-Pro. The full sequence is Met-Glu-His-Phe-Pro-Gly-Pro, giving it seven amino acid residues. The molecular formula is C39H53N9O10S, with a molecular weight of approximately 813.97 Da.

• Amino acid sequence: Met-Glu-His-Phe-Pro-Gly-Pro (MEHFPGP)
• Molecular weight: ~813.97 Da
• Molecular formula: C39H53N9O10S
• Number of amino acids: 7
• Parent peptide: ACTH(4-7) fragment (Met-Glu-His-Phe)
• C-terminal modification: Pro-Gly-Pro tripeptide extension
• Isoelectric point (calculated): ~5.2
• Contains one sulfur atom (from methionine residue)

Selank is a synthetic analog of tuftsin (Thr-Lys-Pro-Arg), a naturally occurring tetrapeptide derived from the Fc domain of immunoglobulin G (IgG). Like Semax, Selank is extended at the C-terminus with the tripeptide Pro-Gly-Pro. The full sequence is Thr-Lys-Pro-Arg-Pro-Gly-Pro, yielding seven amino acid residues. The molecular formula is C33H57N11O9, with a molecular weight of approximately 751.87 Da.

• Amino acid sequence: Thr-Lys-Pro-Arg-Pro-Gly-Pro (TKPRPGP)
• Molecular weight: ~751.87 Da
• Molecular formula: C33H57N11O9
• Number of amino acids: 7
• Parent peptide: Tuftsin (Thr-Lys-Pro-Arg)
• C-terminal modification: Pro-Gly-Pro tripeptide extension
• Isoelectric point (calculated): ~11.0 (highly basic)
• No sulfur-containing residues

Despite both being heptapeptides with a shared C-terminal Pro-Gly-Pro motif, Semax and Selank differ in nearly every physicochemical parameter. Semax is an acidic-to-neutral peptide (pI ~5.2) containing aromatic residues (His, Phe) and a sulfur-containing methionine, while Selank is strongly basic (pI ~11.0) with no aromatic or sulfur-containing residues. The ~62 Da molecular weight difference reflects their distinct amino acid compositions in the first four positions.

• Shared feature: Both contain the C-terminal Pro-Gly-Pro extension for enzymatic stability
• Molecular weight difference: Semax (~814 Da) vs. Selank (~752 Da) — a 62 Da gap
• Charge at physiological pH: Semax is near-neutral; Selank carries a net positive charge (+2 to +3)
• Aromatic residues: Semax contains His and Phe; Selank has none
• Sulfur content: Semax has one Met residue (oxidation-sensitive); Selank has no sulfur
• Solubility: Both are water-soluble; Selank is more readily soluble due to higher charge
• UV absorption: Semax absorbs at 214 nm and weakly at 280 nm (Phe, His); Selank absorbs primarily at 214 nm

Both Semax and Selank were designed using the same structural strategy: appending the Pro-Gly-Pro tripeptide to the C-terminus of a biologically active short peptide. The ACTH(4-7) fragment (Met-Glu-His-Phe) is a four-residue sequence from the N-terminal region of adrenocorticotropic hormone (ACTH, 39 amino acids total). Tuftsin (Thr-Lys-Pro-Arg) is a tetrapeptide corresponding to residues 289-292 of the heavy chain of IgG, released by enzymatic cleavage involving spleen tuftsin-endocarboxypeptidase and leukokininase.

The Pro-Gly-Pro extension serves a specific structural purpose: the proline-rich sequence creates a conformational constraint that impedes recognition and cleavage by aminopeptidases and carboxypeptidases. This modification substantially increases the half-life of both peptides compared to their unmodified parent sequences, which are rapidly degraded by serum proteases.

The Pro-Gly-Pro C-terminal extension confers resistance to carboxypeptidase-mediated degradation in both peptides. However, Semax and Selank differ in their susceptibility to other degradation pathways due to their distinct amino acid compositions.

• Semax: The methionine residue at position 1 is susceptible to oxidation (Met to Met sulfoxide), particularly in the presence of reactive oxygen species, metal ions (Cu2+, Fe3+), or prolonged exposure to air. Oxidized Semax can be detected as a distinct peak in RP-HPLC with a slightly earlier retention time.
• Selank: Lacks oxidation-sensitive residues, making it chemically more stable in solution than Semax. The primary degradation pathway is hydrolysis at the Thr-Lys or Lys-Pro bonds by trypsin-like endopeptidases.
• Lyophilized stability: Both peptides are stable for 24+ months as lyophilized powders when stored at -20 degrees C in sealed, desiccated vials.
• Solution stability: Reconstituted Semax should be used within 2-4 weeks at 2-8 degrees C (due to methionine oxidation risk). Selank solutions are stable for up to 4-6 weeks at 2-8 degrees C.
• Freeze-thaw sensitivity: Both peptides tolerate 3-5 freeze-thaw cycles without significant degradation, though single-use aliquoting is recommended for quantitative research.

Analytical verification of Semax and Selank purity and identity employs standard peptide characterization techniques. Both peptides are well-resolved by reverse-phase HPLC and produce characteristic mass spectra.

• RP-HPLC: Both peptides are analyzed on C18 columns (4.6 x 250 mm, 5 um particle size) with a water/acetonitrile gradient containing 0.1% TFA. Semax typically elutes at 18-22 minutes; Selank at 12-16 minutes (shorter retention due to higher hydrophilicity). Purity specification: 98% or greater for research-grade material.
• Mass spectrometry: ESI-MS (electrospray ionization) produces [M+H]+ ions at m/z 814.0 (Semax) and 752.9 (Selank). MALDI-TOF can also be used for identity confirmation.
• Amino acid analysis (AAA): Hydrolysis followed by derivatization and chromatographic quantification confirms expected amino acid ratios. For Semax: Met:Glu:His:Phe:Pro:Gly = 1:1:1:1:2:1. For Selank: Thr:Lys:Arg:Pro:Gly = 1:1:1:3:1.
• Peptide content (net peptide content): Determined by nitrogen analysis or AAA. Typical values: 70-85% for lyophilized peptides, with the remainder being counter-ions (acetate or TFA salts) and residual moisture.
• Endotoxin testing: LAL (Limulus Amebocyte Lysate) assay for research-grade peptides intended for cell culture applications. Specification: less than 0.25 EU/mg.

• Lyophilized powder: Store at -20 degrees C in sealed vials with desiccant. Protect from light. Stable for 24+ months.
• Reconstituted solution: Store at 2-8 degrees C. Use Semax solutions within 2-4 weeks; Selank solutions within 4-6 weeks.
• Recommended reconstitution solvent: Sterile water or bacteriostatic water for Selank (basic peptide). For Semax, sterile water or 0.1% acetic acid.
• Avoid repeated freeze-thaw cycles. Aliquot reconstituted solutions into single-use volumes before freezing.
• Use low-binding polypropylene tubes to minimize peptide adsorption to container surfaces.
• Semax-specific: Minimize exposure to air and oxidizing agents to prevent methionine oxidation. Purge vial headspace with nitrogen or argon gas after reconstitution.

ChemVerify aggregates pricing data from verified research peptide vendors, allowing researchers to compare cost per milligram for both Semax and Selank across multiple suppliers. Both peptides are widely available from verified vendors in standard quantities (5 mg, 10 mg, 30 mg vials). Selank is generally priced 10-20% lower than Semax due to simpler synthesis (no methionine coupling or oxidation-sensitive protection steps).

Use the ChemVerify price comparison tool to find current pricing from verified vendors. Filter by purity grade, quantity, and shipping region to identify the most cost-effective source for your research needs.

Both Semax and Selank incorporate a C-terminal Pro-Gly-Pro tripeptide extension that was engineered to enhance resistance to aminopeptidase and carboxypeptidase degradation. This modification extends the peptides' stability in biological matrices compared to their unmodified parent sequences (ACTH(4-7) for Semax and tuftsin for Selank) without significantly altering receptor binding properties.

ACTH (adrenocorticotropic hormone) is a 39-amino-acid pituitary hormone involved in cortisol regulation and neuromodulation. Tuftsin is a 4-amino-acid immunostimulatory peptide (Thr-Lys-Pro-Arg) naturally produced from immunoglobulin G. Their distinct biological origins give Semax and Selank different amino acid compositions, charge profiles, and HPLC retention behaviors despite similar molecular weights.

Yes. Despite similar sizes, their molecular weights differ: Semax at ~813 Da and Selank at ~751 Da, a ~62 Da gap that is easily resolved by both ESI-MS and MALDI-TOF. Their distinct amino acid sequences also produce unique fragmentation patterns in MS/MS analysis, providing definitive identification even in mixed samples.

Explore detailed chemical profiles and research guides for compounds discussed in this article:

• Selank: Complete Research Guide → /learn/selank
• Semax: Complete Research Guide → /learn/semax

• Read more: Semax: Complete Research Guide & Chemical Profile | ACTH 4-10 → https://www.chemverify.com/learn/semax
• Read more: Selank: Complete Research Guide & Chemical Profile → https://www.chemverify.com/learn/selank
• Read more: BPC-157 Acetate vs. Arginine Salt: Counterion Comparison → https://www.chemverify.com/learn/bpc-157-acetate-vs-arginine
• Read more: AOD-9604 vs. HGH Fragment 176-191: Structural Comparison → https://www.chemverify.com/learn/aod-9604-vs-hgh-fragment