Selank dissolves readily in bacteriostatic water, and that is the standard solvent for it in research. It is a short, very hydrophilic peptide that carries a net positive charge at the pH of bacteriostatic water, so it needs no acid, no organic solvent and no special technique beyond adding the water slowly and swirling.
Why Selank dissolves so easily
The sequence Thr-Lys-Pro-Arg-Pro-Gly-Pro explains it. None of its seven residues is strongly hydrophobic: there is no leucine, isoleucine, valine, phenylalanine or tryptophan. Threonine carries a hydroxyl group, lysine and arginine carry charged side chains, and the rest are proline and glycine. PubChem’s computed XLogP for Selank is minus 6.1, a strongly hydrophilic value (PubChem CID 11765600).
Charge does the rest. Counting the ionisable groups gives two basic side chains (Lys2, Arg4) plus the N-terminal amine, against the C-terminal carboxyl and no acidic side chain. From that count we calculate a net charge of about plus 2 at pH 7 and an isoelectric point near pH 11.5. These are our calculations from standard pKa values, not a measured figure. A peptide is least soluble at its isoelectric point, where its charges cancel. Bacteriostatic water sits far below pH 11.5, so dissolved Selank molecules carry a like positive charge and repel one another.
The prolines help as well. Proline breaks up the hydrogen-bonded sheets that make some peptides gel, and with three of them in seven residues Selank has no tendency to self-assemble at research concentrations.
Bacteriostatic water first
Bacteriostatic water is sterile water with 0.9% benzyl alcohol, which allows a refrigerated vial to be accessed more than once. For Selank it is the solvent of choice: the peptide dissolves fully in it, and the preservative supports repeated access over the 8 to 12 week fridge window described on how long Selank lasts. Sterile water works for a vial that will be accessed once. The difference is explained in bacteriostatic water explained.
Why not acetic acid?
Laboratories sometimes add a little dilute acetic acid to coax basic peptides into solution, because lowering the pH increases their positive charge. Selank is basic, but it is already fully charged and fully soluble at the pH of bacteriostatic water, so acid adds nothing. It is also unnecessary chemistry: acid speeds backbone hydrolysis in any peptide. Plain bacteriostatic water is enough.
Saline and sodium chloride
Selank also dissolves in 0.9% sodium chloride; the ready-mixed Selank Nasal Spray is made in sterile saline. Salt ions screen the charges on a peptide, which can reduce solubility for peptides that rely on charge repulsion near their isoelectric point. At the pH of saline Selank is far from that point and is hydrophilic to begin with, so this is not a practical concern at research concentrations. This is general peptide chemistry rather than a Selank measurement. Salt does help in one way: it reduces the attraction between a positively charged peptide and a negatively charged glass surface, as described on the Selank handling page. For a multi-access vial, bacteriostatic water remains the choice because of its preservative.
Concentrations that dissolve cleanly
A 10 mg vial is commonly mixed with 1 to 5 mL of bacteriostatic water, giving 10 mg/mL down to 2 mg/mL. With 2 mL the concentration is 5 mg/mL, and with 3 mL about 3.33 mg/mL; these are the figures our Selank reconstitution page uses for its tables. All of them dissolve to a clear, colourless solution within a minute of gentle swirling.
A larger water volume does not change the amount of peptide; it spreads it more thinly. Whatever volume is chosen, write the concentration on the vial.
Cloudiness and gelling: what causes them
Because Selank dissolves so readily, persistent haze is uncommon and usually points to technique or storage rather than the peptide:
- Water jetted onto the cake. A strong stream can trap air and create foam, which looks cloudy. Running the water down the inside wall of the vial avoids it.
- Shaking. Vigorous agitation creates air and liquid interfaces where peptides can unfold and clump. Swirling or rolling is enough for this peptide.
- Cold water onto cold powder. Both at fridge temperature slows dissolving. Letting the vial and the water reach room temperature first helps.
- Haze appearing later in storage. This suggests the vial was kept too warm, frozen and thawed, or contaminated, and the solution is discarded.
Brief cloudiness that clears within 10 to 15 minutes of mixing is normal and usually trapped air. The general causes are covered in why peptides gel or go cloudy, and the mixing method step by step is in how to reconstitute peptides with bacteriostatic water.
Related Selank pages
How the dissolved peptide behaves over time is on the Selank stability page. Semax, which shares the Pro-Gly-Pro tail but carries a methionine and a net negative charge, behaves differently in solution; see the Semax solubility guide.
All products supplied by Vistara Labs are intended for laboratory research use only. They are not drugs, are not approved for human or veterinary use, and are not intended to diagnose, treat, cure or prevent any condition. Nothing on this page constitutes medical advice or guidance for use in humans.
