Dissolving peptides: which solvent for what?
Almost every peptide arrives as a dry powder and has to be dissolved in the lab. Which solvent suits it depends on the chain itself.
The three usual solvents
| Solvent | What it is | What for |
|---|---|---|
| Bacteriostatic water | Water with 0.9% benzyl alcohol | The standard. The alcohol slows bacterial growth, so a vial stays usable longer after first puncture. |
| Sterile water | Water with nothing added | For substances that do not tolerate benzyl alcohol. Intended for single use. |
| Dilute acetic acid | Water with a little acetic acid | For peptides that float or clump in neutral water. |
Why benzyl alcohol makes the difference
As soon as you pierce a rubber cap, air and therefore micro-organisms can get in. In plain water they carry on growing happily. Benzyl alcohol slows that down, which is why a vial of bacteriostatic water may be punctured several times while sterile water is meant for single use.
It is a brake, not a steriliser. It does not kill what is already there; it only slows growth. Clean work is still needed: wipe the cap with alcohol, a fresh needle every time.
Why some peptides refuse to dissolve
Whether a chain dissolves in water depends on the side chains of its amino acids. If many of them repel water, the molecule would rather huddle against its own kind than be surrounded by water. The result is a cloudy liquid or a flake that keeps floating.
Lowering the acidity puts a charge on the acidic side chains. Like charges repel, so the chains stay apart. That is the whole idea behind dilute acetic acid as a solvent. With long or fatty peptides it is often the only way to get a clear solution.
The temptation to shake when a lump is still floating is strong. Resist it. Shaking drives air into the liquid, and the boundary between air and water is exactly where long chains lose their shape. Swirl the vial slowly, or simply let it stand.
How to go about it
- Let the vial come to room temperature first. Cold glass attracts condensation.
- Wipe the rubber cap with an alcohol wipe and let the surface dry.
- Run the solvent down the wall of the vial, not straight onto the powder.
- Swirl the vial gently until the solution is clear. Do not shake.
- If it stays cloudy or particles float, note that and do not use the solution for a measurement.
How long does a solution last?
Shorter than the powder, always. A freeze dried peptide is stable for months to years in a freezer. The same substance in water declines from day one, and the longer the chain the faster that goes. With sensitive substances such as MGF or glutathione, preparing fresh is the difference between a usable and an unusable sample.
So always note the date you dissolved it. Without that date you will not know in three weeks what you are holding.
This site is made by the team behind Apex Bio Research, a Dutch supplier of research peptides with a certificate of analysis per batch.
Visit the shopThis site explains what the scientific literature says about peptides. It is not medical advice and not a set of instructions. The substances discussed here are research compounds: they are not approved for use in humans or animals. If you have a health question, see a doctor.
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