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Dissolving Peptides Properly: Swirl, Vortex or Sonicate? A Charge-Based Method
A peptide that will not dissolve is one of the most common frustrations at the bench. The reflex is to shake harder or add something stronger. A little chemistry first, plus a gentle method, saves most peptides and a lot of material.
Key takeaways
- Estimate the net charge of the peptide at neutral pH to choose a starting solvent.
- Basic peptides tend to dissolve in slightly acidic water; acidic peptides in slightly basic buffer.
- Hydrophobic peptides may need a small amount of organic solvent first.
- Gentle swirling is the default; vigorous shaking creates foam and surface stress.
- Brief, cooled sonication can help, but heat and radicals can damage sensitive residues.
Step one: count charges
Count each lysine and arginine as +1, each aspartate and glutamate as -1, and add +1 for a free N-terminus and -1 for a free C-terminus. Histidine is mostly neutral at pH 7. If the total is clearly positive, the peptide is basic; clearly negative, acidic. A peptide is least soluble near its isoelectric point, as explained in our isoelectric point post.
Step two: pick a starting solvent
- Net positive: try sterile water first, then a little dilute acetic acid if needed.
- Net negative: try water, then a little dilute ammonium bicarbonate. Be careful with cysteine-containing peptides, which oxidize faster at higher pH.
- Neutral or very hydrophobic: dissolve in a small volume of an organic solvent such as DMSO, then dilute slowly, as in our DMSO guide.
- Always test a small portion first before committing the whole vial.
Step three: mix gently
Let the vial reach room temperature before opening. Add the solvent down the wall of the vial rather than onto the powder, then swirl or roll the vial. Allow a few minutes to dissolve. Foam is a warning sign: air-liquid interfaces are hard on peptides and encourage aggregation. Vortexing is acceptable for robust peptides in short bursts, but not for fragile or aggregation-prone ones.
When to use sonication
Short bursts of sonication, about ten seconds at a time, in a cold water bath, can break up stubborn clumps. Long sonication heats the sample and generates reactive species that can oxidize methionine, cysteine and tryptophan, as described in our methionine post and our light sensitivity post. Mild warming, up to about body temperature, is sometimes enough.
Check the result
- The solution should be clear, with no visible particles.
- If it looks hazy, spin it and check whether a pellet forms.
- Avoid filtering small volumes through membranes that can bind peptide; see our syringe filter post.
- Measure concentration if you can, as in our A280 guide.
- Aliquot and freeze, as in our freeze-thaw guide.
The bottom line
Charge first, then solvent, then gentle mixing. Most dissolving problems are solved by thinking about the molecule rather than by force. For lot documentation, see our lab reports.
Related reading
- Isoelectric point and solubility
- DMSO stock solutions
- Why peptides aggregate
- Browse the full Research Library
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For laboratory research use only. Not for human or veterinary use, and not intended to diagnose, treat, cure or prevent any condition. This article is educational and is not medical advice.