Why Won't My Peptide Dissolve?
Not all peptides dissolve easily in water. Some are hydrophobic (water-fearing), some aggregate into clumps, and others have charge properties that make them insoluble at certain pH levels. If your reconstituted solution looks cloudy, has visible particles, or appears gel-like, you have a solubility problem.
Don't panic — most solubility issues can be resolved with the right approach. The key is understanding why it's not dissolving and applying the appropriate solution.
How to Predict Solubility Problems
Before you even reconstitute, look at the peptide's amino acid sequence. Certain patterns predict trouble:
- High hydrophobic content — peptides with many Leu, Ile, Val, Phe, Trp, or Met residues often resist water
- Short peptides (<5 amino acids) with hydrophobic residues — not enough polar groups to balance
- Peptides near their isoelectric point (pI) — solubility is lowest when net charge is zero
Most Certificate of Analysis (CoA) documents list the peptide's pI. If the solution pH is close to this value, solubility drops dramatically.
Troubleshooting Workflow
Step 1: Give It Time
Some peptides dissolve slowly. After adding your solvent, wait 10-15 minutes before concluding there's a problem. Gentle swirling (not shaking) can help. If it's still cloudy after 15 minutes, move to the next step.
Step 2: Try Sonication
Place the sealed vial in a laboratory ultrasonic bath for 5-10 minutes. The sound waves help break up aggregates and encourage solvation.Keep the water bath cool — don't let the peptide solution heat up significantly.
Step 3: Adjust the pH
This is often the most effective intervention:
- For basic peptides (pI > 7, rich in Arg, Lys, His): Add a small amount of dilute acetic acid (1-10%). The acid protonates basic residues, increasing positive charge and repulsion between molecules.
- For acidic peptides (pI < 6, rich in Asp, Glu): Add dilute ammonium hydroxide or sodium hydroxide. This deprotonates acidic residues, increasing negative charge.
Add the acid or base dropwise. You're trying to shift the pH 1-2 units away from the pI, not go to extremes.
Step 4: Use a Co-Solvent
For very hydrophobic peptides, water alone may never work. Try this approach:
- Dissolve the peptide in a small volume of DMSO (dimethyl sulfoxide) — typically 50-100μL
- Once fully dissolved, slowly dilute with water or buffer to your target volume
- Keep final DMSO concentration below 5-10% for most applications
Check Compatibility
DMSO may interfere with some assays or cell culture work. Verify that your research application tolerates the final DMSO concentration before committing to this approach.
What If Nothing Works?
If you've tried everything and the peptide still won't dissolve:
- Check the peptide purity — degraded peptides often have altered solubility
- Verify storage conditions — was the peptide stored properly before you received it?
- Contact the supplier — they may have specific recommendations for that sequence
- Consider the peptide may be aggregated — some sequences naturally form beta-sheets or fibrils