← Knowledge · Updated 6 November 2026 · 5 min read
Solubility depends on sequence, charge and counterion. Test a small portion first, so a failed attempt doesn’t cost you the whole sample.
Estimate the charge
At the relevant pH, count the charged residues: basic residues (Lys, Arg, His, N-terminus) against acidic residues (Asp, Glu, C-terminus).
- Mostly basic: try water first, then dilute acetic acid if needed.
- Mostly acidic: try water first, then dilute ammonia solution or ammonium bicarbonate if needed.
- Neutral or hydrophobic: a small amount of organic solvent such as DMSO or acetonitrile, then dilute stepwise with buffer.
A worked example of charge
Take a peptide with two lysines, one arginine and one aspartic acid at pH 7. Positively charged are the N-terminus, two lysines and the arginine, together +4. Negatively charged are the aspartic acid and the C-terminus, together −2. The net charge is +2, so the peptide is mostly basic. First try: water, then dilute acetic acid if needed.
If the C-terminus is amidated, its negative charge disappears and the net charge rises to +3. An acetylated N-terminus lowers it by one. Such modifications appear in the sequence, as “Ac-” at the start or “-NH2” at the end.
The isoelectric point
The isoelectric point (pI) is the pH at which a peptide carries no net charge. Solubility is usually lowest there, because the molecules no longer repel each other. A solvent whose pH is one or two units away from the pI therefore usually dissolves a peptide better. You can estimate the pI from the sequence with common online calculators.
Step by step
- Let the vial reach room temperature and spin it briefly so the powder collects at the bottom.
- Take a small portion and choose the first solvent from the charge estimate.
- Add the solvent slowly and swirl gently. Do not shake hard.
- If the solution stays cloudy, sonicate briefly or try the next solvent in the list above.
- Only dilute to the final concentration with assay buffer once the solution is clear.
Solvents at a glance
| Solvent | Suitable for | Note |
|---|---|---|
| Water | Short, charged peptides | First try for most sequences |
| Dilute acetic acid, about 10% | Basic peptides | Then dilute with buffer |
| Dilute ammonia or ammonium bicarbonate | Acidic peptides | Not for cysteine peptides, promotes disulphide formation |
| DMSO | Hydrophobic peptides | Oxidises methionine and cysteine, check assay tolerance |
| DMF | Hydrophobic peptides with Met or Cys | Usually unsuitable for cell work |
| Acetonitrile | Hydrophobic peptides, analytical samples | Volatile, concentration changes on standing |
Peptides containing cysteine
Cysteine easily forms disulphide bridges at neutral to basic pH, within one molecule or between two. So dissolve cysteine peptides preferably in slightly acidic solution and with degassed solvent. Whether a reducing agent such as TCEP may be added depends on the experiment. Peptides that carry a disulphide bridge as part of their structure must not be reduced.
Hydrophobic and aggregating peptides
Peptides with many hydrophobic residues such as leucine, isoleucine, valine and phenylalanine tend to aggregate. They often dissolve only in neat organic solvent. Then add the aqueous buffer dropwise to the dissolved peptide, not the other way round. If the peptide comes out of solution, the share of organic solvent is too low.
Choosing the stock concentration
A more concentrated stock has two advantages: it needs less solvent in the final mix, and proportionally less peptide is lost by sticking to the container wall. It must not be too high, though, or it exceeds the solubility limit and the peptide comes out of solution during storage. A common middle ground is a stock between about 1 and 10 mM, depending on the sequence. Write the concentration, solvent, date and lot on every tube. How to calculate the weighed mass is shown in Peptide content vs purity.
Compatibility with the assay
Organic solvents disturb many biological tests even in small amounts. In cell culture, DMSO is often limited to 0.1 to 1% of the final volume at most. Plan the stock concentration so this limit holds after dilution. Run a solvent control without peptide so the solvent’s own effects become visible.
Filtering
If a solution is to be sterile-filtered, syringe filters with a 0.22 µm pore size and a low-binding membrane, such as PES or PVDF, are suitable. Peptides can stick to filter membranes, so the loss is noticeable with small amounts. Rinse the filter with a little solvent if needed and check the concentration after filtering.
Check the concentration
If the peptide contains tryptophan or tyrosine, you can estimate the concentration from absorbance at 280 nm. Without these residues, absorbance around 205 to 214 nm helps, though buffer and solvent also absorb there. Also allow for the peptide content of the weighed mass, as described in Peptide content vs purity.
Practical notes
- DMSO can oxidise methionine and cysteine. With these residues, DMF is often the better choice.
- Check how much organic solvent your assay tolerates.
- Brief sonication can help. Heat only with care.
- Cloudiness means the peptide isn’t fully dissolved, so the real concentration is unknown.
How to keep the finished solution is under Storing peptides. Water and other solvents are in Lab supplies.
Common questions
My peptide won’t dissolve in water. What now? Estimate the net charge. Dilute acetic acid helps with basic peptides, dilute ammonia with acidic ones. If it still won’t dissolve, the peptide is probably hydrophobic, and a small amount of DMSO or DMF is the next step.
Can I use a cloudy solution? Not for quantitative work. The real concentration is unknown, and aggregates can distort the result.
Can I change the solvent afterwards? Only by freeze-drying the peptide again and redissolving it. That is why you test a small portion first.
Does warming help? Gentle warming to room or body temperature can help. Stronger heating, however, speeds up degradation and aggregation. Try sonication or another solvent first.
Why does powder collect on the lid and wall? Lyophilisates are light and often carry static charge. A brief spin before opening brings the powder to the bottom, so nothing is lost when you open the vial.
For laboratory research use only. Not for use in humans or animals.
