> For the complete documentation index, see [llms.txt](https://docs.rplpeptides.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.rplpeptides.com/docs/faq/solubility-faq.md).

# Solubility FAQ

## 1. What solvents are recommended for peptide reconstitution?

| Solvent                           | Best For                    | Notes                                    |
| --------------------------------- | --------------------------- | ---------------------------------------- |
| Sterile Water for Injection (WFI) | Most hydrophilic peptides   | pH \~5–7, first choice for most peptides |
| PBS (pH 7.4)                      | Cell-based assays           | Buffered, physiological pH               |
| 0.9% Saline                       | General research use        | Isotonic                                 |
| 0.1% Acetic Acid                  | Hydrophobic/basic peptides  | Improves solubility for basic peptides   |
| DMSO (≤0.1% final)                | Highly hydrophobic peptides | Avoid >0.1% in cell assays               |

## 2. What is the typical solubility of research peptides?

Solubility is highly sequence-dependent. Most peptides dissolve at 1–10 mg/mL in WFI or PBS. Hydrophobic peptides or those with high β-sheet propensity may require acidic conditions or co-solvents.

## 3. How do I calculate the final concentration?

Concentration (mg/mL) = Mass of peptide (mg) / Volume of solvent (mL)

For molar concentration: Molarity (M) = Concentration (mg/mL) / Molecular Weight (g/mol)

**Example**: 5 mg peptide with MW 1500 g/mol dissolved in 2.5 mL of WFI:

* Concentration = 5 / 2.5 = 2 mg/mL
* Molarity = 2 / 1500 = 0.00133 M = 1.33 mM

## 4. What if the peptide doesn't dissolve?

Try these steps in order:

1. Allow more time (15–30 min with gentle swirling)
2. Warm to 25°C (do not exceed 37°C)
3. Sonicate in a water bath (30 seconds, avoid foaming)
4. Add a small volume of 0.1% acetic acid or DMSO
5. Adjust pH (use 0.1 M HCl or NaOH in small increments)

## 5. Can I use DMSO as the primary solvent?

DMSO is effective for hydrophobic peptides but should be used with caution. Final DMSO concentration in cell-based assays should not exceed 0.1%. For stock solutions in DMSO, prepare at 1000× the final assay concentration.

## 6. How does pH affect solubility?

Peptides are most soluble at a pH away from their isoelectric point (pI). For basic peptides (pI > 7), slightly acidic conditions improve solubility. For acidic peptides (pI < 7), slightly basic conditions are preferred.

***

*For solubility inquiries: <technical@rplpeptides.com>*


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