Research Solution Preparation: Dissolving Peptides Accurately

Preparing a peptide solution looks simple: add liquid, mix, use. The details decide whether the concentration you write on the label is the concentration in the tube, and whether the peptide is still intact when you use it.

Step 1: Know how much peptide you have

A lyophilized peptide powder contains the peptide plus counterions (often trifluoroacetate or acetate from purification) and some water. The net peptide content, measured by amino acid analysis or nitrogen analysis, is often in the range of 60–90% of the powder weight. Purity by HPLC is a different number: it describes how much of the peptide material is the target sequence, not how much of the powder is peptide. See Purity Terminology.

If the COA gives net peptide content, use it:

peptide mass = powder mass × net peptide content

For example, 5.0 mg of powder at 80% net peptide content contains 4.0 mg of peptide. Dissolving it in 4.0 mL gives 1.0 mg/mL of peptide, not 1.25 mg/mL. If the content is not reported, record that the concentration is “as powder” so others know which convention was used.

Step 2: Choose the solvent

Choose from the peptide’s charge and hydrophobicity. If solubility is uncertain, dissolve a small test portion first. Solvent Selection covers this in detail.

Step 3: Dissolve

1. Equilibrate the closed vial to room temperature in a desiccator.
2. Tap or briefly centrifuge the vial so the powder collects at the bottom.
3. Add solvent slowly down the inside wall, rather than jetting it onto the powder.
4. Swirl or invert gently until dissolved. Brief sonication in a water bath can help stubborn material; avoid prolonged sonication, which heats the sample.
5. Allow time. Some peptides take several minutes to dissolve fully.

Step 4: Check the solution

• It should be clear and colourless, with no particles, haze or gel.
• If it is cloudy, the peptide is not fully dissolved or is aggregating. Do not filter it to “fix” it, because that removes peptide. Try the solvent strategies in the next guide.
• Check the pH if it matters for the assay or for stability.
• For accurate work, measure the concentration by UV absorbance at 280 nm (for Trp- and Tyr-containing peptides) or by amino acid analysis.

Step 5: Filter if required

For cell work, sterile-filter through a 0.2 µm filter with a low-protein-binding membrane such as PES or PVDF. Expect some loss, especially with small volumes and dilute solutions, and flush or pre-wet the filter as the manufacturer recommends.

Step 6: Label, record, store

Label the container and record compound, lot, mass, solvent, volume, concentration, date and preparer. Then aliquot and freeze, or store at 2–8 °C for immediate use. See Aliquoting and Solution Stability.