How to reconstitute lyophilized research peptides

Step by step

1. 1Let the vial warm up. Take the sealed vial from -20°C storage and leave it at room temperature until no longer cold to the touch. Opening a cold vial lets ambient moisture condense onto the powder.
2. 2Choose the solvent. Bacteriostatic water is the default for peptides intended for repeat withdrawals. Sterile water suits single-use preparations. A small number of hydrophobic sequences need a co-solvent first — check the batch documentation before assuming water will dissolve the cake.
3. 3Calculate the volume. Divide the vial's peptide mass by the concentration you want. A 5mg vial plus 2mL of solvent gives 2.5mg/mL; the same vial plus 5mL gives 1mg/mL.
4. 4Add the solvent slowly. Direct the stream down the inside wall of the vial so it runs onto the cake rather than striking it. Adding solvent directly onto lyophilized powder is a common cause of foaming.
5. 5Swirl, do not shake. Rotate the vial gently until the cake dissolves. Mechanical agitation is the single most common cause of degradation in peptide handling — shaking denatures far more product than any storage mistake.
6. 6Inspect before use. A properly reconstituted solution is clear. Cloudiness or visible particulate means incomplete dissolution or degradation; do not use it as though the nominal concentration still holds.
7. 7Aliquot if the solution will be used repeatedly. Splitting into single-use volumes avoids repeated freeze-thaw cycling of the whole stock.

The concentration calculation

Peptide vials are labelled by mass, not by concentration, so the working concentration is set entirely by how much solvent is added. The arithmetic is one division, but it is worth writing down, because the same vial can legitimately be prepared at very different concentrations for different assays.

For conjugated products the calculation needs care. PEG-MGF, for example, carries a PEG chain that adds substantial mass, so a molar concentration must be derived from the conjugate rather than from the peptide portion alone.

Common mistakes

• Shaking to speed dissolution. It works, and it degrades product. Swirl and wait instead.
• Opening a cold vial. Condensation onto a hygroscopic cake starts hydrolysis before the solvent is even added.
• Reducing buffers with disulfide-containing peptides. Oxytocin and related sequences depend on an intact disulfide bridge; a reducing environment inactivates them and produces a confusing null result.
• Omitting a required cofactor. Thymulin is inactive without zinc. Run it in a zinc-free buffer and it will read as inert at any concentration.
• Assuming the label concentration after storage. Reconstituted peptide degrades over time. Validate the working life for your own conditions rather than trusting a generic figure.