How to Calculate Peptide Concentration After Reconstitution
When you dissolve a lyophilized peptide for an in vitro experiment, the first number you need is its concentration. The math is simple, but small errors carry through every experiment that follows.
The formula
Concentration = amount of peptide ÷ volume of diluent
Keep your units consistent. If the amount is in milligrams and the volume is in milliliters, the result is in milligrams per milliliter.
Worked example 1
A vial contains 10 milligrams of lyophilized peptide. You add 2 milliliters of diluent.
10 ÷ 2 = 5 milligrams per milliliter
Worked example 2
A vial contains 5 milligrams. You add 2.5 milliliters of diluent.
5 ÷ 2.5 = 2 milligrams per milliliter
Converting to micrograms
Many assays work in micrograms. One milligram equals 1,000 micrograms.
5 milligrams per milliliter = 5,000 micrograms per milliliter, which is the same as 5 micrograms per microliter.
Calculating molar concentration
Receptor binding and kinetics work often requires molar units. You need the peptide's molecular weight, which is listed on the COA.
Molarity = (concentration in grams per liter) ÷ (molecular weight in grams per mole)
For example, a solution of 1 milligram per milliliter is 1 gram per liter. If the molecular weight is 1,000 grams per mole, the solution is 0.001 molar, or 1 millimolar.
Tips for accuracy
- Use the measured content when available. Some COAs report the actual peptide content, which can differ slightly from the label.
- Account for net peptide content. Lyophilized peptides can contain counter ions and residual moisture, so net peptide content may be below 100 percent.
- Use calibrated pipettes and record the exact volume added.
- Add diluent slowly down the side of the vial and swirl gently rather than shaking.
- Label the vial with the final concentration and date.
For more on diluents, read reconstituting lyophilized peptides with bacteriostatic water.
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