Accurate dosage calculation is fundamental to reproducible peptide research. Converting between mass, moles, volume, and concentration requires understanding molecular weight, net peptide content, and proper unit handling.
Basic Calculation Framework
The foundation of peptide dosage calculation is the relationship: Concentration = Mass / Volume. For peptide research, mass is expressed in milligrams (mg) or micrograms (mcg), volume in milliliters (mL) or microliters (uL), and concentration in mg/mL, mcg/mL, or molar units (mM, uM, nM). To convert between mass-based and molar concentrations, use the molecular weight from the certificate of analysis: Molarity = (mass in grams) / (molecular weight × volume in liters).
A critical step often overlooked is adjusting for net peptide content. If a vial contains 10 mg of lyophilized material with 80% net peptide content, you have 8 mg of actual peptide. All concentration calculations should use this adjusted mass. Failing to account for counterions, salts, and moisture in the gross weight leads to systematic overestimation of peptide concentration and inconsistent experimental results.
Reconstitution Volume Calculations
To determine how much solvent to add: Volume (mL) = Net peptide mass (mg) / Desired concentration (mg/mL). For example, a 5 mg vial with 75% net peptide content contains 3.75 mg of peptide. To prepare a 1 mg/mL stock solution, add 3.75 mL of solvent. For molar stock solutions: Volume (mL) = [Net peptide mass (mg) × 1000] / [Molecular weight (g/mol) × Desired concentration (mM)].
Prepare stock solutions at a concentration 10–100 times higher than the working concentration to minimize the volume of solvent introduced into experimental systems. This also reduces the number of dilution steps and associated pipetting errors. Verify your stock concentration by UV absorbance if the peptide contains tryptophan or tyrosine residues.
Serial Dilution for Dose-Response Studies
Dose-response experiments require a series of concentrations spanning several orders of magnitude. A standard approach is half-log dilutions (approximately 3.16-fold per step) or log dilutions (10-fold per step). Prepare these by serial dilution from the stock solution using calibrated pipettes and fresh tips at each step. Mix thoroughly between dilutions. For highest accuracy, prepare each concentration independently from the stock rather than sequentially, though this uses more stock material.
Frequently Asked Questions
How do I convert mcg/kg to a practical injection volume?
First calculate the total mass needed: Total mcg = dose (mcg/kg) × body weight (kg). Then determine the injection volume based on your stock concentration: Volume (mL) = Total mass (mg) / Stock concentration (mg/mL). For example, a 100 mcg/kg dose for a 0.025 kg mouse equals 2.5 mcg total. From a 0.5 mg/mL stock, this requires 0.005 mL (5 uL). Use our dosage calculator for automated conversions.
Why is net peptide content important for calculations?
Lyophilized peptide powder is not pure peptide — it includes counterions (TFA or acetate salts), residual moisture, and residual salts. Net peptide content, typically 60–85% of the gross weight, tells you the actual mass of peptide molecule in the powder. Using gross weight instead of net peptide content overestimates your actual peptide concentration by 15–40%.