Glossary
Molarity
Molarity counts moles of dissolved substance per litre. Why in-vitro peptide work is reported in nM or µM, and how to get there from a mg/mL concentration.
Molarity (M) expresses the moles of dissolved substance in each litre of solution. It is the relevant concentration unit whenever molecules interact with other molecules — enzyme kinetics, affinity measurements, receptor occupancy — because such events scale with how many molecules are present, not with how much they weigh. That is why in-vitro peptide publications quote nanomolar or micromolar values, whereas vial labels state milligrams.
Converting from mass to molarity
Moving from one to the other requires the molecular weight. Example: take a peptide of 1,419 Da, put 5 mg into 2 mL of bacteriostatic water, and the result is 2.5 mg/mL; 2.5 g/L divided by 1,419 g/mol equals 1.76 mmol/L, i.e. 1.76 mM. A thousand-fold dilution of that stock brings it to 1.76 µM. Bear in mind that equal mg/mL values give unequal molar concentrations as soon as two peptides differ in mass — exactly why comparing an analog with its parent on a mass basis can be misleading.
Two caveats: the reference volume is the finished solution rather than the solvent, and the value drifts with temperature as liquids expand. When high precision is needed, molality sidesteps both problems. Our calculator and the guide on moles and molarity do the sums for you.
Related terms
See also reconstitution, mcg vs mg, and the calculator for reconstitution