Molarity (M) is moles of solute per litre of solution. It is the concentration unit that matters whenever the question is about molecules interacting with molecules — receptor occupancy, binding affinity, enzyme kinetics — because those depend on particle count, not on mass. Published in-vitro peptide work is therefore reported in nanomolar or micromolar terms, while vial labels are in milligrams.
Why molarity matters in peptide research
Converting between the two requires the molecular weight. A worked example: 5 mg of a 1,419 Da peptide reconstituted in 2 mL of bacteriostatic water gives 2.5 mg/mL; dividing 2.5 g/L by 1,419 g/mol yields 1.76 mmol/L, or 1.76 mM. Diluting that stock one thousand-fold gives 1.76 µM. Note that two peptides at the same mg/mL are at different molarities whenever their masses differ, which is exactly why mass-matched comparisons between an analog and its parent can mislead.
Two cautions: molarity is per litre of solution, not of solvent, and it is temperature-dependent because volume expands. Where precision matters, molality avoids both issues. Our molarity calculator and the moles and molarity guide handle the arithmetic.
Related terms
micrograms vs milligrams · reconstitution · reconstitution calculator