What is Molarity?
Also called: Molar concentration, mol/L, mM, µM
Concentration expressed as moles of solute per litre of solution (mol/L, written M); peptide work usually uses millimolar (mM), micromolar (µM) or nanomolar (nM).
By the APL Research Team · Updated
Molarity counts molecules rather than mass: a 1 M solution holds one mole of solute per litre. Receptor occupancy, enzyme kinetics and concentration–response curves all depend on the number of molecules present, so molar units are the standard for comparing peptides, while mg/mL remains the natural unit for weighing and reconstitution.
The conversion
Molarity (mol/L) = mass concentration (g/L) ÷ molecular weight (g/mol)
Because 1 mg/mL is the same as 1 g/L, a 1 mg/mL solution of any peptide is 1,000 ÷ MW millimolar.
| Peptide | MW (g/mol) | 1 mg/mL equals |
|---|---|---|
| GHK | 340.38 | 2.94 mM |
| Ipamorelin | 711.87 | 1.40 mM |
| Semaglutide | 4113.6 | 0.243 mM (243 µM) |
Equal mass concentrations of these three peptides differ twelve-fold in molarity. Testing them side by side at "1 µg/mL each" would expose cells to very different numbers of molecules, which is why concentration–response experiments are designed in molar units.
Worked example: from stock to 100 nM
A semaglutide stock at 1.0 mg/mL is 243.1 µM. To make 10 mL of a 100 nM working solution, C1V1 = C2V2 gives:
- In one step: V1 = (100 nM × 10 mL) ÷ 243,100 nM = 4.1 µL of stock. Volumes that small are hard to pipette accurately, so any error in them passes straight into the final concentration.
- Through an intermediate: dilute the stock 1:100 to 2.431 µM, then take 411 µL of that into a final volume of 10 mL.
Now suppose the powder's net peptide content were 80%. The "1.0 mg/mL" stock would really be 194.5 µM and the working solution 80 nM, not 100 nM. Any EC50 read from a curve built on it would be shifted by the same 20%. The dilution calculator handles the C1V1 = C2V2 step and the molarity calculator the mass-to-moles step.
Units ladder
| Unit | mol/L | Where it typically appears |
|---|---|---|
| mM (millimolar) | 10⁻³ | Stock solutions |
| µM (micromolar) | 10⁻⁶ | Working solutions, many cell assays |
| nM (nanomolar) | 10⁻⁹ | Potent receptor agonists |
| pM (picomolar) | 10⁻¹² | Very high-affinity binding |
Common mistakes
- Mixing units mid-calculation. 1 mg in 500 µL is 2 mg/mL, not 0.002 mg/mL; converting everything to mg and mL first avoids this.
- Using the free-peptide MW for a salt. A trifluoroacetate or acetate salt weighs more per mole of peptide; see counter-ion.
- Confusing molarity with molality. Molarity is per litre of solution, molality per kilogram of solvent. For dilute peptide solutions the numbers are almost identical, but the definitions are not.
- Assuming the stock stays at its nominal value. Adsorption to plastic and degradation lower the real concentration over time, while evaporation from a poorly sealed tube raises it; single-use aliquots limit both.
The molarity calculations guide works through more cases, including salt and net-content corrections.