A peptide solution's concentration can be expressed as mass-per-volume (milligrams per milliliter, mg/mL) or as molarity (micromolar, µM, or nanomolar, nM); converting between them uses the peptide's molecular weight. This guide explains both.
Mass-per-volume (mg/mL)
This is the simplest expression: how many milligrams of peptide are in each milliliter of solution. It is mg/mL = peptide mass (mg) ÷ solution volume (mL) and is what you use to measure out a mass.
Molarity (µM, nM)
Molarity expresses concentration as the number of molecules per volume, which many in-vitro assays specify. A micromolar (µM) is one-millionth of a mole per liter; a nanomolar (nM) is one-billionth.
How to convert
Converting mass-per-volume to molarity requires the peptide's molecular weight (MW, in g/mol or daltons) from its Certificate of Analysis. Molarity (mol/L) = concentration (g/L) ÷ MW (g/mol). Because peptides are large, a few mg/mL often corresponds to micromolar or nanomolar concentrations.
Why molarity matters for assays
Many assays are defined by the number of molecules present, not the mass, so a protocol will specify a concentration in µM or nM. Knowing the molecular weight lets you translate a prepared mg/mL stock into the molarity an assay calls for.
Related research guides
- Peptide concentration math guide
- Stock & working solutions (serial dilution)
- mcg, mg & molarity units primer
Research-use note
This describes laboratory concentration for in vitro research use only.
Related research: LIPO-C | MIC-C.
