Molarity and concentration in peptide research

Once a lyophilised peptide is reconstituted, the important question for laboratory work is: what is its concentration? There are a few ways to express that, and in-vitro assays usually care about molarity rather than mass. Here's how the units relate, with worked examples.

Mass concentration: mg/mL and µg/mL

The simplest way to describe a solution is mass per volume. If you dissolve 10 mg of peptide in 2 mL of solvent, the concentration is:

10 mg ÷ 2 mL = 5 mg/mL (equivalently, 5,000 µg/mL).

Mass concentration is easy to calculate and useful for record-keeping, but it doesn't directly tell you how many molecules are present — and biological assays are usually designed around molecule count.

Molar concentration: mM and µM

Molarity expresses concentration as moles per litre, which is proportional to the number of molecules. To convert from mass concentration to molarity you need the peptide's molecular weight (MW):

molarity (mol/L) = mass concentration (g/L) ÷ MW (g/mol)

Worked example: our 5 mg/mL solution is 5 g/L. If the peptide's MW is 1419.5 g/mol:

5 ÷ 1419.5 = 0.00352 mol/L = 3.52 mM (or 3,520 µM).

Because different peptides have very different molecular weights, two solutions at the same mg/mL can have quite different molarities — which is exactly why assays specify molar concentrations.

Let the calculator do it

You don't need to run these by hand. Our reconstitution & molarity calculator takes the vial mass, the solvent volume and the molecular weight (which it can autofill for our products) and returns mg/mL, µg/mL and molar concentration in one step. It's the fastest way to prepare a solution to a target concentration for in-vitro work.

A note on molecular weight

Every DNR product page lists the molecular weight, and each Certificate of Analysis confirms the compound's identity by mass spectrometry — so the MW you're calculating with is the right one for that exact lot. You can verify a batch here, and our reconstitution guide covers the practical handling.

The takeaway

Mass concentration (mg/mL) is the easy number; molar concentration (mM/µM) is the one assays are built around, and converting between them just needs the molecular weight. Use the calculator to skip the arithmetic, and start from a lot whose identity you've verified. For laboratory and research use only.