Calculating Protein Concentration

Tom Anderson via (by ucgatan from
Thu Jan 17 14:40:08 EST 2008

On Thu, 17 Jan 2008, DK wrote:

> In article <mailman.219.1200581980.2451.methods from>, "Yvonne Couch" <yvonne.couch from> wrote:
> >I have two proteins of different molecular weights, how do I calculate
> >the molarity of any solution I dissolve them in?  For example Protein 1
> >is 15kDa and 1mg is dissolved in 1ml of water, how do I calculate how
> >many moles this is?
> For this calculation you'll need to learn how molarity is defined...

Which, since Yvonne is a technician in a hurry, i shall (try to) explain.

Molarity, aka concentration, is the number of moles of something divided
by the volume it's in.

If you don't know the number of moles, but you know how much of the thing
you have by mass, and the molecular mass of it, you can work it out.
Molecular mass is defined as the mass of one mole of something (kinda),
with a dalton (Da) being one gram per mole, so you just divide the mass by
the molecular mass.

So, 1 mg of a 15 kDa protein is 0.001 g / 15000 g/mol = 0.000000067 mol,
or 67 nmol. 67 nmol / 0.001 l = 67 uM.

67 nanomoles may not sound like a lot, but don't worry, it's still 40
thousand million million molecules.


Tom Anderson, MRC Laboratory for Molecular Cell Biology, UCL, London WC1E 6BT
(t) +44 (20) 76797264   (f) +44 (20) 76797805   (e) thomas.anderson from

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