Hi there,

you have to know the MW of your protein. mass of protein in gram = number of mole*MW in gram/mole. So if you know your protein concentration in µM then multiply it by the MW to get it in µg per L.

Now protein MW is referred in kDa (kilo daltons). 1 kDa = 1000 g/mole.

Feed this in the equation above.

Recall that "micromolar" is a unit of concentration, i.e., quantity in moles per unit volume.  And recall that "microgram" is a unit of "quantity in grams."  So you need to know both the molecular weight (MW) of your compound and the volume of solution it is dissolved in.  Multiplying "micromolar" with volume will give you "quantity in moles."  Multiplying "quantity in moles" with the MW (in grams per mole) will give you "quantity in grams."  This is general chemistry stuff you learn early in high school. 

Dear Ramesh K Jha and Dominique Liger,

many thanks for your helpful and nice comments;)