How do I choose the right molecular weight cut-off (MWCO) for protein?

The molecular weight cut-off (MWCO) is a term used in membrane filtration to denote the pore size distribution and retention capability of a membrane. In general, for optimal recovery of most macromolecules, the recommendation is to use an ultrafiltration membrane with an MWCO that’s 1/3 to 1/2 of the target molecule’s molecular weight. 

MWCO Selection for Protein

• Determine the average molecular weight of an amino acid.
• Calculate the protein's molecular weight based on its amino acid composition.
• Select an ultrafiltration membrane with an appropriate MWCO, typically around one-third of the protein's weight.

For instance, the average molecular weight of an amino acid residue in a peptide chain is approximately 110 Da. 

Immunoglobulin IgG has 2 heavy chains and 2 light chains. Each heavy chain comprises an average of 500 amino acid residues. Each light chain comprises approximately 214 amino acid residues. Hence, the molecular weight of IgG can be estimated at (500 x 2 + 214 x 2) = 1420 x 110 Da = 156, 200 Da = 156 kDa. 

In this case, you would require a membrane with an MWCO of approximately 52 kDa (1/3 of 156).