Precipitation by Non-ionic Polymers
This form of precipitation involves the addition of a non-ionic polymer to the
protein solution. This works because the addition of the polymer reduces the
amount of water available to interact with the protien.
The polymers that have received the most attention have
been dextrans and polyethylene glycols. An expression to explain this is
The protein self interaction term has an effect at high concentrations of
protein, and at pH values away from the isoeletric point.
The type of non-ionic polymer to be added to a protein solution depends on the molecular weight of the protein. The protein feed itself should be aqueous and at the isoelectric pH. At this pH it can again be noted (same as for isoelectric precipitation) that the solubility is reduced. The non-ionic solvents are generally organic solvents such as ethanol or acetone. Organic solvents other than these are not used, they could be too harsh and cause denaturation. The precipitation is caused by dilution.
Here are five tips for this precipitation:
- Low temperature precipitation increases yield and reduces denaturation.
- Ionic strengths of 0.05 to 0.2 M. Concentrations higher than these will use excess solvent, while a more diluted solution will lead to a poor separation.
- The higher the molecular weight of the solute, the less solvent required for the precipitation. For acetone initiated precipitations, Scopes (1982), has the following useful equation:
[(v/v)%] = 1.8- 0.12 ln [MW]
where (v/v)% = volume % required for precipitation
where MW = molecular weight of the solute
- If two proteins are present, the solubility of one protein will be decreased because of the other protein's presence.
- A precipitated solute that doesn't redissolve is most likely denatured. This denaturation can hurt the overall yield, but it can offer further purification.
These tips taken from
- "Bioseparations" by Belter, Cussler, and Hu
The advantage of using
non-ionic polymers as precipitants is that they stabilize proteins and may
be used around room temperature.
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