Isoeletric Precipitation
Fractional precipitation can be achieved by varying the pH of the medium. At
low pH's, proteins have a net positive charge because the amide gains an extra
proton. At high pH's, they have a net negative charge due to the carboxyl on
the protein backbone losing its proton. At their pI value, a protein has no
net charge. This leads to reduced solubility because the protein is unable
to interact with the medium and will then fall out of solution. This phenomena
can be explained using the Cohn equation which is the same for salt induced precipitation since the constant in the equation in the
curve is also pH dependent. Here is an example of solubility curves for two
different proteins.
This method is commonly used for soya protein which has a pI of about 4.6.
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