In general does the pH effect the precipitation, e.g. would a pH of 6 cause less precipitation than pH 7.5. Or are they unrelated?
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I think it would affect some proteins, but ammonium sulfate precipitation usually requires such large amounts of salt, that you would have to add a lot of base to adjust the pH.
When I have done ammonium sulfate precipitation, its to take a rough cut of proteins out of a whole cell/whole organ lysate - its more like a hammer than a pair of tweezers as far as protein purification is concerned. I would rather use a charged plastic resin or a sizing column if I were to care about the isoelectric purification.
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$\begingroup$ Thanks. I'm actually trying to stop precipitation when incubating a protein in ammonium acetate, and I wondered if an acidic pH would reduce this. Do you have any suggestions $\endgroup$– AnakeCommented Oct 9, 2012 at 12:30
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1$\begingroup$ Do you know the pI of your protein? Proteins are least soluble at pH=pI. $\endgroup$ Commented Oct 9, 2012 at 13:10
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$\begingroup$ also consider detergent - might the protein be a bit hydrophilic? $\endgroup$– shigetaCommented Oct 9, 2012 at 13:51
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$\begingroup$ @Anake, I would look up en.wikipedia.org/wiki/Hofmeister_series $\endgroup$ Commented Oct 10, 2012 at 8:45
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$\begingroup$ the protein isn't hydrophobic, it is the ammonium's precipitating tendency as in the Hoffmeister series. I HAVE to use an ammonium salt, so the hoffmeister series cannot help me in that respect $\endgroup$– AnakeCommented Oct 10, 2012 at 13:48