Following the concentration of the column fractions, I dialyze from 100 mM imidazole/500 mM NaCl/20 mM Tris ph 7.9 into 50 mM NaCl/20 mM Tris pH 7.4 overnight. The protein precipitates out of solution and I have been unable to resuspend it. I suspect the aggregation is due to hydrophobic interactions, since the wild type tryptophan protein does not precipitate.
I have tried increasing the NaCl concentration to 500 mM but this has not increased solubility. I would appreciate any suggestions as to what changes in the buffer might prevent protein precipitation.