Recently, we attempted to purify his-tag heme protein by Ni-NTA resin. Everything goes well, and we got elution fractions like wine. Elution buffer contains 250 mM imidazole, 50 mM tris-Hcl (pH 7.5) and 400 mM NaCl. But the protein started to precipitate when it came to the dialysis step that aims to remove imidazole. After the overnight dialysis at 4 degree, proteins with red color almost precipitated. It seems that imidazole is essential for the enzyme stability. Do you think is it possible? Any suggestions are welcome!