I am purifying a protein having zinc fingers using pet28a his tag.During induction I add Znso4 to the LB media.The protein has 8 cysteine residues and pI is 6.

I have tried purifying my protein using Hepes and tris buffer at pH6,6.5,7 and 8. What when I remove immidazole keeping the salt concentration high (IM NaCl) during dialysis or by Gel filtration chromatography it starts precipitating.Should i add dtt to my sample protein?would it have any impact on crystallization also I have to perform ubiquitination assays..will it interfere with these assays....need suggestions

thanks