I am purifying a basic protein with pI 7.8. I used 50mM tris 8.8, 300 mm NaCl, 5% glycerol,  0.1mM PMSFand 5mM beta ME  in the buffer and performed affinity purification. Eluted with 200mM imidazole.  While elution I could see slight turbid in eluted protein (I get pure protein, single band on SDS-PAGE). However, when I centrifuged I couldn't see any pellet. After some time(a day) protein got degraded. In other method I tried cation exchange purification of lysate with MES buffer pH 6.5. Here also I saw slight precipitation and later degradation.

After this again I tried with MES buffer pH 5.5 and PIPES buffer pH 6.0, here also same problem I faced.

Please suggest me a method where I can get stable protein.