I expressed the protein(46kDa, PI=9.04) with His tag in E.coli and purified it with cobalt beads. However, when I was using Amicon Ultra 15 mL Centrifugal Filters to concentrate elution flowed through the cobalt column, protein aggregated. The whole process is on the context of 4 degrees celcius and buffer is 20 mM Tris, 150 NaCl, pH7.5. Could someone give me some suggestions to solve protein aggregation?