how to purify a protein with PI=4.8

[xuyihui]

now we have a fusion protein expressed in Ecoli GI 724,it has a PI4.8 and
40KD.between the proteins we have a POLY_his,but the usual method for
his-tag proteins is not efficient for it .and I use the DEAE chromotography
,it is not efficient yet,How can I do?
   I'll be very greatful for your help!
yihuixu


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