Hello everyone!
I have listened someone suggested that,  first use affinity chromatography 
(Ni-NTA),  then use SEC (superdex200 increase),  and finally used ion exchange 
(monoQ),   to purified protein,  which will be used to crystallization.
My question  is why  the monoQ used in the finally step,  why not the SEC used 
at the finally step?

sincerely
Liuqing Chen

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