I am trying to get pure GDP-bound tubulin. Presumably, ordering purified tubulin from bovine or porcine brain will have a mixture of GDP and GTP bound tubulin. I really need a pure GDP-bound tubulin for an SPR experiment. Does anyone have any ideas for how I might cause GTP hydrolysis. I was thinking if the buffer contained MgCl2 it might work but I am unsure.