tokens: Crystals were grown by the hanging - drop vapor diffusion method at 24 degrees C by mixing equal volumes ( 2 mu l each ) of the protein solution ( at 19 mg ml - 1 concentration in a buffer consisting of 50 mM Tris - HCl ( pH 7 . 4 ) , 1 mM dithiothreitol , 1 mM EDTA and 200 mM sodium chloride ) and the reservoir solution .