tokens: The dimer fractions were concentrated to ~ 6 mg ml - 1 concentration using an YM10 ultrafiltration membrane ( Millipore - Amicon ) and further purified by gel filtration on a HiLoad XK 16 Superdex 200 prep - grade column ( GE Healthcare ) , which was previously equilibrated with a buffer of 50 mM Tris - HCl ( pH 7 . 4 ) , 200 mM sodium chloride , 1 mM dithiothreitol and 1 mM EDTA .