Rigorous filtration for protein crystallization

Centrifugation or filtration of protein samples through 0.22 mu m mesh size filters is standard procedure before setting up trials for crystallization. This note concerns other, mostly unused filters, namely 0.1 mu m and 100 000-300 000 molecular weight cut-off filters, which have proved to aid in obtaining single high-quality crystals. It serves to pool together information concerning filtration which has been scattered, and somewhat concealed, in various publications over the years and to add information that has not yet been published. Filtration is relevant to all methods of crystallization for both screening and optimization.