Determination of Bragg peaks from a flat single crystal using a rotation photograph

This procedure is most effective when the precipitant cannot pass through the dialysis membrane, e.g. polyethylene glycol (PEG), or for isoelectric crystal l izations. In the first case the final concentration of protein is easily control led by varying the init ial PEG concentration in the protein solution, and may be used to concentrate a di lute protein solution. In experiments using salts or small molecules as precipitants, which can easily pass through the membrane, after the init ial movement of water, the level of the solution in the dialysis cell wil l seek the level of the external solution. In this case crystal l izat ion may stil l occur, but the protein concentrat ion wil l not be well defined because there is no constraint on the final volume occupied by the protein solution. This procedure was developed for crystal l iz ing viral components of polyoma virus and has been tested using southern bean mosaic virus. An example is shown in Fig. 2; the experiment which used 2 /.tl of material c learly shows wellformed microcrystals of a form not previously characterized.