Abstract:
In attempts to produce diffraction-quality crystals of Fabs from two human IgM cryoglobulins (Pot and Yvo), we
observed unexpected interconversions in crystal morphologies. The Pot Fab crystallized in two forms when
polyethylene glycol (PEG) 6,000 was used as the precipitating agent. Broad, relatively short rods with hexagonal crosssections
were our crystals of choice. However, long, thin rods that were often curvilinear (i.e., "fibril-like" crystals) also
appeared in profusion. A third form crystallizing in water had similar but not identical morphology to the fibril-like
crystals. In the presence of NaC!, the hexagonal rods were shortened to hexagonal bipyramids and the growth rates of
the fibril-like crystals were also greatly reduced. Under the same conditions, we observed an unusual phenomenon, in
which hexagonal pyramids grew along the fibril-like crystals, like candy crystals growing on strings. PEG 6,000 was also
used to crystallize the Yvo Fab, but the early crystals were highly twinned and imperfectly formed. Among the additives
tested to improve the quality of Yvo Fab crystals, 1% (v/v) dimethylsulfoxide (DMSO) provided the most dramatic
effect. Initial clusters of small, irregular crystals were replaced with large crystals suitable for X-ray analysis over a
period of twelve days.
