Dehydration leads to a phase transition in monoclinic factor XIII crystals

Monoclinic factor XIII crystals have been transferred to a solution contain ing increasing amounts of the precipitant PEG 6000. At a concentration of a bout 36%(w/v) PEG 6000, a phase transition was observed. The space group of the crystals was preserved on the transition, but half of the 2(1) screw a xes were lost, which meant that the unit-cell volume and the content of the asymmetric unit were doubled. The structure of factor XIII in the new crys tal form was solved by molecular replacement. About 80% of the changes acco mpanying the transition can be explained by a rigid-body rotation of half o f the factor XIII dimers in the lattice by about 5 degrees. The remaining c hanges are mostly small interdomain movements of the four domains which con stitute one factor XIII monomer.