Rice University

In addition to the diffraction intensity observed in the Bragg spots, many crystals show pronounced diffuse scattering. While the Bragg spots contain information about the time and space averaged structure, the diffuse scatter contains information about disorder and dynamical processes in the crystals. The buildup of diffuse features accompanies the reduction in scattering in the Bragg peaks. We are developing methods to accurately visualize the non-Bragg scattering in reciprocal space in order to learn more about the disorder and dynamics captured in the diffraction pattern. We want to explore of the effects of conformational heterogeneity on the diffraction pattern. An understanding of these effects is needed for XFEL studies as crystals become smaller and approach small cluster sizes.

Crystals of the I349V mutant of methionine adenosyltransferase sMAT from Sulfolobus solfataricus show strong diffuse features. Analysis of the crystal packing shows that there are 2 protomers of the biological tetramer in the asymmetric unit with the tetramer generated by crystallographic symmetry.  One of the protomers in the asymmetric unit (half of the tetramer with its symmetry mate) makes extensive crystal contacts with neighboring tetramers, while the second protomer primarily makes contact with the other chains within the tetramer.  This allows this second protomer to move, breaking crystal lattice symmetry and increasing the diffuse scattering. This is also evident in the mean B-factors refined for the two protomers where one chain has a mean B of ~80 and the other chain has a mean B of ~180.