E0036

Mosaic Modeling of Protein Crystals: Segregated Mosaic Domain Model. Tae-Sung Yoon, Jurgen Sygusch, Département de biochimie, Université de Montréal, Montréal, PQ, H3C 3J7, Canada.

Mosaicity of protein crystals has been used to characterize protein crystal quality and to optimize the quality of protein data reduction. Synchrotron radiation, by minimizing instrumental broadening effects ¹, has afforded routine investigation of mosaic spread in diffraction profiles while fine-sliced data collection has allowed a detailed representation of the rocking curve profile as the protein crystal sweeps through the sphere of reflection ². X-ray topographical studies have indicated that the common mosaic block model represents an oversimplification ¹ that is best replaced by mosaic structure composed of distinct mosaic domains each possessing its own distinct rocking curve profile ³. Multiple Gaussian fitting of 1-D rocking curve has been used with some success to analyze mosaic spreads from rocking curve profiles in the presence of multiple mosaic domains. To maximize resolution of the mosaic domains and to discern anisotropic broadening effects due to mosaicity, fine-sliced synchrotron diffraction images of protein crystals were analyzed in terms of a linear combination of 3-D anisotropic Gaussian profiles. A graphical interface, written in MATLAB and C languages, was developed that allows fitting of 3-D Gaussian profiles to the diffracted intensity distribution. The 3-D profile analysis of mosaic structure was based on the segregated mosaic-domain model ³ where each domain is represented by a 3-D double Gaussian profile. Profile fitting is performed by non-linear least squares minimization and includes positional refinement of each mosaic domain as well as anisotropic refinement of mosaic spreads from each domain. Relative populations of each double Gaussian representation are constrained identically for all Bragg reflections while relative magnitudes of the eigenvectors describing the anisotropic half-widths are restrained. The use of constraints and restraints ensure identical domain populations for all Bragg reflections from a single crystal. Mosaic spread analysis of microgravity grown protein crystals from the Canadian Advanced Protein crystallization Experiment (CAPE) ³ and STS-95 mission indicated the feasibility of such an approach. Results from the study of space- and earth- grown crystals will be presented.

¹Nave, C. (1999). Acta Cryst. D55, 1663-1668.

²Bellamy, H. D. et al. (2000). Acta Cryst. D56, 986-995.

³Yoon, T.-S., Tetreault, S., Bosshard, H. E., Sweet, R. M. & Sygusch, J. (2001). J. Crystal Growth, in press.