W0204

MRSAD: A Simple, SAD-Based Verification Strategy for Difficult Molecular Replacement Problems. Jonathan P. Schuermann and John J. Tanner, Depts. of Chemistry and Biochemistry, Univ. of Missouri-Columbia, Columbia, MO 65211, USA.

A strategy for assessing the correctness of trial molecular replacement (MR) solutions, based on single wavelength anomalous dispersion (SAD) from S atoms collected at Cu Kalpha, is presented. For each MR solution to be tested, SAD phases are calculated using the coordinates of a subset of the S atoms of the MR solution, and these initial SAD phases are then used to calculate an anomalous difference Fourier map. If the map has strong peaks corresponding to the S atoms omitted from the SAD phase calculation, then the trial MR solution is confirmed as being correct. Otherwise, the trial MR solution is likely to be wrong. Since the method does not use anomalous difference Patterson maps or dual-space methods to obtain the initial S atom substructure, the method is very fast and easy to implement, and it is applicable to cases in which the inherent anomalous signal is low and the crystal quality is marginal. This approach is demonstrated using a 2.5 Å data set collected from a Fab crystal that has a theoretical anomalous signal of only 0.8 %. The favorable results obtained from this realistic test case suggest that anomalous differences from S atoms should be routinely collected and used in MR structure determination.