THE ANALYTICAL APPROACH OF PHASING BY MULTIWAVELENGTH ANOMALOUS DIFFRACTION (MAD). Hao Wu & Wayne A. Hendrickson, 630 W. 168Th Street, Dept. Of Chemistry, Columbia University, New York, NY 10032

The de novo phasing of macromolecular crystal structures by MAD relies on the mathematical relationships among the structure factors of both Friedel mates at particular wavelengths. The least-squares solutions to these equations give rise, among other parameters, to the structure factor amplitudes of the anomalous scatterers alone which are optimal for locating the anomalous scattering centers by either Patterson synthesis or direct method. The desired phase angles for the crystal can then be determined explicitly or represented by phase probability distributions using the Hendrickson-Lattman coefficients.

The MADSYS package, which was developed over the years to perform analytical MAD phasing, was recently implemented to maximize automation and self-containment while improving performance. The sampling in derived structure factor amplitudes should also give better phase probabilities.