MacCHESS is involved in phasing of macromolecular data using MAD phasing techniques, direct methods and more recently multiple-beam diffraction. In order to implement MAD phasing at CHESS we have an ongoing effort for hardware and software development. Station F-2 was equipped with double crystal, focusing optics, a rotation camera, CCD detector, liquid nitrogen cooling apparatus and scintillation detectors for fluorescent measurements during energy scans of sample and reference materials. New software was implemented to rapidly change energies while maintaining the alignment of the x-ray beam relative to the sample. At F-2, the energy resolution is about 3-4 eV and exposure times are typically 20-30 seconds per degree of crystal rotation. These parameters are well suited to MAD phasing experiments. During the past year we have focused on analyzing MAD structure of greater and greater complexity. The include 5'-methylthioadenosine phosphorylase (9 Se atoms), thiamin phosphate synthase (16 Se atoms), glycinamide ribonucleotide reductase (14 Se atoms), aminoimidazole ribonucleotide reductase (28 Se atoms) and S-adenosylmethionione decarboxylase (24 Se atoms). All five of these structures have been determined using MAD data. More recently, we have located used MAD data to locate the 66 Se atoms of a previously unknown epimerase structure. The phasing process is just now underway. We have also used direct methods for direct structure determination. We have developed procedures for measuring high resolution data using CHESS stations A-1 or F-1, cryocrystallography, CCD detectors. Using 1.0 ? resolution data for triclinic lysosyme (1200 atoms including ordered solvent), we have determined the structure with the Shake and Bake procedure developed by Hauptman, et al. We have also measured data for xylan esterase (1500 atoms) and the structure analysis is currently underway. Experimental measurement of phases using multiple beam diffraction is just now underway at CHESS. Preliminary experiments have lead to a new procedure for data collection in which the crystal is rotated about a reference beam and analysis of the results are encouraging.
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