DIRECT METHODS PHASING OF A 450 ATOM STRUCTURE. Gil Privé¹ and David Eisenberg². ¹Ontario Cancer Institute, 610 University Ave., Toronto, Canada M5G 2M9. ²Molecular Biology Institute, UCLA, Los Angeles, California 90024-1570, USA

The Shake-and-Bake method can generate automatic solutions for large structures provided that atomic resolution data are available. We have solved the structure of a triclinic crystal containing four independent copies of a twelve residue [[alpha]]-helical peptide using this method. The final structure contains 408 peptide atoms, 2 ethanolamine molecules, 1 MPD, 1 chloride ion and 25 water molecules, for a total of 450 non-hydrogen atoms. The structure has been refined with SHELXL-93 to R=.112 and R_free=0.132 for all data to 0.92 Å (23,681 reflections). The success depended critically on the accurate collection of high resolution data, and special care was taken to measure a complete dataset for the P1 crystals. A single crystal was flash frozen at beamline X12C at the NSLS, and data were collected on the 30 cm MAR image plate detector. Different detector 2[[Theta]] offsets were used to collect the high angle data, and several crystal orientations were required to get complete coverage of the unique hemisphere of reciprocal space. This posed special problems for the data collection strategy because of the very short crystal-to-detector distance used. The structure of alpha-1 reveals an unusual arrangement of helices packed head-to-tail into quasi-infinite columns, which in turn associate laterally into extended sheets of tightly packed [[alpha]]-helices. The sheets then form layers with alternating hydrophobic and hydrophilic interfaces. Certain elements of the intended 4-helix bundle design are found in the structure, as well as many novel features.