1. Department of Chemistry, University of Virginia, Charlottesville, Virginia 22904, The Joint Center for Structural Genomics and Department of Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, and Departments of Physics and Applied Physics, Biophysics Program, and Stanford Synchrotron Radiation Laboratory, Stanford University, 476 Lomita Mall, Stanford, California 94305