1. Department of Chemistry and Center for Macromolecular Crystallography, University of Alabama at Birmingham, Birmingham, Alabama 35294, BioCryst Pharmaceuticals, Inc., 2190 Parkway Lake Drive, Birmingham, Alabama 35244, Department of Biochemistry and Molecular Biology, University of Oklahoma, Oklahoma City, Oklahoma 73190, and John Curtin School of Medical Research, Australian National University, Canberra 2601, Australia