1. Department of Chemistry, The Ohio State University, Columbus, Ohio 43210, Department of Molecular Biology and Biochemistry, Simon Fraser University, Burnaby, British Columbia V5A 1S6, Canada, The Skaggs Institute for Chemical Biology, Department of Cell Biology and Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, and Sandia National Laboratories, Livermore, California 94551-0969