1. Department of Chemistry, The Skaggs Institute for Chemical Biology, Department of Immunology and Microbial Science, Department of Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, and the Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, La Jolla, California 92093