1. Department of Chemistry, New York University, 100 Washington Square East, New York, New York 10003, Department of Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, Department of Molecular and Cell Biology, University of California, Berkeley, California 94720, Genomics Institute of the Novartis Research Foundation, 3115 Merryfield Row, Suite 200, San Diego, California 92121, and Department of Chemistry, University of California, Berkeley, California 94720