1. Department of Chemistry, University of Illinois at Urbana−Champaign, 600 South Mathews Avenue, Urbana, Illinois 61801, Howard Hughes Medicinal Institute, The Salk Institute for Biological Studies, Jack H. Skirball Center for Chemical Biology & Proteomics, 10010 Torrey Pines Road, La Jolla, California 92037, and Department of Chemistry, University of California San Diego, 9500 Gilman Drive, La Jolla, California 92093