1. Department of Chemistry and The Skaggs Institute of Chemical Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, Departments of Chemistry, Biology, and Chemical and Biological Engineering, Rensselaer Polytechnic Institute, Biotechnology Building, 110 8th Street, Troy, New York 12180-3590, and Department of Cell and Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037