1. Department of Chemistry, Northwestern University, 2145 N. Sheridan Road, Evanston, Illinois 60208-3113, Department of Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, Department of Chemistry and Biochemistry, Utah State University, 0300 Old Main Hill, Logan, Utah 84322-0300, and The Department of Biochemistry, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061-0002