1. Departments of Molecular Biology and Chemistry, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, California 92037, Biotechnology Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, Department of Chemistry and Biochemistry, University of Notre Dame, Notre Dame, Indiana 46556, and Department of Chemistry and Biochemistry, University of California, Los Angeles, California 90095-1569