1. Departments of Biochemistry and Chemistry, University of Illinois at Urbana-Champaign, 600 S. Mathews Avenue, Urbana, Illinois 61801, Department of Biochemistry, Albert Einstein College of Medicine, Bronx, New York 10461, and and Department of Biopharmaceutical Sciences, School of Pharmacy and California Institute for Quantitative Biomedical Research, University of California, 1700 4th Street, San Francisco, California 94158