1. Department of Biological Sciences, Purdue University, West Lafayette, Indiana 47907-1392, Departments of Biochemistry and Chemistry, and the Rosenstiel Basic Medical Sciences Research Center, Brandeis University, Waltham, Massachusetts 02254-9110, Department of Pharmaceutical Chemistry, University of California, San Francisco, California 94143-0446, and Department of Biochemistry, University of Illinois, Urbana, Illinois 61801