1. Department of Biochemistry, School of Medicine, Vanderbilt University, Nashville, Tennessee 37232, Novartis Institute for Biomedical Research, CH-4002 Basel, Switzerland, Department of Chemistry and Biochemistry, Concordia University, Montreal, Quebec H4B 1R6, Canada, and Department of Chemistry and Applied Biosciences, Institute of Pharmaceutical Sciences, Swiss Federal Institute of Technology, CH-8093 Zürich, Switzerland