1. Department of Pharmacology, University of Medicine and Dentistry of New Jersey, Robert Wood Johnson Medical School, Piscataway, New Jersey 08854, Department of Chemistry, Rutgers-The State University of New Jersey, Piscataway, New Jersey 08854-8087, The Cancer Institute of New Jersey, New Brunswick, New Jersey 08901, and Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125