1. Laboratory for Bioorganic Chemistry, Sloan-Kettering Institute for Cancer Research, 1275 York Avenue, New York, New York 10021, Department of Chemistry, Columbia University, Havemeyer Hall, 3000 Broadway, New York, New York 10027, and Department of Physiology and Biophysics, Weill Medical College of Cornell University, 1300 York Avenue, New York, New York 10021