1. Department of Chemistry and Biochemistry, Hunter College of The City University of New York, 695 Park Avenue, New York, New York 10021, Department of Medical and Molecular Parasitology, New York University School of Medicine, 341 E. 25th Street, New York, New York 10010, and Aaron Diamond AIDS Research Center, The Rockefeller University, 455 First Avenue, New York, New York 10016