1. Department of Chemistry, Yale University, 225 Prospect Street, P.O. Box 208107, New Haven, Connecticut 06510-8107, Laboratory of Structural Biology, Institute of Biotechnology AS CR, v.v.i., 14200 Prague 4, Czech Republic, Macromolecular Crystallography Laboratory, 539 Boyles Street, National Cancer Institute at Frederick, Frederick, Maryland 21702, and Department of Pharmacology, Yale University School of Medicine, 333 Cedar Street, New Haven, Connecticut 06520