1. Departments of Structural Biology and Neurobiology, Weizmann Institute of Science, Rehovot 76100, Israel, Instituto di Ricerche di Biologia Molecolare P. Angeletti, Pomezia (RM) 00040, Italy, Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061, and Computer-Aided Molecular Design Laboratory, Mayo Clinic College of Medicine, Rochester, Minnesota 55905.