1. Department of Chemistry, State University of New York at Buffalo, Buffalo, New York 14260-3000, Department of Chemistry, Wake Forest University, Box 7486, Winston-Salem, North Carolina 27109, Departments of Radiology and of Physiology and Pharmacology, Wake Forest University School of Medicine, Winston-Salem, North Carolina 27157, and Targacept, Inc., P.O. Box 1487, Winston-Salem, North Carolina 27102-1487