1. Contribution from the Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106-7078, Department of Biochemistry, University of Georgia, Athens, Georgia 30602, Department of Chemistry, Chemical Instrumentation Center, The Ohio State University, Columbus, Ohio 43210, Department of Neurology, Duke University Medical Center, Durham, North Carolina 27710, and Suntory Institute of Bioorganic Research, Wakayamadai, Shimamoto-cho, Mishima-gun, Osaka 618, Japan