1. Contribution from the Departments of Structural Biology and Neurobiology, Weizmann Institute of Science, Rehovot 76100, Israel, Institut de Chimie des Substances Naturelles, C.N.R.S., Avenue de la Terrasse F-91190 Gif-sur-Yvette CEDEX, France, and BioStrX Ltd., 2a Katzir Street, Ramat Gan, 52656, Israel