Department of Chemistry, Georgia State University, Atlanta, Georgia 30303, The Cancer Research UK Biomolecular Structure Group, The School of Pharmacy, University of London, London WC1N 1AX, UK, INSERM U-524 IRCL, Place de Verdun, 59045 Lille, France, and Swiss Tropical Institute, CH-4002 Basel, Switzerland (At 2002-10-29)