1. Instituto de Química Médica, CSIC, Juan de la Cierva, 3, 28006 Madrid, Spain, IBLS, Division of Infection & Immunity, University of Glasgow, Glasgow G12 8QQ, U.K., Swiss Tropical Institute, Socinstrasse 57, CH-4002 Basel, Switzerland, and Department of Chemistry, Georgia State University, Atlanta, Georgia 30303-3083