1. European Molecular Biology Laboratory, Meyerhofstrasse 1, 69117 Heidelberg, Germany, Departamento de Fisiología y Farmacología, Universidad de Alcalá, 28871 Alcalá de Henares, Madrid, Spain, Laboratorios Menarini, SA, R & D Department, Alfonso XII 587, 08912 Badalona, Spain, and Department of Chemistry, University of Perugia, Via Elce di Sotto, 8, 06100 Perugia, Italy