1. European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany, Universität Ulm, Sektion Polymere, Albert-Einstein-Allee 11, D-89081 Ulm, Germany, and Merck Farma-Química, Laboratori de BioInvestigació, C/Casp 108, 08010 Barcelona, Spain