1. Laboratory of Physical Chemistry of Polymers and Membranes, Chemistry Department, Swiss Federal Institute of Technology, CH-1015 Lausanne, Swizerland, Geneva Biomedical Research Institute, Glaxo Wellcome Research and Development S.A., Chemin des Aulx 14, CH-1228 Plan-les-Ouates, Geneva, Swizerland, and F. Hoffmann-La Roche, Research Laboratories, CH-4070 Basel, Switzerland