1. Université de Mons-Hainaut, Laboratoire de Physicochimie des Polymères, Place du Parc, 20, B-7000 Mons, Belgium; European Molecular Biology Laboratory, Hamburg Outstation, EMBL c/o DESY, Notkestrasse, 85, D-22603 Hamburg, Germany; and Division of Chemistry, National Research Council of Canada, Ottawa, Ontario K1A OR9, Canada