1. van 't Hoff Institute for Molecular Sciences, University of Amsterdam, Nieuwe Achtergracht 166, 1018WV Amsterdam, The Netherlands
2. Dutch Polymer Institute P. O. Box 902, 5600 AX Eindhoven, The Netherlands
3. School of Chemistry, University of St. Andrews, St. Andrews, Fife, Scotland KY16 9ST, UK, Fax: +44‐1334‐463‐808
4. Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht Univerity, Padualaan 8, 3584 CH Utrecht, The Netherlands