1. Contribution from the Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Laboratory of Macromolecular and Organic Chemistry, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands, Niels Bohr Institute, University of Copenhagen, DK-2100 Copenhagen, Denmark, and Department of Biotechnology Engineering, Ben Gurion University of the Negev, P.O. Box 653, 84105, Beer-Sheva, Israel