1. Contribution from the Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Department of Biophysics, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands, and Max-Planck-Institute für Polymerforschung, Postfach 3148, 55021 Mainz, Germany