1. Max-Planck-Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany, Department of Chemistry, Life Sciences Department, Paul Scherrer Institut, CH-5232 Villigen PSI, Switzerland, and Department of Biochemistry B, The Panum Institute, IMBG, Blegdamsvej 3c, DK-2200, Copenhagen N, Denmark