1. Max-Planck-Institute of Biophysics, Department of Structural Biology, Max-von-Laue-Str. 3, D-60438 Frankfurt, Germany, Philipps-University of Marburg, Department of Chemistry, Hans-Meerwein-Str. Bldg. H, D-35032 Marburg, Germany, and Material Sciences Center, D-35032 Marburg, Germany