1. Institut für Physikalische und Theoretische Chemie, Marie-Curie-Strasse 11, Johann Wolfgang Goethe-Universität Frankfurt, 60439 Frankfurt am Main, Germany, Max-Planck-Institute of Biophysics, Department of Biophysical Chemistry, Marie-Curie-Strasse 15, 60439 Frankfurt am Main, Germany, and Max-Planck-Institute of Molecular Physiology, Department of Physical Biochemistry, Otto-Hahn-Strasse 11, 44227 Dortmund, Germany