1. Institute of Physical and Theoretical Chemistry, Institute of Biophysics, Johann Wolfgang Goethe-University Frankfurt, Max von Laue-Strasse 7, 60438 Frankfurt am Main, Germany
2. Max-Planck-Institute of Molecular Physiology, Otto-Hahn-Strasse 11, 44139 Dortmund, Germany
3. Max-Planck-Institute of Biochemistry, Am Klopferspitz 18, 82152 Martinsried, Germany.