1. Department Molecular Membrane Biology, Max-Planck-Institute of Biophysics, 60438 Frankfurt am Main, Germany, Institute for Biophysics, Johann-Wolfgang-Goethe-University, 60438 Frankfurt am Main, Germany, Institute for Physical and Theoretical Chemistry and Center for Biomolecular Magnetic Resonance, Johann-Wolfgang-Goethe-University, D-60439 Frankfurt am Main, Germany, and Wolfson Institute for Biomedical Research, University College London, London WCIE6BT, U.K.