1. Institute of Physical and Theoretical Chemistry and Center for Biomolecular Magnetic Resonance, J. W. Goethe University of Frankfurt, D-60439 Frankfurt, Germany, Institute of Physical Chemistry III, Darmstadt University of Technology, D-64287 Darmstadt, Germany, and Institute of Biophysics and Physical Biochemistry, University of Regensburg, 93053 Regensburg, Germany