1. Contribution from the Junior Research Group “Solid-State NMR Studies of Membrane-Associated Proteins”, Biotechnological-Biomedical Center of the University of Leipzig, D-04107 Leipzig, Germany, Max Planck Institute of Molecular Physiology, D-44227 Dortmund, Germany, Institute of Medical Physics and Biophysics, University of Leipzig, D-04107 Leipzig, Germany, and Departments of Chemistry and Physics, University of Arizona, Tucson, Arizona 85721.