1. Research Group for NMR Signal Enhancement, Max Planck Institute for Biophysical Chemistry Am Fassberg 11 37077 Göttingen Germany
2. Center for Biostructural Imaging of Neurodegeneration Von-Siebold-Str. 3A 37075 Göttingen Germany
3. Department of Chemistry and Biochemistry University of California at Los Angeles Los Angeles CA 90095-1569 USA