1. Leiden Institute of Chemistry, Gorlaeus Laboratoria, University of Leiden, 2300 RA Leiden, The Netherlands, Department of Biophysics, Huygens Laboratory, University of Leiden, 2300 RA Leiden, The Netherlands, and University of Nijmegen, NWO/CW HF-NMR Facility, Toernooiveld, 6525 ED Nijmegen, The Netherlands