1. Laboratory of Molecular Physics and Wageningen NMR Centre, Department of Biomolecular Sciences, Wageningen University, Dreijenlaan 3, 6703 HA Wageningen, The Netherlands, and Research Center of Nucleic Acid and Peptide Chemistry, Institute of Organic Chemistry, University of Tübingen, Auf der Morgenstelle 18, 72076 Tübingen, Germany