1. Stratingh Institute for Chemistry University of Groningen Nijenborgh 4 9747 AG Groningen The Netherlands
2. NMR Spectroscopy group Bijvoet Center for Biomolecular Research Utrecht University Padualaan 8, 3584 CH Utrecht The Netherlands
3. Current address: Structural and Computational Biology Unit European Molecular Biology Laboratory Meyerhofstraße 1 69117 Heidelberg Germany