1. Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands
2. Current address: Bijvoet Center of Biomolecular Research, Department of NMR Spectroscopy, Utrecht University, Padualaan 8, 3584 CH, Utrecht, The Netherlands
3. Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH, Utrecht, The Netherlands