1. Department of Molecular Physics, Huygens Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands, Chemical Biology & Organic Chemistry, Bijvoet Center for Biomolecular Research, Utrecht University, Utrecht, The Netherlands, and Medicinal Chemistry and Chemical Biology, Utrecht Institute for Pharmaceutical Sciences, Department of Pharmaceutical Sciences, Faculty of Science, Utrecht University, Utrecht, The Netherlands