1. Contribution from the Laboratory of Supramolecular Chemistry and Technology, MESA+ Research Institute, University of Twente P.O. Box 217, 7500 AE Enschede, The Netherlands, Department of Biomolecular Mass Spectrometry, Utrecht Institute for Pharmaceutical Sciences and Bijvoet center for Biomolecular Research, Utrecht University, Sorbonnelaan 16, 3584CA Utrecht, The Netherlands, and Department of Organic Chemistry, University of Leuven, Celestijnlaan, Leuven, Belgium