1. Contribution from the Department of Biophysical Organic Chemistry, Leiden Institute of Chemistry, Leiden University, Einsteinweg 55, P.O. Box 9502, 2300 RA Leiden, Division of Medicinal Chemistry, Leiden Amsterdam Center for Drug Research, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, and Department of Biochemistry, RUNMC-286, Nijmegen Centre for Molecular Life Sciences, Radboud University Nijmegen Medical Centre, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands