1. Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, PO Box 9502, 2300 RA Leiden, The Netherlands, Department of Medical Biochemistry, Academic Medical Center, University of Amsterdam, 1105 AZ Amsterdam, The Netherlands, DSM Pharma Chemicals - ResCom, Donaustaufer Strasse 378, 93055 Regensburg, Germany, and Syncom BV, Kadijk 3, PO Box 2253, 9704 CE Groningen, The Netherlands