1. Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Department of Biochemistry, Institute of Cellular Signalling, University of Nijmegen, P.O. Box 9101, 6500 HB Nijmegen, The Netherlands, and Fachbereich Chemie, Universitaet Rostock, D-18055 Rostock, Germany