1. Institute of Pharmaceutical Chemistry, University of Düsseldorf, Universitätsstr. 1, D-40225 Düsseldorf, Germany, Department of Biophysical Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands, and Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada