1. Department of Molecular and Cellular Neurobiology, Graduate School Neurosciences Amsterdam, Research Institute Neurosciences, Vrije Universiteit, Faculty of Biology, De Boelelaan 1087, NL-1081 HV Amsterdam, The Netherlands, Department of Chemistry, University of North Carolina, Chapel Hill, North Carolina 27599-3290, and Institute of Medical Physics and Biophysics, University of Münster, Robert-Koch-Strasse 31, D-48149 Münster, Germany