1. Institut für Chemie; Biologische Chemie; Technische Universität Berlin; Müller-Breslau-Strasse 10 10623 Berlin Germany
2. Present address: The Scripps Research Institute; Department of Chemistry; Jupiter FL 33458 USA
3. Institute of Chemistry and Biochemistry; Freie Universität Berlin; Takustrasse 6 14195 Berlin Germany