1. Kekulé-Institute of Organic Chemistry and Biochemistry, University of Bonn, Gerhard-Domagk-Strasse 1, 53121 Bonn, Germany
2. Interfaculty Institute of Microbiology and Infection Medicine, Deptartment of Microbial Bioactive Compounds, Auf der Morgenstelle 28, 72076 Tübingen, Germany