1. Department of Chemistry; Ludwig-Maximilians-Universität München; Butenandtstr. 5-13 81377 Munich Germany
2. Present address: Faculty of Chemistry; University of Belgrade; Studentski trg 12-16 11158 Belgrade Serbia
3. Helmholtz Institute for Pharmaceutical Research Saarland (HIPS); Helmholtz Center for Infection Research and Pharmaceutical Biotechnology; Saarland University; Campus, Building E8.1 66123 Saarbrücken Germany
4. Department of Chemistry; New York University; 100 Washington Square East New York USA