1. Organic Chemistry and Chemical Biology; Goethe University Frankfurt; Max-von-Laue-Strasse 7 60438 Frankfurt am Main Germany
2. German Cancer Consortium; DKTK; German Cancer Research Center; DKFZ; Im Neuenheimer Feld 280 69120 Heidelberg Germany
3. Institute for Biophysics; Goethe University Frankfurt; Max-von-Laue-Strasse 1 60438 Frankfurt am Main Germany
4. Present address: Francis Bitter Magnet Laboratory; Massachusetts Institute of Technology; Cambridge MA 02139 USA