1. Institute of Biophysics and Physical Biochemistry, University of Regensburg, D-93040, Regensburg, Germany, Division of Biophysical Chemistry, Biozentrum Basel, University of Basel, CH-4056 Basel, Switzerland, Department of Organic Chemistry, University of Barcelona, Spain, and Abteilung Biologie der Mikroorganismen, Ruhr-Universität Bochum, D-44780 Bochum, Germany