1. Institute of Pharmacy and Food Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany, Department of Physiological Chemistry II, Theodor-Boveri-Institute, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany, Institute of Organic Chemistry, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany, and Institute of Virology and Immunology, University of Würzburg, Versbacher Strasse 7, D-97078 Würzburg, Germany