1. Institute of Immunology and Environmental Hygiene, Philipps University Marburg, Pilgrimstein 2, D-35037 Marburg, Germany, Lehrstuhl für Bodenkunde, Technische Universität München, D-85350 Freising- Weihenstephan, Germany, and Department of Chemistry, Philipps University Marburg, Hans-Meerwein-Strasse, D-35032 Marburg, Germany