1. Universität WürzburgInstitut für Organische Chemie Am Hubland 97074 Würzburg Germany
2. International Max Planck Research School Molecular BiologyUniversity of Göttingen Göttingen Germany
3. Present address: Department of PhysicsTechnische Universität München München Germany
4. Department of Molecular BiologyUniversity Medical Center Göttingen Humboldtallee 23 37073 Göttingen Germany