1. Department of Chemistry, Organic & Bioorganic Chemistry, University of Graz, Heinrichstrasse 28, A-8010 Graz, Austria, Institute of Biochemistry, Graz University of Technology, Petersgasse 12, A-8010 Graz, Austria, Center for Medical Research, Medical University of Graz, Stiftingtalstrasse 24, A-8010 Graz, Austria, and Institute of Molecular Biosciences, Structural Biology, University of Graz, Humboldtstrasse 50/III, A-8010 Graz, Austria