1. Contribution from the Max-Planck Society, Research Unit for Enzymology of Protein Folding, Weinbergweg 22, D-06120 Halle/Saale, Germany, Department of Biochemistry, Faculty of Biosciences, Pharmacy, and Psychology, University of Leipzig, Talstrasse 33, D-04103 Leipzig, Germany, and Department of Organic Chemistry, Faculty of Chemistry, University of Leipzig, Johannisallee 29, D-04103 Leipzig, Germany