1. Department of Medical Biochemistry, Leiden Institute of Chemistry, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands
2. Department of Bioorganic Synthesis, Leiden Institute of Chemistry, Leiden University, P. O. Box 9502, 2300 RA Leiden, The Netherlands
3. York Structural Biology Laboratory, Department of Chemistry, The University of York, Heslington, York, YO10 5DD, UK