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