1. Contribution from the Leiden Institute of Chemistry, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands, Departments of Biochemistry and Molecular Biology and of Microbiology and Immunology, University of British Columbia, Vancouver, BC V6T 1Z3, Canada, Institute of Analytical Chemistry, The Danish University of Pharmaceutical Sciences, 2100 Copenhagen, Denmark, and The Weizmann Institute of Science, Rehovot 76100, Israel