1. Contribution from Organic and Bioorganic Chemistry, Lund University, P. O. Box 124, SE-221 00 Lund, Sweden, Section MIG, Department of Laboratory Medicine, University of Lund, Sölvegatan 23, SE-223 62 Lund, Sweden, and Departments of Medical Genetics and Microbiology and Biochemistry, University of Toronto, Toronto, Ontario, M5S 1A8, Canada