1. Division of Pharmaceutical Chemistry, Faculty of Pharmacy, P.O. Box 56, FI-00014 University of Helsinki, Finland, Laboratory of Inorganic Chemistry, Department of Chemistry, P.O. Box 55, FI-00014 University of Helsinki, Finland, and Department of Natural Sciences, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark