1. Department of Animal Product Quality, Danish Institute of Agricultural Sciences, Research Centre Foulum, P.O. Box 50, DK-8830 Tjele, Denmark; Chemistry Department, Bioinorganic Group, The Royal Veterinary and Agricultural University, Thorvaldsensvej 40, DK-1871 Frederiksberg C, Denmark; and Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark