1. Department of Chemistry, University of Natural Resources and Applied Life Sciences, Muthgasse 18, A-1190 Vienna, Austria, Research Institute for Biochemical Pharmacology and Toxicology, University of Veterinary Medicine Vienna, Veterinärpl. 1, A-1210 Vienna, Austria, Dipartimento di Chimica e Chimica Industriale, Università di Pisa, Via Risorgimento 35, 56126 Pisa, Italy, and Research and Development, DSM Nutritional Products, P.O. Box 3255, CH-4002 Basel, Switzerland