1. Institut für Physiologische Chemie, Tierärztliche Fakultät, Ludwig-Maximilians-Universität, Veterinärstrasse 13, 80539 München, Germany, The School of Chemistry, University of Bangor, Alun Roberts Building, Deiniol Road, Bangor, Gwynedd LL57 2UW, U.K., and Centre for Synthesis and Chemical Biology, University College Dublin, Belfield, Dublin 4, Ireland