1. Chemistry Research Laboratory; University of Oxford; Mansfield Road Oxford OX1 3TA UK
2. Present address: Department of Molecular and Cellular Physiology; University of Stanford; Stanford CA 94305-5345 USA
3. Present address: School of Chemical Sciences; The University of Auckland; Private Bag 92019 Auckland 1142 New Zealand
4. Present address: Roche Innovation Center Munich; Roche Diagnostics GmbH; Nonnenwald 2 82377 Penzberg Germany
5. Present address: Institute of Pharmaceutical Sciences; Albert-Ludwigs-Universität Freiburg; 79104 Freiburg Germany