1. Chemistry Department, University College London, 20 Gordon Street, London WC1H 0AJ, U.K., School of Chemistry, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9ST, Scotland, U.K., Department of Biochemistry and Molecular Biology, University College London, Gower Street, London WC1E 6BT, U.K., and Department of Chemistry, The University of Auckland, Auckland, New Zealand