1. Department of Chemistry, Birkbeck College, University of London, Gordon House and Christopher Ingold Laboratories, 29 Gordon Square, London WC1H 0PP, U.K., MRC Biomedical NMR Centre, National Institute for Medical Research, Mill Hill, London NW7 1AA, U.K., Department of Biochemistry and Molecular Biology, University of British Columbia, Vancouver, BC V6T 1Z3, Canada, and Department of Biochemistry, University of Vermont College of Medicine, Burlington, Vermont 05405