1. Departments of Biochemistry and Chemistry, University of Leicester, University Road, Leicester, LE1 7RH, U.K., Biomedical Research Centre, University of Dundee, Ninewells Hospital & Medical School, Dundee, DD1 9SY, U.K., and Biological NMR Centre and Department of Biochemistry, University of Leicester, P.O. Box 138, Leicester, LE1 9HN, U.K.