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