1. Department of Chemistry, University of Edinburgh, The King's Buildings, West Mains Road, Edinburgh, EH9 3JJ U.K., Zeneca Life Science Molecules, Biomolecules, P.O. Box 2, Belasis Avenue, Billingham, Cleveland, TS23 1YN U.K., and Departments of Molecular Biology and Biochemistry, and Physiology and Biophysics, University of California, Irvine, California 92697-3900