1. Department of Physiology, Birmingham University, Birmingham B15 2TJ, U.K.
2. Department of Biochemistry, Sheffield University, Sheffield S10 2TN, U.K.
3. Centre de Biochimie Macromoleculaire, CNRS, 34033 Montpellier, France
4. Inorganic Chemistry Laboratory, Oxford University, Oxford OX1 3QR, U.K.