1. Laboratory of Molecular Biophysics, Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, U.K., Oxford Centre for Molecular Science, South Parks Road, Oxford OX1 3QT, U.K., and Department of Crystallography, Birkbeck College, University of London, Malet Street, London WC1E 7HX, U.K.