1. Northern Institute for Cancer Research, School of Natural SciencesChemistry, Bedson Building, University of Newcastle, Newcastle upon Tyne, NE1 7RU, U.K., Northern Institute for Cancer Research, Paul O'Gorman Building, Medical School, Framlington Place, University of Newcastle, Newcastle upon Tyne, NE2 4HH, U.K., and Laboratory of Molecular Biophysics and Department of Biochemistry, University of Oxford, Oxford, OX1 3QU, U.K.