1 Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, UK; Department of Physics and Astronomy, University College London, Gower St., London, WC1E 6BT, UK.
2 School of Biological and Chemical Sciences, Queen Mary University of London, London, E1 4NS, UK; QB3 University of California, Berkeley CA94720, USA.
3 Institut für Biochemie, 79104 Freiburg, Germany.
4 Clarendon Laboratory, University of Oxford, Oxford OX1 3PU, UK.
5 School of Biological and Chemical Sciences, Queen Mary University of London, London, E1 4NS, UK.
6 QB3 University of California, Berkeley CA94720, USA.
7 Biological Physical Sciences Institute (BPSI), University of York, York YO10 5DD, UK. Electronic address: mark.leake@york.ac.uk.