1. Research Complex at Harwell, Rutherford Appleton Laboratory, Harwell Oxford, Didcot OX11 0FA, U.K.
2. Department of Chemistry, University of Oxford, Oxford OX1 3QR, U.K.
3. Division of Molecular Microbiology, School of Life Sciences, University of Dundee, Dundee DD1 5EH, U.K.