Department of Crystallography/Biological Sciences, Institute of Structural and Molecular Biology, Birkbeck College, University of London, London WC1E 7HX, UK. a.pandurangan@mail.cryst.bbk.ac.uk|Department of Crystallography/Biological Sciences, Institute of Structural and Molecular Biology, Birkbeck College, University of London, London WC1E 7HX, UK. a.pandurangan@mail.cryst.bbk.ac.uk