Institute of Structural and Molecular Biology, School of Crystallography, Birkbeck College, Malet Street, London, WC1E 7HX, UK. m.halling-brown@mail.cryst.bbk.ac.uk|Institute of Structural and Molecular Biology, School of Crystallography, Birkbeck College, Malet Street, London, WC1E 7HX, UK. m.halling-brown@mail.cryst.bbk.ac.uk|Institute of Structural and Molecular Biology, School of Crystallography, Birkbeck College, Malet Street, London, WC1E 7HX, UK. m.halling-brown@mail.cryst.bbk.ac.uk