School of Crystallography and Institute of Structural and Molecular Biology, Birkbeck College, University of London, Malet Street, London WC1E 7HX UK. r.kabiljo@mail.cryst.bbk.ac.uk|School of Crystallography and Institute of Structural and Molecular Biology, Birkbeck College, University of London, Malet Street, London WC1E 7HX UK. r.kabiljo@mail.cryst.bbk.ac.uk|School of Crystallography and Institute of Structural and Molecular Biology, Birkbeck College, University of London, Malet Street, London WC1E 7HX UK. r.kabiljo@mail.cryst.bbk.ac.uk