[1] ESRF, F-38043 Grenoble, France
[2] CLRC Daresbury Lab, Warrington WA4 4AD, Cheshire, England
[3] Rutherford Appleton Lab, Diamond Light Source, Chilton OX11 0QX, Oxon, England
[4] DESY Hamburg, D-22607 Hamburg, Germany
[5] MRC Lab Mol Biol, Cambridge CB2 2QH, England
[6] EMBL Hamburg, D-22603 Hamburg, Germany