LADI - A NEW FACILITY FOR NEUTRON LAUE PROTEIN CRYSTALLOGRAPHY
D.A.A. Myles, F. Cipriani, J.C. Castagna & C. Wilkinson
EMBL
Outstation, Av. des Martyrs, F38042 Grenoble, France,
myles@embl-grenoble.fr
A novel 2-1 neutron sensitive image plate detector, optimised for cold neutron Laue crystallography of biological macromolecules at medium resolution (>1.5 A), is now fully operational at ILL, Grenoble.
Neutron protein crystallography can offer a powerful adjunct to X-ray analysis at the medium resolutions (>1.5A) typical of X-ray protein structure determinations by enabling key details of the hydrogen structure to be revealed. Hydrogen and/or deuterium atoms can be more readily located in a corresponding neutron analysis because the scattering amplitudes of H/D for neutrons are closely similar to those of other biological atoms. Moreover, neutron diffraction is uniquely able to distinguish between hydrogen (negative fermi density) and deuterium (positive fermi density ) exchanged positions in deuterated crystals, thus providing an elegant probe of group accessibility, mobility and exchange dynamics. However, as neutron fluxes are relatively weak, the sample volumes (>5mm3) and time scales (weeks or months) required for conventional (monochromatic) neutron crystallography of even modest biological systems are often prohibitive - such studies are rarely performed.
Laue (white beam) diffraction can provide a more rapid and efficient survey of reciprocal space, maximising the flux at the sample and stimulating large numbers of reflections simultaneously over all incident wavelengths. A neutron Laue diffractometer (LADI) designed to exploit this advantage for protein crystallography has been jointly developed at ILL/EMBL-Grenoble[1] and is now installed on cold guide H142 at ILL. The detector consists of a large Gd2O3 doped neutron sensitive image plate (400x800mm) mounted on a cylindrical camera (318mm diameter) that completely encircles the sample. Narrow (Quasi-Laue) band-passes (l=3.5A, dl/l=8-20%) can be selected using Ni/Ti multilayer filters in order to reduce the reflection overlap and incoherent background from large unit cell biological crystals. As a consequence, almost all reflections (>99%) recorded on the LADI detector are singlets and thus amenable to standard crystallographic analysis [2,3].
The LADI diffractometer began scheduled operation in January 98. The instrument is optimised for single crystal studies of small protein systems (<40kDa) at medium resolution (>1.5A), sufficient to locate individual hydrogen atoms of special interest, water structures or other small molecules that can be marked with deuterium to make them particularly visible. The order of magnitude gains in data collection rates that have been achieved [2,3] make feasible studies of larger biological complexes and/or smaller crystals than has previously been possible. Instrument performance will be illustrated by some recent results.