BioDiff - a neutron diffractometer optimized for crystals with largeunit cell dimensions

The neutron single crystal diffractometer BIODIFF at the research reactor Heinz Maier-Leibnitz (FRM II) is especially designed to collect data from crystals with large unit cells. The main field of application is the structural analysis of proteins, especially the determination of hydrogen atom positions. BIODIFF is a joint project of the Jülich Centre for Neutron Science (JCNS) and the FRM II. Typical scientific questions addressed are the determination of protonation states of amino acid side chains and the characterization of the hydrogen bonding network between the protein active centre and an inhibitor or substrate. BIODIFF (see scheme below) is designed as a monochromatic instrument with a narrow wavelength spread of less than 3 %. By using a highly orientated pyrolytic graphite monochromator (PG002) the diffractometer is able to change its central wavelength in the range between 2.4 Å and 5.6 Å. Contaminations of higher order wavelengths are removed by a neutron velocity selector. To cover a large solid angle the main detector of BIODIFF consists of a neutron imaging plate in a cylindrical geometry with online read-out capability. With a radius of 200 mm and a height of 450 mm it covers a solid angle of approximately 2π with a spatial resolution of up to 125 µm. A fast Li/ZnS scintillator CCD camera is available for additional detection abilities. An optical CCD-camera pointing at the sample position is used to quickly align the sample with respect to the neutron beam. The main advantage of BIODIFF is the possibility to adapt the wavelength to the size of the unit cell of the sample crystal while operating with a clean monochromatic beam that keeps the background level low. BIODFF is equipped with a standard Oxford Cryosystem “Cryostream 700+” which allows measurements in the temperature range from 90 K up to 500 K (see picture below). A standard closed cycle cryostat is also available for measurements down to 4 K in vacuum. This may be used for the determination of magnetic unit cells of inorganic crystals in zero magnetic field. Recent applications of BIODIFF also include clay powder diffraction and diffraction from lipid membranes in a controlled humidity environment.