Neutron Laue diffraction studies of coenzyme cob(II)alamin

Using a recently designed neutron single-crystal diffractometer utilizing a narrow-band Laue concept (LADI), diffraction data were collected from a crystal of the coenzyme cob(II)alamin (B_12r), crystallized from a mixture of D₂O and perdeuterated acetone. The instrument was placed at the end of a cold neutron guide at the Institute Laue Langevin (ILL, Grenoble, France), and data collection with neutrons of 1.8-8.0 Å wavelength to a crystallographic resolution of 1.43 Å was complete after about 36 h. This compares favourably with a previous experiment utilizing the same crystal specimen, where more than four weeks were required to collect monochromatic diffraction data to about 1 Å resolution. Using the Laue data, the structure was solved by molecular replacement with the known X-­ray crystal structure. Difference density maps revealed the atomic positions (including deuterium atoms) of seven ordered solvent water molecules and two (partially disordered) acetone molecules. These density maps were compared with corresponding maps computed with monochromatic neutron-diffraction data collected to 1.0 Å resolution using the same crystal specimen, as well as to maps derived from high-resolution (0.90 Å) synchrotron X-­ray data. In spite of the better definition of atomic positions in the two high-resolution maps, the 1.43 Å LADI maps show considerable power for the determination of the location of hydrogen and deuterium positions.