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 cr ystal 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 c old neutron guide at the Institute Laue Langevin (ILL, Grenoble, France), a nd data collection with neutrons of 1.8-8.0 Angstrom wavelength to a crysta llographic resolution of 1.43 Angstrom was complete after about 36 h. This compares favourably with a previous experiment utilizing the same crystal s pecimen, where more than four weeks were required to collect monochromatic diffraction data to about 1 Angstrom resolution. Using the Laue data, the s tructure was solved by molecular replacement with the known X-ray crystal s tructure. Difference density maps revealed the atomic positions (including deuterium atoms) of seven ordered solvent water molecules and two (partiall y disordered) acetone molecules. These density maps were compared with corr esponding maps computed with monochromatic neutron-diffraction data collect ed to 1.0 Angstrom resolution using the same crystal specimen, as well as t o maps derived from high-resolution (0.90 Angstrom) synchrotron X-ray data. In spite of the better definition of atomic positions in the two high-reso lution maps, the 1.43 Angstrom LADI maps show considerable power for the de termination of the location of hydrogen and deuterium positions.