The IBARAKI biological crystal diffractometer (iBIX) was used in single-crystal time-of-flight neutron diffraction experiments on manganese catalase from Thermus thermophilus. The unit-cell dimensions were 133 × 133 × 133 Å, which is close to the designed maximum limitation of iBIX (135 × 135 × 135 Å). The optimum integration box sizes were set and the degree of integration box overlap was calculated for each Laue spot. Using the overlap ratio as the criterion, the selection of the diffraction intensity data was performed to give a minimum R
p.i.m.. Subsequently, diffraction intensity data from Laue spots with overlap ratios ≤0.1 were selected and a complete reflection data set with d
min = 2.35 Å was obtained. Joint X-ray and neutron structure refinements were also successfully performed. It was difficult to determine the structures and protonation states of all the oxygen atoms in the manganese cluster owing to the disordered structure. No hydrogen atom was observed on the ordered μ-bridging oxygen atom O1003. Instead, this oxygen atom probably forms a hydrogen bond with Thr39. In addition, the refinements clearly showed the protonation states of the amino acid residues and hydrogen bonds, as observed in Tyr192, Glu167 and Glu280. This first neutron crystal structure of manganese catalase shows that iBIX can provide acceptable diffraction data for neutron single-crystal analyses of at least 2.4 Å resolution within the original targeted unit-cell dimensions of 135 × 135 × 135 Å.
