Abstract
Difference Fourier maps were calculated at 2.8 .ANG. resolution by using neutron diffraction data obtained from a single crystal of [bovine pancreatic] RNase A. The phases were derived from a model resulting from the joint refinement of X-ray and neutron data at 2.0 .ANG. and 2.8 .ANG. resolution, respectively. The orientation of histidine-48 assumed during the refinement of the X-ray model at 2.5 .ANG. was confirmed, whereas the other 3 histidines had to be rotated around C.beta.-C.gamma. bonds to agree with the neutron difference Fourier maps. In the final model, histidine-12 is clearly hydrogen bonded to the carbonyl oxygen of threonine-45 and to the O2 of the inorganic phosphate, and histidine-119 is bonded to another O2 of the phosphate and to the oxygen OD1 of aspartic acid 121.