title: Orientation of histidine residues in RNase A: neutron diffraction study.
abstract: Difference Fourier maps have been calculated at 2.8-A resolution by using neutron diffraction data obtained from a single crystal of RNase A. The phases were derived from a model resulting from the joint refinement of x-ray and neutron data at 2.0-A and 2.8-A resolution, respectively. The orientation of histidine-48 assumed during the refinement of the x-ray model at 2.5 A was confirmed, whereas the other three histidines had to be rotated around C beta--C gamma bonds in order 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 oxygen of the inorganic phosphate, and histidine-119 is bonded to another oxygen of the phosphate and to the oxygen OD1 of aspartic acid-121.
mesh_terms: D003460:Crystallization; D004720:Endonucleases; D005583:Fourier Analysis; D006639:Histidine / Q000032:analysis*; D009501:Neutron Activation Analysis; D011487:Protein Conformation; D012259:Ribonuclease, Pancreatic; D012260:Ribonucleases; D014961:X-Ray Diffraction
chemical_list: D006639:Histidine; D004720:Endonucleases; D012260:Ribonucleases; D012259:Ribonuclease, Pancreatic