title: Nuclear magnetic resonance studies of residual structure in thermally unfolded ribonuclease A.
abstract: A proton nuclear magnetic resonance study of the four histidine residues of thermally unfolded ribonuclease A has provided evidence that two of the residues are in regions of residual structure, whereas the other two are freely exposed to solvent. Histidine-48 and, tentatively, histidine-105 occupy an environment at 69 degrees characterized by residual structure and display a pK value of 5.75 and a spin-lattice relaxation time of about 0.8 sec at pH 5.5. Histidine-12 and, tentatively, histidine-119 are in an environment at 69 degrees which is freely accessible to solvent and show a pK value of 5.96 and a spin-lattice relaxation time of about 1.1 sec at pH 5.5.
mesh_terms: D003201:Computers; D005583:Fourier Analysis; D006639:Histidine / Q000032:analysis; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009682:Magnetic Resonance Spectroscopy; D011487:Protein Conformation; D012260:Ribonucleases / Q000032:analysis
chemical_list: D006639:Histidine; D012260:Ribonucleases