NUCLEAR MAGNETIC RESONANCE STUDIES OF STRUCTURE AND BINDING SITES OF ENZYMES .7. SOLVENT AND TEMPERATURE EFFECTS ON IONIZATION OF HISTIDINE RESIDUES OF RIBONUCLEASE

The chemical shift of the nuclear magnetic resonance absorption of the C2 hydrogen of histidine is a measure of the protonation of the imidazole ring. By this means the titration curves of the four histidine residues of ribonuclease have been obtained under a variety of conditions. Comparisons are made between the titration curves obtained in H2O or D2O and 0.2 m sodium acetate or 0.2 m sodium chloride, and thermodynamic data are reported for the ionization of histidine residues 12, 119, and 105. Evidence for an isomerization of the enzyme involving histidine residue 48 is discussed. © 1969, American Chemical Society. All rights reserved.