Proton Nuclear Magnetic Resonance Studies of Ribonuclease A in H 2 O

Proton Nuclear Magnetic Resonance Studies of Ribonuclease A in H₂O

Dinshaw J. Patel*,
Clare K. Woodward†, and
Frank A. Bovey*

^*Bell Telephone Laboratories, Incorporated, Murray Hill, New Jersey 07974
^†Department of Laboratory Medicine, University of Minnesota Medical School, Minneapolis, Minn. 55455

Abstract

A resonance (designated a) due to an exchangeable proton titrates (pK_a = 6.1) between 11.5 and 13 ppm in the nuclear magnetic resonance spectrum of RNase A-0.2 M NaCl in H₂O at 20°. Comparison with models has permitted assignment to a ring-nitrogen proton of histidine in slow exchange with solvent H₂O. The pH and temperature-dependent line-width changes of resonance a are analyzed in terms of an exchange between histidine and protonated histidine, without the necessity to invoke any exchange processes associated with protein conformational changes. Several other resonances due to exchangeable protons are observed between 10 and 15 ppm in the nuclear magnetic resonance spectrum of RNase A in H₂O.