Thermal Unfolding Transition of Ribonuclease A Measured by 2′-CMP Binding

We report an approach to the problem of detecting and characterizing intermediates in the unfolding of ribonuclease A. Two distinct properties of the protein are compared at equilibrium within the unfolding transition zone: (1) a physical property of the protein, the absorbance of buried tyrosine residues, and (2) a functional property, the ability to bind the specific ligand, 2′-CMP. A direct comparison of these two properties is made within the pH 5.8 transition zone, and an indirect comparison is made by using the stopped-flow instrument to sample rapidly the equilibrium properties of the pH 2.0 transition. At both pH 2.0 and pH 5.8, the results indicate that there are no intermediates in folding which have the physical properties of the native enzyme but which have lost the ability to bind a specific ligand.