Refolding of ribonuclease in the presence and absence of ammonium sulfate pulses. Comparison between experiments and simulations.

Experiments have been carried out on ribonuclease A in which refolding in high concentrations of guanidine hydrochloride is either preceded or not preceded by a short ammonium sulfate pulse. Application of the pulse causes the rapid formation of the nativelike intermediate, and the effect of this pulse was determined by using three different methods for monitoring the subsequent refolding reaction: direct absorbance, direct fluorescence, and a double-jump fluorescence unfolding assay which is specific for the isomerization of proline-93. The effect of the pulse is quite different depending on the method of detection. With absorbance detection, the pulse causes a large reduction in the refolding amplitude with no change in the kinetics of the decay curve, while with the fluorescence unfolding assay, the pulse causes no change in the refolding amplitude but produces a large acceleration in the decay kinetics. The results with direct fluorescence are intermediate with some reduction seen in the refolding amplitude and some acceleration in the decay kinetics. The results of these experiments are simulated by using the simple model of Lin and Brandts (1984) [Lin, L.-N., & Brandts, J. F. (1984) Biochemistry 23, 5713] in which proline-93 must be in the correct cis configuration before folding to the native or nativelike state can occur. In all cases, the simulations accurately predict the experimental results for all three methods of detection, without any adjustment of parameter values from those published earlier.(ABSTRACT TRUNCATED AT 250 WORDS)