Direct NMR evidence for an intermediate preceding the rate-limiting step in the unfolding of ribonuclease A

It is commonly believed that there are no detectable intermediates in the kinetic unfolding reactions of small proteins. If such intermediates could be found, they would give important information about the nature of the transition state for unfolding, which is thought to occur close to the native state. We report here that one-dimensional proton magnetic resonance spectra recorded during the unfolding of ribonuclease A provide direct evidence for at least one unfolding intermediate in which side chains are free to rotate. This intermediate appears to be a 'dry molten globule' of the kind hypothesized by Shakhnovich and Finkelstein.