Evidence concerning rate-limiting steps in protein folding from the effects of trifluoroethanol

The refolding kinetics of 13 proteins have been studied in the presence of 2,2,2-trifluoroethanol (TFE). Low concentrations of TFE increased the folding rates of all the proteins, whereas higher concentrations have the opposite effect. The extent of deceleration of folding correlates closely with similar effects of guanidine hydrochloride and can be related to the burial of accessible surface area during folding. For those proteins folding in a two-state manner, the extent of acceleration of folding correlates closely with the number of local backbone hydrogen bonds in the native structure. For those proteins that fold in a multistate manner, however, the extent of acceleration is much smaller than that predicted from the data for two-state proteins. These results support the concept that for two-state proteins the search for native-like contacts is a key aspect of the folding reaction, whereas the rate-determining steps for folding of multistate proteins are associated with the reorganization of stable structure within a collapsed state or with the search for native-like interactions within less structured regions.