The folding and unfolding of proteins is generally assumed to be so co-operative that the overall process may be followed by a single probe, such as tryptophan fluorescence, Folding kinetics of three mutants of barnase and chymotrypsin inhibitor 2 (CI2) were studied by real-time NMR. Rate constants for changes in individual residues during the unfolding or refolding of the mutants studied by real-time NMR are all within experimental error of the overall process of folding/unfolding measured by stopped-flow measurements of tryptophan fluorescence. Folding of these mutants is thus highly cooperative. Changes in the tryptophan fluorescence give accurate measurements of the protein folding process.
Killick, T. R., Freund, S. M. V., & Fersht, A. R. (1998). Real-time NMR studies on folding of mutants of barnase and chymotrypsin inhibitor 2. FEBS Letters, 423(1), 110–112. https://doi.org/10.1016/S0014-5793(98)00075-1