The transition-path is the tiny fraction of an equilibrium molecular trajectory when a transition occurs between two states. It contains all the mechanistic information on how a protein folds and unfolds, which is predicted to be heterogeneous. Transition-paths are readily observed in atomistic molecular dynamics simulations for systems with fast kinetics, but have never been observed experimentally for any system in the condensed phase. By carrying out a collective photon-by-photon analysis (likelihood calculations) of a very large number of transitions collected at high illumination intensities, we determined a transition-path time of ~2 �s for fast-folding WW domain and an upper bound of ~15 �s for slow-folding protein GB1. The transition-path times for the two proteins differ by less than 10-fold while the folding rates differ by a factor of 20,000. This result shows that a slow-folding protein can fold almost as fast as a fast-folding protein when folding actually occurs!

Measurement of average transition-path time for protein folding in single molecule FRET experiments

243rd National Meeting (2012)

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