Photon-by-photon trajectories of single protein molecules folding and unfolding

The transition path time in kinetics is the tiny fraction of an equilibrium trajectory for a single molecule when the transition actually happens and has not been measured for any molecular process in solution. From measurements of photon-by-photon trajectories for fluorophore-labeled single protein molecules undergoing folding and unfolding transitions we have determined that the upper bound for the transition path time is more than 10,000-fold less than the mean first passage time, consistent with a Kramers' analysis of diffusive barrier crossings.