Absolute comparison of simulated and experimental protein-folding dynamics

Protein folding is difficult to simulate with classical molecular dynamics. Secondary structure motifs such as α-helices and β-hairpins can form in 0.1-10 μs (ref. 1), whereas small proteins have been shown to fold completely in tens of microseconds². The longest folding simulation to date is a single 1-̀s simulation of the villin headpiece³; however, such single runs may miss many features of the folding process as it is a heterogeneous reaction involving an ensemble of transition states^{4, 5}. Here, we have used a distributed computing implementation to produce tens of thousands of 5-20-ns trajectories (700 μs) to simulate mutants of the designed mini-protein BBA5. The fast relaxation dynamics these predict were compared with the results of laser temperature-jump experiments. Our computational predictions are in excellent agreement with the experimentally determined mean folding times and equilibrium constants. The rapid folding of BBA5 is due to the swift formation of secondary structure. The convergence of experimentally and computationally accessible timescales will allow the comparison of absolute quantities characterizing in vitro and in silico (computed) protein folding⁶.