Computational estimation of ms-sec atomistic folding times

Despite the development of massively parallel computing hardware including inexpensive graphics processing units (GPUs), it has remained infeasible to simulate the folding of atomistic proteins at room temperature using conventional molecular dynamics (MD) beyond the s scale. Here we report the folding of atomistic, implicitly solvated protein systems with folding times {tau}f ranging from ~100 s to ~10s using the weighted ensemble (WE) strategy in combination with GPU computing. Starting from an initial structure or set of structures, WE organizes an ensemble of GPU-accelerated MD trajectory segments via intermittent pruning and replication events to generate statistically unbiased estimates of rate constants for rare events such as folding; no biasing forces are used. Although the variance among atomistic WE folding runs is significant, multiple independent runs are used to reduce and quantify statistical uncertainty. Three systems were examined: NTL9 at low solvent viscosity (yielding {tau}f ~ 5s), NTL9 at water-like viscosity ({tau}f ~ 40s), and Protein G at low viscosity ({tau}f ~ 10s). In all cases the folding time, uncertainty, and ensemble properties could be estimated from WE simulation; for protein G, this characterization required significantly less overall computing than would be required to observe a single folding event with conventional MD simulations. Our results suggest discrepancies with experimental folding times that should enable improvement of force fields and solvent models.