  When described by a low-dimensional reaction coordinate, the rates of protein
folding are determined by a subtle interplay between free-energy barriers and
friction. While it is commonplace to extract free-energy profiles from
molecular trajectories, a direct evaluation of friction is far more elusive,
and one typically evaluates it indirectly via memoryless reaction rate
theories. Here, using memory-kernel extraction methods founded on a generalised
Langevin equation (GLE) formalism, we directly calculate the memory-dependent
friction for eight fast-folding proteins, taken from a published set of
large-scale molecular dynamics protein simulations. Our results reveal that,
contrary to common expectation, friction is more important than free energy
barriers in determining protein folding rates, particularly for larger
proteins. We also show that proteins fold in a regime where the finite decay
time of friction significantly reduces the folding times, in some instances by
as much as a factor of 10, compared to predictions based on memoryless
friction.
